G Klotz

B Marquet

D C Mos

P Rogers

Keith

V De Mazancourt

New Insights on Biodiversity

Introduction

Atyid shrimps (Crustacea: Decapoda) are essential components of tropical freshwater ecosystems, playing a role of cleaner by shredding fallen leaves or filtering organic particular matter and being preys to a number of organisms [START_REF] Covich | The role of benthic invertebrate species in freshwater ecosystems[END_REF][START_REF] Crowl | Freshwater shrimp effects on detrital processing and nutrients in a tropical headwater stream[END_REF]. Among them, genus Caridina H. Milne Edwards, 1837 is particularly diversified, comprising more than 300 described species making it the most diversified of the infra-order Caridea [START_REF] De Grave | Dead Shrimp Blues: A Global Assessment of Extinction Risk in Freshwater Shrimps(Crustacea: Decapoda: Caridea)[END_REF]. In Indo-Pacific Islands, the majority of the species of Caridina have an original diadromous lifestyle, with a planktonic marine larval phase and a benthic freshwater adult phase; amphidromy [START_REF] Mcdowall | On amphidromy, a distinct form of diadromy in aquatic organisms[END_REF][START_REF] Bauer | Amphidromy in shrimps: a life cycle between rivers and the sea[END_REF]. This strategy allows the species to colonize isolated habitats, such as the rivers of volcanic islands, and to survive to the instability of these environments (floods, droughts, volcanism, etc.) by keeping a stock of larvae in the ocean, ready to recolonize depopulated rivers [START_REF] Keith | Vanuatu freshwater fish and crustaceans[END_REF]. Amphidromy, by isolating populations, has also contributed to establishing the high diversity that exists among caridean shrimps. This diversity, however, is the source of an extreme taxonomic confusion that impedes the monitoring of the species and the establishment of appropriate conservation programs [START_REF] Klotz | To "bee" or not to be -on some ornamental shrimp from Guangdong Province, Southern China and Hong Kong SAR, with descriptions of three new species[END_REF]. Recently, with the development of new molecular biology techniques, integrative taxonomy is being increasingly used by systematicians attempting to achieve more precise species delineations by combining morphological data with genetic, ecological and/or biogeographical information [START_REF] Mazancourt (de) | When morphology and molecules work together: lines of evidence for the validity of Caridina buehleri Roux, 1934 (Crustacea: Decapoda: Atyidae) and Caridina gueryi Marquet, Keith & Kalfatak, 2009 as its junior synonym[END_REF]. In this chapter, we 1) review the current state of knowledge of amphidromous Caridina diversity in the light of the latest results of integrative taxonomy on this model, 2) review the threats faced by Caridina in Indo-Pacific Islands, and 3) discuss how integrative taxonomy approaches can inform and support programs designed to limit the impacts of threatening processes on the diversity of Caridina.

Biodiversity

Taxonomy

There is substantial confusion surrounding the taxonomy of many species of Caridina that has resulted in the creation of several species complexes (N.B.: We consider a species complex a monophyletic species group that share a common taxonomic history in having been synonymized at one point, and/or that can be identified by morphological characters). In Indo-Pacific islands, amphidromous shrimps have been grouped into seven main complexes, (1) Caridina nilotica (P. Roux, 1833) complex [START_REF] Johnson | Distributional and other notes on some freshwater prawns (Atyidae and Palaemonidae) mainly from the Indo-West Pacific region[END_REF][START_REF] Holthuis | A collection of Decapod Crustacea from Sumba, Lesser Sunda islands, Indonesia[END_REF][START_REF] Jalihal | Five new species of freshwater atyid shrimps of the genus Caridina H. Milne Edwards from Dharwar area (Karnataka State, India). Records of the Zoological Survey of India[END_REF][START_REF] Choy | The atyid shrimps of Fiji with description of a new species[END_REF][START_REF] Richard | Caridina nilotica (P. Roux, 1833) (Crustacea: Decapoda: Caridea: Atyidae) from East Africa, with descriptions of four new species[END_REF][START_REF] Karge | Süßwassergarnelen aus aller Welt[END_REF], (2) C. weberi De Man, 1892 complex [START_REF] Richard | A systematic report on the fresh water prawns of the atyid genus Caridina H. Milne Edwards 1837, from Madras (Tamilnadu: India)[END_REF]Cai & Shokita, 2006a), (3) C. gracilirostris De Man, 1892 complex [START_REF] Cai | The freshwater decapod crustaceans of Halmahera, Indonesia[END_REF][START_REF] Mcdowall | On amphidromy, a distinct form of diadromy in aquatic organisms[END_REF], (4) C. typus H. Milne Edwards, 1837 complex [START_REF] Karge | Süßwassergarnelen aus aller Welt[END_REF][START_REF] Bernardes | The complex evolutionary history and phylogeography of Caridina typus (Crustacea: Decapoda): long-distance dispersal and cryptic allopatric species[END_REF], ( 5) C. serratirostris De Man, 1892 complex (Cai & Shokita, 2006b), ( 6) C. brevicarpalis De Man, 1892 complex [START_REF] Short | Freshwater Crustacea of the Mimika Region, New Guinea[END_REF]) and ( 7) C. propinqua De Man, 1908 complex (WK, unpublished). Little work has been done to differentiate among the species clustered within these complexes, possibly due to the difficulty and costs associated with genetic analyses and the highly similar morphology of shrimps from different populations.

To recognize amphidromous species, the egg size is often a good indicator. Small eggs indicate that the species has an indirect development with several planktonic stages that are often marine.

Conversely, species with large eggs are most likely landlocked, with a direct development limiting their dispersal abilities, meaning that they are often endemic. However, some species can have small eggs and still be endemic. This can be due to particular conditions that limit the dispersal of marine larvae. For example, oceanic currents or physical barriers (e.g. closed coral lagoons) may prevent dispersal to nearby islands even if larvae are capable of surviving in oceanic conditions. It is not unusual therefore for different types of larval development to occur within one species group.

Moreover, in some cases, like in C. meridionalis (see [START_REF] Mazancourt (de) | Integrative taxonomy helps separate four species of freshwater shrimps commonly overlooked as Caridina longirostris (Crustacea: Decapoda: Atyidae) in Indo-West Pacific islands[END_REF] or C. gracilipes (WK, unpublished), egg size varies within a single species, depending on the habitat, with generally larger eggs found in upstream localities and smaller eggs near the estuary, suggesting the possibility of facultative amphidromy.

Phylogeny

A recent work [START_REF] Mazancourt (de) | The complex study of complexes: The first well-supported phylogeny of two species complexes within genus Caridina (Decapoda: Caridea: Atyidae) sheds light on evolution, biogeography, and habitat[END_REF], object of a presentation at the 9 th International Crustacean Congress in Washington DC (May 2018), focused on producing a molecular phylogeny of two species complexes from the Indo-Pacific islands, C. nilotica and C. weberi complexes. This work demonstrated the importance of an integrative taxonomy approach in understanding the diversity of Caridina. Non-integrated approaches are likely to underestimate the as the number of species for a locality, with many cryptic/pseudocryptic species confused under a single name, or more rarely, morphotypes of a single variable species described as separate species. As a result, geographical ranges and ecological data concerning poorly delineated species are often misleading, which impedes the establishment of programs that provide sufficient protection and management. The work also found that habitat was often the most important factor leading to the separation of complexes. Species belonging to different complexes live in different habitat types. Identification to species level (or at least species complex level) and monitoring of Caridina diversity over time may therefore provide an early warning system that provides information about changes in their environment.

Habitat

(Figure 1 here) Habitat is a critical factor influencing the distribution of Caridina species and it can often be used as an identification tool to separate species [START_REF] Mazancourt (de) | The complex study of complexes: The first well-supported phylogeny of two species complexes within genus Caridina (Decapoda: Caridea: Atyidae) sheds light on evolution, biogeography, and habitat[END_REF]. Caridina shrimps inhabit different kinds of freshwater ecosystems, divided between lentic environments, such as lakes (n°7 in Fig. 1; Fig. 2L) or swamps (n°8 in Fig. 1; Fig. 2K), and lotic (river) environments for surface waters, as well as for subterranean waters (n°9 in Fig. 1). Amphidromous shrimps, due to their ability to migrate between freshwater and seawater are mainly found in rivers or lentic environments connected to the sea.

On the tropical mountainous islands that are typical in the Indo-Pacific, rivers are subject to important natural disturbances like droughts, floods, volcanic eruptions, or earthquakes. The flow of rivers can therefore be subject to great temporal variations (strong during floods or zero for temporary waterways) as well as spatial variations (depending on the slope, substrate type, or plant cover). The morphology of rivers, for example, plays an important role in determining the variability of environmental conditions that are often unique to the specific watershed that form discontinuous units isolated from each other by the sea and geological formations, such as ridgelines. The length of rivers depends on island morphology, ranging from short streams a few metres in length to large rivers of several kilometres. Examples of the great diversity of river lengths in the Indo-Pacific include the Diahot in New Caledonia and the Sigatoka in Fiji.

To better characterize the variable environmental conditions experienced by shrimp in different rivers, [START_REF] Keith | Vanuatu freshwater fish and crustaceans[END_REF] proposed an altitudinal zonation divided into five zones with flexible boundaries depending on the island studied. The five zones are the spring zone (n°1 in Fig. 1; Fig. 2I), the higher course (n°2 in Fig. 1; Fig. 2A andB), the middle course (n° 4 in Fig. 1; Fig. 2C andD), the low part of the stream (n°5 in Fig. 1; Fig. 2E andF), and the lower course (n°6 in Fig. 1; Fig. 2G andH). An additional sub-zonation exists depending on the current velocity, distinguishing calm zones (Fig. 2C andE) or pools (Fig. 2B) from riffles (Fig. 2A, D andF). Usually, the altitudinal zonation is further simplified into three functional levels (Fig. 1): -The higher course, characterized by a steep slope (generally >10%), a fast current and a boulder substrate.

-The middle course, characterized by an intermediate slope (<10%) and a substrate of pebbles and rocks or sand in calm areas. This level is typically separated from the higher course by a topographical feature, like a cascade (n°3 in Fig. 1; Fig. 2J).

-The lower course, characterized by a very low or zero slope, with a slow current and a substrate of pebbles whose size decreases when approaching the estuary. The estuary is the lowest end of the river where environmental conditions are influenced by the tidal flux of seawater, as distinguished from the purely limnic area. This chemical boundary, between seawater and freshwater, is essential in terms of habitat, for some amphidromous species. The size of the estuary can vary, from very broad and long for large rivers (like the Jordan in Santo, Vanuatu) to absent in some small streams flowing directly from the mountains to the sea.

Species living in rivers are subject to a range of biotic and abiotic conditions depending on the zone of the river where they live. Differences in biotic and abiotic conditions often form gradients that apply over the entire length of rivers (Tab. 1):

-A current velocity gradient mainly linked to elevation. Water currents are typically fastest at high altitudes and slowest in the estuary, although local variations at a given altitude are also important, depending on the configuration of the river (calm zones vs riffles).

-A temperature and oxygen gradient, with cool and oxygen-rich waters typical of the higher course and warm and low-oxygen waters typical of the lower course.

-A width and depth gradient, from small rivulets in the spring zone to large rivers near the mouth.

-A salinity gradient that is typically limited to the lower course. Salinity tends to decline rapidly outside of the areas influenced by tidal flux.

-A sediment size gradient, from bedrock in the spring zone, to boulders and rocks in the higher course, pebbles in the middle course, and sand and silt in the lower course.

-A particulate organic matter gradient, with clear, oligotrophic waters in the spring area to turbid, eutrophic waters in the estuary.

-A riparian vegetation gradient, with riverbanks consisting of naked rock in the higher course switching to dense riparian vegetation in the lower course.

-A predation gradient, with the greatest density and diversity of predatory species (birds, fish) present in the lower course. Predators of shrimp in the higher course are mostly limited to eels (Anguilla spp.) or insects (dragonfly larvae, etc.) (Table 1 here) Gradients in abiotic and biotic factors along rivers overlap in many combinations to create a considerable variety of micro-habitats [START_REF] Gehrke | Chapter 7: Vulnerability of freshwater and estuarine fish habitats in the tropical Pacific to climate change[END_REF] to which species of Caridina have adapted. We can observe a vertical zonation of species depending on the variation of these factors linked to the elevation, as well as horizontal variables linked to the river configuration, according to the lotic or lentic facies in the area, or micro-habitats (sunken wood, roots, aquatic macrophytes, rocks, etc.). Indeed, different species assemblages are observed at the different levels of the rivers and in the different flowing facies. In the lower course will be encountered species of the C. nilotica species complex [START_REF] Mazancourt (de) | Integrative taxonomy helps separate four species of freshwater shrimps commonly overlooked as Caridina longirostris (Crustacea: Decapoda: Atyidae) in Indo-West Pacific islands[END_REF], the C. gracilirostris species complex or species allied to C. serratirostris and C. brevicarpalis. Higher in the river, two species groups are found, C. weberi complex in lotic areas and C. nilotica complex in lentic areas. Species of the C. typus group seem to have a broader range of habitats, present virtually in the whole length of the rivers (pers. obs.).

As observed for fish in tropical rivers by [START_REF] Pouilly | Changes of taxonomic structure of fish assemblages along an environmental gradient in the Upper Beni watershed (Bolivia)[END_REF] and [START_REF] Lorion | Altitudinal gradients in stream fish diversity and the prevalence of diadromy in the Sixaola River basin, Costa Rica[END_REF] in continental systems and by [START_REF] Keith | Indo-Pacific Sicydiine Gobies: biodiversity, life traits and conservation[END_REF] in insular systems, there seems to be an altitudinal diversity gradient in Caridina, with a maximum diversity in the lower course, decreasing with elevation. This diversity gradient may be explained by the greater concentration of organic particulate matter in the lower reaches, allowing the establishment of a higher biomass [START_REF] Angermeier | Fish communities along environmental gradients in a system of tropical streams[END_REF].

Alternatively, diversification may be promoted by higher heterogeneity in habitats [START_REF] Gorman | Habitat Structure and Stream Fish Communities[END_REF] and stronger predator pressure in the lower course [START_REF] Deacon | Gradients in predation risk in a tropical river system[END_REF]. The proximity to the ocean of the lower course also presents the advantage of reducing the distance that larvae of amphidromous species have to travel to the sea as larvae and to the upper course as juveniles. In higher altitudes, in contrast, resources are more limited but predator pressure is weaker, which often leads to low biodiversity but high abundance [START_REF] Leberer | Factors affecting the distribution of atyid shrimps in two tropical insular rivers[END_REF]. Some species living in these environments have adopted a life cycle completed entirely in freshwater, with reduced larval stages or even direct development, avoiding the hazards of migration [START_REF] Hancock | The relationship between egg size and embryonic and larval development in the freshwater shrimp Paratya australiensis Kemp (Decapoda: Atyidae)[END_REF].

It is important to note patterns in diversity across zones can sometimes be more nuanced than the zonation framework proposed by [START_REF] Keith | Vanuatu freshwater fish and crustaceans[END_REF] may suggest. In some cases, patterns in diversity can be dominated by a single parameter, such as the nature of the bedrock. In New Caledonia, ultramafic rocks of the South of the island leach high concentrations of metals (nickel, chromium, cobalt) into the water of the rivers. These metals are toxic for most species, but some species that have evolved to tolerate high levels of dissolved metals live there [START_REF] Marquet | Atlas des poissons et des crustacés d'eau douce de Nouvelle-Calédonie[END_REF]. These rivers flowing over ultramafic substrate thus exhibit a different fauna than rivers on sedimentary substrate, with different altitudinal zonations.

(Figure 2 here)

Biogeography

The genus Caridina can be found in virtually all the rivers of the tropical Indo-Pacific and Africa, ranging from West Africa [START_REF] Richard | African Caridina (Crustacea: Decapoda: Caridea: Atyidae): redescriptions of C. africana Kingsley, 1882, C. togoensis Hilgendorf, 1893, C. natalensis Bouvier, 1925 and C. roubaudi Bouvier, 1925 with descriptions of 14 new species[END_REF], Egypt [START_REF] Hussein | External morphology of the freshwater prawn Caridina africana collected from Egypt[END_REF][START_REF] Richard | Caridina nilotica (P. Roux, 1833) (Crustacea: Decapoda: Caridea: Atyidae) from East Africa, with descriptions of four new species[END_REF]Mazancourt et al., in press) and South Africa [START_REF] Mirimin | Genetic and morphological characterization of freshwater shrimps (Caridina africana Kingsley, 1882) reveals the presence of alien shrimps in the Cape Floristic Region, South Africa[END_REF] to mainland Japan [START_REF] Saito | Factors affecting distribution of freshwater shrimps and prawns in the Hiwasa River, southern Japan[END_REF], Polynesia [START_REF] Keith | Poissons et crustacés d'eau douce de Polynésie[END_REF] and South Australia [START_REF] Davie | Crustacea: Malacostraca: Phyllocarida, Hoplocarida, Eucarida (Part 1)[END_REF], stretching to Middle East localities [START_REF] Christodoulou | Freshwater Shrimps (Atyidae, Palaemonidae, Typhlocarididae) in the Broader Mediterranean Region: Distribution, Life Strategies, Threats, Conservation Challenges and Taxonomic Issues[END_REF] (Fig. 3).

(Figure 3 here)

As expected for such a wide-ranging genus, regional differences in diversity of Caridina exist in the Indo-Pacific. When focusing on endemic species of Caridina (Fig. 4A), the maximum diversity occurs around the Coral Triangle [START_REF] Veron | Delineating the Coral Triangle. Galaxea[END_REF], between the Philippines, Indonesia, and Papua-New Guinea. The number of endemic species tends to be substantially lower in rivers further away from this area, although large continental islands such as Madagascar or Sri Lanka, and archipelagos such as the Solomon Islands, constitute local hotspots of endemism. Some of these hotspots have been exposed by previous studies on freshwater shrimps, including Madagascar [START_REF] De Grave | Global diversity of shrimps (Crustacea: Decapoda: Caridea) in freshwater[END_REF][START_REF] De Grave | Dead Shrimp Blues: A Global Assessment of Extinction Risk in Freshwater Shrimps(Crustacea: Decapoda: Caridea)[END_REF][START_REF] Cumberlidge | Updated extinction risk assessments of Madagascar's freshwater decapod crustaceans reveal fewer threatened species but more Data Deficient species[END_REF] and Sulawesi [START_REF] De Grave | Global diversity of shrimps (Crustacea: Decapoda: Caridea) in freshwater[END_REF][START_REF] Klotz | Three new species of Caridina (Decapoda: Atyidae) from Central Sulawesi and Buton Island, Indonesia, and a checklist of the islands' endemic species[END_REF][START_REF] De Grave | Dead Shrimp Blues: A Global Assessment of Extinction Risk in Freshwater Shrimps(Crustacea: Decapoda: Caridea)[END_REF]. Islands of relatively old geological origin like New Caledonia, Fiji, or Mauritius also appear to harbor a high diversity of endemic species. In contrast, younger and often isolated islands like those of Polynesia or Micronesia harbor fewer endemic species. Localities at the northern and southern limits of the distribution area (Japan and South Australia for example) show low endemism.

(Figure 4 here) It appears regional patterns in the diversity in Caridina of the Indo-Pacific are explained by the age, size, and latitude of islands. Diversity will likely be high in a large continental island close to the equator and low in a small young volcanic island or in an area away from the equator [START_REF] Paulay | Biodiversity on Oceanic Islands: Its Origin and Extinction[END_REF][START_REF] Willig | Latitudinal gradients of biodiversity: Pattern, Process, Scale, and Synthesis[END_REF]. Geologically old islands have had more opportunity to be colonized by shrimps, and had time for colonizers to adapt and specialize to the different habitats and form new species. [START_REF] Chen | Speciation and endemism under the model of island biogeography[END_REF] showed that the number of speciations in islands, and thus their biodiversity, increases with time, whereas the number of colonizations decreases. According to the same study, the number of endemic species correlates to the size of the island, due to the greater diversity of habitats (geological or climatic differences, for example), which agrees with our observations for large islands such as Madagascar, Sri Lanka, or New Caledonia.

The patterns in the diversity of endemic species (Fig. 4C) vary considerably from the patterns in the diversity of amphidromous species (Fig. 4A). The greatest diversity of amphidromous species is found in the Solomon Islands and Vanuatu. This may also include the Coral Triangle, as the diversity of Caridina from the area remains poorly known. The high diversity of Caridina in this region may be explained by the complex geological history of the area, which lies at the convergence of three tectonic plates. The region contains islands of both continental and volcanic origin, as well as historical connections to Papua in the North.

The lowest diversity of amphidromous species is found in Polynesia and at the borders of the distribution area in general (Fig. 4C). Important areas of endemism, such as Madagascar, are not hotspots of diversity for amphidromous species. Localities with low endemism, like the Polynesian or Micronesian islands, often have a higher proportion of amphidromous species.

Patterns in the diversity of amphidromous species may be explained by the recent colonization of the islands by amphidromous shrimps with great dispersal abilities, allowing them to colonize these isolated habitats without becoming reproductively isolated. Small islands often exhibit unstable conditions in which endemic landlocked populations could not maintain themselves. In contrast, amphidromous species can repopulate islands following disturbance as they maintain a stock of larvae in the ocean, ready to recolonize depopulated habitats [START_REF] Mcdowall | On amphidromy, a distinct form of diadromy in aquatic organisms[END_REF].

The landlocked or amphidromous lifestyle of a species is not always linked to it being endemic or widespread, with examples of amphidromous species that are known from a single island for instance. The patterns of diversity of landlocked species are shown in Fig. 4B. Globally, the highest diversity of landlocked species is found in the Coral Triangle with a diminution when one moves away from it, until they completely disappear at the borders of the distribution area of the genus.

However, the differences between landlocked and endemic or amphidromous species is that landlocked species are mainly absent from volcanic islands such as Polynesia, Micronesia, Solomon Islands, or Vanuatu, where the majority of species are amphidromous. In contrast landlocked species often constitute the majority of the species in continents (India, China, Australia, South Africa) or large continental islands (Madagascar). This supposes that habitats for landlocked species need to have been available for sufficient time for new species to evolve a landlocked lifestyle from amphidromous ancestors, or for species to diverge from landlocked ancestors that were already present.

Conservation

Taxonomic confusion was until now a substantial obstacle to the establishment of programs aimed at the conservation of species of Caridina. Some ill-delineated species like C. longirostris or C. weberi were thought to be widespread, however, studies using an integrative taxonomy approach [START_REF] Mazancourt (de) | Integrative taxonomy helps separate four species of freshwater shrimps commonly overlooked as Caridina longirostris (Crustacea: Decapoda: Atyidae) in Indo-West Pacific islands[END_REF] showed that these 'species' are actually comprised of several species with more restricted distributions (some even being endemic). Furthermore, a better knowledge of systematics allows greater understanding of the habitat and ecology of these species, since each species is adapted to a different environment. These new data will impact the way in which the conservation status of these species is assessed. Species with small distributions and specific habitat needs are more vulnerable to extinction than widespread and tolerant species [START_REF] Purvis | Predicting extinction risk in declining species[END_REF].

Threats to amphidromous Caridina

The main threats faced by Caridina almost always originate from anthropization of their environment.

Habitat degradation. Many anthropic activities induce habitat degradation. The direct impacts of habitat degradation occur through impact through pollution of the rivers by pesticides (Kumar et al., 2010a ;Kumar et al., 2010b), industrial accidents [START_REF] Dominique | Constat de pollution suite à un déversement d'effluent industriel au sein du creek de la Baie Nord[END_REF], illegal poisoning [START_REF] Greathouse | Recovery of a tropical stream after a harvest-related chlorine poisoning event[END_REF], use of the rivers as dumps [START_REF] Keith | Atlas des poissons et des crustacés d'eau douce des Comores, Mascareignes et Seychelles[END_REF], eutrophication by fertilizers, poisoning and causing the proliferation of algae clogging the rivers, or increased sedimentation from mining, gravel extraction, or constructions (dams, roads, etc.) smothering the stream bed, and increasing water turbidity [START_REF] Boseto | Biodiversity and conservation of freshwater fishes in selected rivers on Choiseul Island, Solomon Islands[END_REF][START_REF] Keith | Poissons et crustacés d'eau douce de Polynésie[END_REF]. Habitat destruction also occurs through the removal of water from river systems for agriculture, industry or tourism, causing physical changes (e.g. lower flow rates) and the disappearance of lotic zones [START_REF] Keith | Poissons et crustacés d'eau douce de Polynésie[END_REF] that are the exclusive habitat of some species (e.g. Caridina weberi species complex). The cause and effects of habitat destruction can also be associated with human activities that do not impact rivers directly but have flow-on effects that impact shrimp habitats and threaten the viability of populations (i.e. indirect effects). For example, the disappearance of perennial rivers and associated loss of habitat for shrimp due to deforestation, like in Rodrigues Island [START_REF] Keith | Atlas des poissons et des crustacés d'eau douce des Comores, Mascareignes et Seychelles[END_REF], in Madagascar for rosewood, in Anjouan [START_REF] Mirhani | De la problématique de l'eau au modèle numérique d'aménagement en milieu tropical humide insulaire : le bassin versant d'Ouzini-Ajaho (Anjouan -Comores)[END_REF] for firewood and Ylang-ylang plantations, in the Marquesas islands and Rapa for goats and livestock, or in New Caledonia due to bushfires. As some species of amphidromous shrimps live in mangroves (like those of the C. gracilirostris complex), they are threatened during both their larval and adult stages by the degradation of their habitat by aquaculture activities. Together, the impacts of aquatic and terrestrial habitat destruction are well documented to affect the physiology, reproduction and migration of caridean shrimp and remain one of the most important threatening processes for carideans and other freshwater species [START_REF] De Grave | Dead Shrimp Blues: A Global Assessment of Extinction Risk in Freshwater Shrimps(Crustacea: Decapoda: Caridea)[END_REF].

Reduced connectivity. The construction of instream barriers suppresses habitat connectivity [START_REF] Dudgeon | Large-Scale Hydrological Changes in Tropical Asia: Prospects for Riverine Biodiversity: The construction of large dams will have an impact on the biodiversity of tropical Asian rivers and their associated wetlands[END_REF][START_REF] March | Damming Tropical Islands Streams: Problems, Solutions and Alternatives[END_REF]. The construction of dams, such as the Yaté hydroelectric plant in New Caledonia, are one such example of instream barriers that result in a rapid disappearance of amphidromous species in the whole watershed upstream. Indeed, shrimps need to be able to migrate along the river course depending on the water level in order to not get trapped in pools during the dry season. For amphidromous species, it is critical to have a continuum between the adult habitat and the sea in order to maintain populations.

Introduced species. The introduction of exotic species can lead to local extinctions of indigenous species due to predation, competition, habitat alteration, or disease transmission (Keith, 2002a(Keith, , 2002b;;[START_REF] De Grave | Dead Shrimp Blues: A Global Assessment of Extinction Risk in Freshwater Shrimps(Crustacea: Decapoda: Caridea)[END_REF]. For caridean shrimp, another important but little studied threat associated with introduced species, is the potential for hybridization with non-native caridean species. The ease with which caridean shrimp hybridize is well noted in the aquarium literature.

There have been no reports of hybridization of introduced and native carideans in the wild, but the extinction of local populations due to hybridization following a translocation within a catchment has already occurred in other genera within the family Atyidae [START_REF] Hughes | Translocation causes extinction of a local population of the freshwater shrimp Paratya australiensis[END_REF][START_REF] Fawcett | Consequences of a translocation between two divergent lineages of the Paratya australiensis (Decapoda:Atyidae) complex: reproductive success and relative fitness[END_REF].

Overharvesting. Shrimps are harvested for human consumption. In the Philippines, Madagascar, Indonesia, India, and China, caridean shrimp are caught using nets in large quantities to be eaten fresh, dried, or salted, and to feed animals or used as fertilizer [START_REF] Holthuis | FAO species catalogue. Volume 1-Shrimps and prawns of the world (An annotated catalogue of species of interest to fisheries Vol. 1). Food and Agriculture Organization of the United Nations[END_REF], or in Reunion Island where they are called "chevaquines" (GM, pers. comm.). A unique threat for Caridina spp. is the capture of individuals for the aquarium trade, particularly for the colorful species living in Sulawesi [START_REF] De Grave | Dead Shrimp Blues: A Global Assessment of Extinction Risk in Freshwater Shrimps(Crustacea: Decapoda: Caridea)[END_REF] or "Bee shrimps" from mainland China, some species being known only from a single stream of about 150m in length (WK pers. comm.). While most of the species impacted are lacustrine landlocked, in the future growing global demand could see the expansion of the collection of amphidromous species. Currently, only a few amphidromous species appear in the trade, but whether these are collected legally, poached, or bred in captivity is difficult to determine due to long and complex supply chains. It is unclear how many shrimp are traded globally, but there is substantial anecdotal evidence suggesting local populations experience severe declines due over-collection, particularly where a local colour or pattern variant becomes highly sort after in the trade. For example, [START_REF] Klotz | Breeders'n'Keepers Wildshrimp China Special[END_REF] reported the disappearance of Caridina trifascata from locales near Zuhai, China over a period of less than 12 months, likely associated with over-harvesting for the aquarium trade.

Climate change. Climate change is likely already impacting and will continue to impact caridean shrimp through a variety of direct and indirect mechanisms (Table 1). Unfortunately, whilst crustaceans generally appear to be robust to some changes in their environment associated with climate change (e.g. ocean acidification), little research has been done to address the gaps in our knowledge about indirect effects of climate stressors, and the effects of interactions of climate stressors (e.g. temperature) with non-climate stressors (e.g. pesticide pollution). This is concerning given studies that have examined the indirect and interactive effects of climate stressors on marine taxa have highlighted how these effects may be more important than the direct effects of climate stressors (e.g. [START_REF] Kamya | Indirect effects of ocean acidification drive feeding and growth of juvenile crown-of-thorns starfish Acanthaster planci[END_REF][START_REF] Rosenblatt | Climate change, nutrition and bottom-up and top-down food web processes[END_REF][START_REF] Kroeker | Ocean acidification causes ecosystem shifts via altered competitive interactions[END_REF][START_REF] Boyd | Modes of interactions between environmental drivers and marine biota[END_REF].

(Table 2 here)

Conclusions and recommendations

The integrative study of Caridina led to the discovery of unsuspected diversity in Indo-Pacific Islands, with species complexes that are now recognized and present differences in habitat.

Therefore, when making inventories of the freshwater fauna of rivers, if it is not possible to identify specimens to species level, the species complex can be sufficient to provide interesting data. Keys and diagnoses of the different species complexes will be presented in an upcoming study (Mazancourt et al. submitted). Now that species are better delineated thanks to integrative taxonomy, their distribution (including micro and macro-habitats) can be more informative and their specific conservation status (threats, IUCN status, etc.) needs to be re-assessed.

Our recommendations to protect these amphidromous species of Caridina revolve around three main points: 1) Taking biological and ecological specificities into consideration Arguably the best way to enable conservation is by changing the mind-set of people directly implicated through education and example. For example, managing caridean species requires managers to have a minimum knowledge of their biology (i.e. amphidromous lifecycle) and to take their specificities into consideration when making decisions. Understanding of the lifecycles of amphidromous species highlights the importance of maintaining the natural flow of water ways in a good state. Maintaining population connectivity through natural water flows allows populations to be comprised of a range of life stages. This limits the potential for local or regional extinctions as breeding populations depend on the pool of larvae to replace them (and conversely, the pool of larvae depends on the breeding populations). Successful implementation of conservation measures designed to maintain and enhance natural water flows may be best achieved through the inclusion of a range of stakeholders in projects (e.g. farmers, fishers, water management authorities, local government, community conservation groups, international conservation organisations, etc.).

2) Limiting anthropic impacts on amphidromous species

The most efficient way to protect amphidromous species is to protect their environment by limiting anthropic impacts. This requires an integrated management approach that takes into account the role of both terrestrial and aquatic impacts. For example, water catchments should always have a minimum flow in line with their natural seasonal variations, considering the upstream migration of the juveniles is often triggered by greater freshwater arrival in the sea during floods. Therefore the effects of a variety of management decisions, such as avoiding man-made structures altering water flow, design of roads and dams that maintain the connectivity from the sea to the mountains, maintaining riparian vegetation in a good state, and preserving estuarine habitats, need to be considered in an integrated manner.

2) Establishing priorities for species conservation

As conservation status is not known for many species, the priority is to protect their habitats. It would thus be useful to identify those habitats that need special protection and make sure to apply to them the recommendations of the previous point. Caridean shrimps seem to be good indicators of water and habitat quality. They could thus be a very useful tool for managers as indicators of anthropic effects and the impact of climate change in tropical rivers. As exotic species are often harmful to native fauna, preventing their introduction would be an important recommendation to preserve freshwater shrimps. Finally, supporting research on the biology and ecology of these species would help to improve our knowledge and assess the threats they face.

Figures captions:

Figure

  Figure 1. Schematic representation of the different freshwater habitats. 1: Spring zone; 2: Higher

1 .

 1 Figure 1. Schematic representation of the different freshwater habitats. 1: Spring zone; 2: Higher course; 3: Waterfall; 4: Middle course; 5: Lower course; 6: Estuary; 7: Lake; 8: Swamp; 9: Subterranean waters.

Figure 2 .

 2 Figure 2. Examples of habitats types. A: Higher course riffle, lotic mode (River Poitete, Kolombangara Island, Solomon Islands, credit PK); B: Higher course pool, lentic mode (Ciu waterfall, New Caledonia, credit VM); C: Middle course, lentic mode (River Wénou, New Caledonia, credit VM); D: Middle course riffle, lotic mode (River Nekouri, New Caledonia, credit

Figure 3 .

 3 Figure 3. Distribution area of Caridina.

Figure 4 .

 4 Figure 4. Heat maps representing the species richness by locality. A: numbers of amphidromous species (eggs < 0.8mm); B: numbers of landlocked species (eggs > 0.8mm); C: numbers of endemic species (known from a single locality/area).


Kevin M Smith

Rinaldo Poli
email: rinaldo-poli@u-bourgogne.fr

Jeremy N Harvey
email: jeremy.harvey@bris.ac.uk

] K M Smith

Poli Harvey Smith

A Computational Study of Ethylene C-H Bond Activation by [Cp*Ir(PR 3 )]

Keywords: Ab initio Calculations, C-H activation, Iridium, Reaction Mechanisms, Spin Crossover

)] species (R = H and CH3). The study consists of full optimizations of local minima, first order saddle points, and minimum energy crossing points (MECP). The experimental results are best accounted for by considering both singlet and triplet spin surfaces. The relative energies of singlet [(C5R5)Ir(PR3)(CH3)H], [(C5R5)Ir(PR3)(CH=CH2)H], and [(C5R5)Ir(PR3)(H2C=CH2)] are in good agreement with experiment, as is the calculated barrier for the conversion from the vinyl hydride to the  2 -alkene complex. However, the singlet surface alone fails to explain the experimentally observed product ratio, or the intermediate inferred from experimental isotope effect studies. Locating the MECP between singlet and triplet surfaces indicates that the thermolysis of the singlet alkyl hydride precursor directly forms triplet [(C5R5)Ir(PR3)]. The weak van der Waals adduct of triplet [(C5R5)Ir(PR3)] and ethylene is proposed to be the key intermediate in the overall reaction.

The interchanging of the available ethylene CH bonds in this triplet -complex accounts for the observed kinetic isotope effects, and partitioning between alkene -complexation and CH bond activation may also occur from this common intermediate. The possible role of steric factors and molecular dynamics are also discussed.

Introduction

The formation of [Cp*Ir(PMe3)(C6H11)H] by photolysis of [Cp*Ir(PMe3)(H)2] in cyclohexane demonstrated the first intermolecular oxidative addition of saturated alkane C-H bonds by a homogeneous organometallic complex. [1] In the years that followed this pioneering report, the reactivity of [Cp*Ir(PMe3)] and related [Cp*ML] species (M = Co, Rh, Ir; L = PR3, CO) have been extensively studied experimentally using a variety of kinetic, thermodynamic, and labeling techniques. [2] Simplified [CpML] model complexes have also been subjected to several theoretical analyses, permitting the comparison of different computational techniques. Early work using Extended Hückel molecular orbital (EHMO) theory [3] was followed by studies using density functional theory (DFT), MP2 and other techniques. [4] While the [CpRh(CO)] system has been the most extensively studied, [5] [CpIr(PH3)] models for the original Bergman system have also been examined, [5a,6] with the most recent contributions focusing on the later [CpIr(PH3)(CH3)] + cationic variants. [7] Scheme 1. Thermolysis of [Cp*Ir(PMe3)(C6H11)H] in cyclohexane.

The reaction shown in Scheme 1 provided the impetus for the current theoretical work.

Thermolysis of [Cp*Ir(PMe3)(C6H11)H] in cyclohexane generates [Cp*Ir(PMe3)], which reacts in situ with H2C=CH2 to form [Cp*Ir(PMe3)(CH=CH2)H] and [Cp*Ir(PMe3)(H2C=CH2)] in a 2:1 ratio. [8] While [Cp*Ir(PMe3)(CH=CH2)H] is stable under the thermolytic conditions of its formation, at higher temperatures it is converted cleanly to [Cp*Ir(PMe3)(H2C=CH2)]. This remarkable reaction indicates that in contrast to many similar reactions, the ethylene adduct is the thermodynamic product, and can not be an intermediate to the vinyl hydride species.

Many reactions of transition metal compounds involve multiple electronic states. We have an ongoing interest in exploring how the associated spin state changes affect the reactivity of organometallic complexes [9] (This has sometimes been referred to as Two-State Reactivity [START_REF] Schröder | For other work in this area[END_REF] ). It has long been recognized that the active [CpML] intermediates in CH bond activation processes could possess ground or low-lying excited triplet spin states. [11] Due to the difficulties associated with the theoretical treatment of spin crossover problems, however, the kinetic ramifications of this phenomenon have not been addressed in previous studies. We recently demonstrated the utility of minimum energy crossing point (MECP) locating techniques in evaluating the role of spin state change in organometallic reactions. [12] The system shown in Scheme 1 is of particular interest because the archetypal [Cp*Ir(PMe3)]-based system is of long-standing and ongoing significance for the study of C-H bond activation, and a wealth of experimental and theoretical results are available for these species. [2,4] Furthermore, [CpIr(PH3)] was recently calculated to possess a triplet ground state at various levels of theory. [6] We hereby present extensive B3LYP computations using polarized basis sets on the model [CpIr(PH3)] + C2H4, which, together with additional B3LYP calculations on the full [Cp*Ir(PMe3)(C2H4)] system, provide an explanation for the surprising experimental observations that have resisted rationalization for well over a decade. Our results illustrate the problems involved in using model compounds in computational studies; more importantly, they also demonstrate the importance of singlet-triplet crossover and the powerful way in which locating MECPs can predict its importance.

Computational Details

Most of the calculations on the [CpIr(PH3)] model, as well as all those relating to the [Cp*Ir(PMe3)] system, were performed using the pseudospectral Jaguar 4.0 package, [13] with a flexible polarized basis set on all atoms. Thus, the Iridium is treated using the Los Alamos relativistic ECP, [14] together with the associated LACV3P++ basis developed for use with Jaguar. [13] The latter basis is a valence triple-zeta contraction of the original double-zeta Los Alamos basis set, [14] augmented by one set of diffuse d functions. All other atoms are described by the standard 6-31G basis, [15] with polarization functions (6-31G** basis) on all atoms which can bond to Ir (i.e., the ring C atoms of Cp and Cp*, the P atom, and the whole of the CH4 or C2H4 group). These calculations will be simply referred to as B3LYP/TZV*. Most of the [CpIr(PH3) . (CH4)] and [CpIr(PH3)(C2H4)] species were also investigated at the B3LYP level using the smaller standard LanL2DZ basis set together with the Gaussian94 program package. [START_REF] Frisch | Gaussian 94[END_REF] The results of these preliminary calculations, where significant, will be briefly mentioned below and referred to as B3LYP/LanL2DZ. The geometries of the minima and transition states were fully optimized without symmetry restrictions, so that convergence to saddle-points during searches for minima, or to higher-order saddle-points during transition state searches should not occur. For transition states, the nature of the transition state was verified by inspecting the unique eigenvector of the approximate Hessian generated during optimization. In a few cases, the exact Hessian was computed, and the expected number of imaginary frequencies was found in all cases. The geometries of the crossing points were also fully optimized, using the method developed by one of the authors, [START_REF] Aj | [END_REF] adapted for use with Gaussian or Jaguar. For open-shell species, the Gaussian and Jaguar calculations used unrestricted and restricted open-shell methods, respectively. In all cases, energies are given in kcal mol -1 relative to triplet [CpIr(PH3)] or [Cp*Ir(PMe3)] and CH4 or C2H4, and do not include a correction for zero-point energy (where available, this correction is found to be small, ~ 1 kcal mol -1 ).

Results

Our analysis of this reaction relied on a thorough study of the singlet and triplet potential energy surfaces, and of their crossings, using the B3LYP hybrid density functional, together with flexible polarized basis sets. The B3LYP method has proven to be very reliable for a broad range of organometallic systems similar to that studied here. [4] To describe our results, we adopt the following order. First, we briefly summarize the essential experimental observations which we aim to explain, so as to make clear what are the difficulties facing our analysis. We then discuss the electronic structure of the [(C5R5)Ir(PR3)] (R = H, Me) intermediate, folllowed by a discussion of the singlet reactive potential energy surface, which, as we will show, is unable to explain the experimental results. Next, we will describe the features of the triplet surface, and finally, we will discuss a realistic scenario for reaction on both surfaces, informed by our results concerning the regions where these surfaces cross. Because our main focus is on the reaction of ethylene with the

Background.

Thermolysis of [Cp*Ir(PMe3)(C6H11)H] in the presence of ethylene affords the C-H oxidative addition product [Cp*Ir(PMe3)(CH=CH2)H] and the ethylene coordinative addition product [Cp*Ir(PMe3)(C2H4)] in a 66:34 ratio which is independent of temperature between 130 and 160°C, and is furthermore invariant during the reaction. This observation [8] suggests that the products are obtained via two different transition states and that ∆∆H ‡ is near zero. Both compounds are thermally stable under the reaction conditions, but thermolysis of the pure vinyl hydride in cyclohexane or benzene at temperatures above 180° results in quantitative conversion to the ethylene complex [Cp*Ir(PMe3)(C2H4)]. This confirms that the initially obtained product mixture arising from the activation of ethylene is under kinetic control, and that the ethylene complex cannot be an intermediate along the formation of the C-H oxidative addition product. Also, no phenyl hydride product is observed when the vinyl hydride is converted to the ethylene complex in benzene solvent. This proves that the rearrangement does not involve formation of free C2H4 and [Cp*Ir(PMe3)], because independent experiments show that the latter is able to competitively activate benzene and ethylene, and because the benzene activation product, [Cp*Ir(PMe3)(C6H5)H],

is thermally stable at 180°.

A more detailed kinetic study [18] of the reaction led to some important conclusions. Thus, an activation barrier ∆H ‡ of 34.6±1.2 kcal mol -1 was found for the rearrangement of [Cp*Ir(PMe3)-(CH=CH2)H] between 180 and 220°C (with ∆S ‡ = 2.6±2.6 eu). The reaction rate was found to be unaffected by the presence of free PMe3, ruling out the involvement of phosphine dissociation.

From independent calorimetric studies, a barrier of at least 41 kcal mol -1 was estimated for the ethylene reductive elimination from the vinyl hydride, thus confirming that the isomerization process takes place without ethylene reductive elimination.

Intermolecular isotope effects were determined by allowing [Cp*Ir(PMe3)] (generated at 145°) to compete for C2H4 and C2D4, yielding kH/kD = 1.49±0.08 for the formation of the vinyl hydride product and  = 0.95±0.02 for the formation of the  complex,  being the secondary isotope effect that each deuterium has on the rate (for instance kH/kD =  4 = 0.82±0.05 for the competition between C2H4 and C2D4). The intramolecular isotope effects for insertion into C-H and equilibrium isotope effects, because the independently synthesized complex

[Cp*Ir(PMe3)(CH=CH2)D] was not found to scramble the label under the same conditions.

The relatively small values of kH/kD for the oxidative addition process are consistent with previous reports and with the involvement of an early transition state, where little C-H bond breakage has occurred. The kH/kD value for the addition process, when compared with available equilibrium isotope effects, indicates that the ethylene structure is perturbed only slightly in the transition state from that found in the free ligand. What is most interesting, however, is that the inter-and intramolecular kH/kD for oxidative addition are not equivalent. Under the assumption that this difference is not caused by a secondary isotope effect of the non-reacting C-D bonds, this non-equivalence requires an intermediate on the reaction pathway from which partitioning can occur. These observations led the authors to propose [18] the existence of an intermediate having the however, remained perplexed [18] that this isomerization process seems so facile compared with C-H insertion and -complex formation.

Figure 1.

Proposed structures [18] for the intermediate on the pathway leading to C-H oxidative addition.

In parallel with the experimental work, some molecular orbital calculations [19] on the model [CpIr(PH3)] + C2H4 system were performed, and led to important insights, but also increased the mystery surrounding it. Whilst the vinyl hydride complex [CpIr(PH3)(CH=CH2)H] was found to lie A Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3) p. 9/30 slightly higher in energy than the olefin complex [CpIr(PH3)(C2H4)], in agreement with experiment, the initial ethylene coordination step leading to the latter was found to be essentially barrierless, in disagreement with the kinetic preference for the vinyl hydride product. Steric effects and the intervention of multiple spin states were suggested as possible explanations of the results, but could not be explored with the then available computational power.

Electronic Structure of [CpIr(PH3)] and [Cp*Ir(PMe3)].

The assumed intermediate in the reaction studied here is the [Cp*Ir(PMe3)] complex, whose electronic structure and geometry are thus of central importance. The qualitative electronic structure of this type of species, based on ab initio and density-functional calculations on [CpIr(PH3)], has been discussed before in great detail. [6] However, one important feature which has not been fully resolved concerns the relative stability of the singlet and triplet states of this intermediate. A number of theoretical studies, summarized in Table 1, have examined the 16-electron [(C5H5)Ir(PH3)] model system, which is relevant to many C-H oxidative addition processes. Because the first ab initio study by Ziegler et al. predicted a singlet ground state, all subsequent calculations along the methane oxidative addition profile were only carried out on the singlet surface. [5a] This is despite the fact that all subsequent calculations indicate a more stable triplet state. [6] However, no calculations of triplet alkane adducts or transition states or crossing points along the oxidative addition path have been carried out starting from the 16-electron iridium fragment in the triplet state, to the best of our knowledge.

[Ir]( 1 -C2H4) / 0.8 / 3 [Ir]( 1 -C2H4) -4.6 -3.1 -1.5 3 [Ir]( 1 -C2H4) to  2 -TS 0.07 -0.3 / 3 [Ir]( 2 -C2H4) -12.7 -10.8 -5.1 3 [Ir](H-C2H3) TS / 22.8 / 3 [Ir]H(C2H3) / 9.5 / MECP nr. 3 [Ir] . C2H4 3.3 2.3 / MECP nr. 3 [Ir]( 1 -C2H4) -2.1 -2.5 -1.8 MECP nr. 3 [Ir]( 2 -C2H4) -12.4 -10.4 / a)
The -complex is not a minimum at this level.

b) The total energies (in Hartrees) for this compound are given for reference purposes.

Our results for [(C5R5)Ir(PR3)] (R = H, Me), as well as all the other calculated B3LYP relative energies are collected in at the B3LYP/TZV* level. This suggests that crossing to the singlet surface should be relatively facile.

The Singlet Potential Energy Surface.

The singlet potential energy surface for the Cp / PH3 model system is shown in Figure 2. ]. [20] Our calculations place the 43.1, [6b] and 46.3 [6b] obtained at other levels of theory). The real system [Cp*Ir(PMe3)(CH3)H] is calculated as 38.1 kcal mol -1 more stable than CH4 and singlet [Cp*Ir(PMe3)] at the B3LYP/TZV* level. We find the barrier for formation of [CpIr(PH3)(CH3)H] from the encounter complex on the singlet surface to be very small, as has also been shown by previous studies. [4] We note that with the single point MP4//MP2 energies of ref.

[6b], there is in fact no barrier since the transition state lies lower in energy than the complex. With B3LYP/LanL2DZ, there is a barrier of 0.5 kcal mol -1 , whereas with B3LYP/TZV*, we do not obtain a distinct [CpIr(PH3)(CH4)] alkane complex: the structure obtained using the smaller basis leads directly to [CpIr(PH3)(CH3)H] upon reoptimization with the larger basis. Within the expected accuracy of our computational method, we cannot predict for sure whether or not there is a barrier of this type, but it would clearly be very small. We note that the intermediacy of a [Cp*Ir(PMe3) . (C6H12)] -complex has been deduced from kinetic measurements. [21] This does not however prove that there is a barrier on the singlet surface, because the observed complex may be a triplet, with the surface crossing (see below) leading to the observed barrier between it and the alkyl hydride structure. It is also conceivable that there is no barrier to the formation of the encounter complex from the separated 16-electron singlet species and CH 4 . Thus, the calculated energy for the oxidative addition process would also correspond to the activation barrier for the reductive elimination process to afford the singlet intermediate. No such experimental data has been reported for the elimination of methane, whereas a ∆H ‡ value of 35.6 kcal mol -1 has been reported for the elimination of cyclohexane from [Cp*Ir(PMe3)(C6H11)H]. [20] Addition of Ethylene to [CpIr(PR3)]. As already discussed by others, [19] there is no barrier to -addition of ethylene to [CpIr(PH3)] on the singlet surface. To check that this is the case, we performed partial geometry optimizations on the singlet surface whilst fixing the Ir-C distance at successively longer distances. A monotonously increasing potential energy profile is obtained, with the energy computed for fixed r(Ir-C) = 3.56 Å, the longest distance considered, still 5.0 kcal mol -1 below the energy of separated [CpIr(PH3)] and ethylene. This effect is larger than could be expected from any computational error and indicates that the interaction between the two moieties is indeed attractive.

Unlike in Hoffmann's computations, [19] however, and as discussed below, there is also no barrier to -addition of ethylene to form the vinyl hydride [CpIr(PH3)H(C2H3)]. This is because the saddle point for interconversion of this species and the ethylene complex lies well below separated [CpIr(PH3)] and ethylene, and is structurally very similar to a -complex between the iridium intermediate and ethylene. This finding echoes the result above, whereby methane adds to singlet [CpIr(PH3)] without a barrier.

Based on these findings concerning the singlet potential energy surfaces, it is difficult to explain the experimental observations by Bergman et al. The main observation, i.e. the overall 2:1 selectivity for formation of vinyl hydride and ethylene complex, can, it is true, be explained on dynamical grounds as discussed by Hoffmann et al. [19] This explanation would loosely run as follows: upon encounter of [CpIr(PR3)] and ethylene, attractive interactions pull the two reagents together. If one of the C-H bonds approaches the iridium center first, and especially if vibrational motion leads to that C-H bond being somewhat more extended than at the equilibrium position, then the system will plunge into the vinyl hydride minimum. If, however, it is mostly the -bonding orbital of the ethylene which approaches the iridium first, then the ethylene complex will be formed.

The preferential formation of vinyl hydride would be accounted for, within this explanation, by subtle features of the "transition region" (meant in the broad sense of the term, since there is no barrier) of the overall attractive [CpIr(PR3)]ethylene potential energy surface. Molecular dynamics simulations, e.g. using a Car-Parrinello generated ab initio potential energy surface, [START_REF] Woo | For a review of this approach[END_REF] could be used to test this model.

However, even if this model were able to reproduce the observed ratio of products, it would fail to explain the experimental isotope effects. As extensively discussed by Bergman et al., [18] the inter-and intramolecular isotope effects for C-H vs. C-D insertion with C2H4, C2D4, 1,1-and cis and trans 1,2-C2H2D2 require that an intermediate [CpIr(PR3) … C2H4] complex is formed during the reaction. than experimentally found. This discrepancy has also been recently noted by Hall et al. [23] and attributed to a disorder problem in the experimental structure. A similar phenomenon also seems to occur for the recently reported structure of [Cp*Ir(PMe3)(CH=CH2)Cl]. [24] To our knowledge, no experimental structure of a [( 5 -C5R5)Ir(PR3)(R2C=CR2)]-type compounds is available. For both the real and the model system, the ethylene  complex is more stable than the vinyl hydride by several kcal mol -1 , in agreement with experiment. From calorimetric data, Bergman has estimated a ∆H of 41 kcal mol -1 between [Cp*Ir(PMe3)(CH=CH2)H] and [Cp*Ir(PMe3)] + H2C=CH2. [18,24] Our calculations provide 42.8 and 41.2 kcal mol -1 for the real and model system, This may be because the compound still maintains a certain degree of interaction with the C-H bond while it has already started to establish the interaction with the C-C  bonding electrons. This is suggested by the Ir-C-C angle of 106.75° and by the Ir … C distance of 2.956 Å. It is also notable that the geometry of the ethylene moiety is essentially planar and scarcely perturbed with respect to free ethylene. Thus, the transition state can also be viewed as a complex between ethylene and The calculated energy for this transition state is 7.7 kcal mol -1 below triplet [CpIr(PH3)] + C2H4 (16.1 kcal mol -1 below the corresponding singlet), in agreement with the experimental finding that the rearrangement does not involve ethylene dissociation. This transition state is 33.5 kcal mol -1 higher than the vinyl hydride species, in remarkable agreement with ∆H ‡ of 34.6 ± 1.2 kcal mol -1 for the conversion of [Cp*Ir(PMe3)(CH=CH2)H] to [Cp*Ir(PMe3)(H2C=CH2)]. [18] The Triplet Potential Energy Surface

Conversion of Vinyl

The triplet potential energy surface for the Cp-PH3 model system is shown in Figure 6. [CpIr(PH3)] forms a weak van der Waals-type complex with ethylene (see Figure 7) which can best be described as a -complex. The closest contact between Ir and an ethylene H atom is over 3 Å and the geometry of the [CpIr(PH 3 )] moiety is essentially unperturbed relative to the free fragment. The ethylene molecule approaches the metal from the crowded side of the coordination sphere. The closest distance of the ethylene H atom is in fact to a phosphine H atom (3.245 Å). The weak binding energy (1.1 kcal mol -1 ) of this complex means that the contribution of basis set superposition error (BSSE) to its geometry and binding energy is likely to be substantial.

Nevertheless, irrespective of their precise geometry and interaction energy, van der Waals complexes of this type between [Cp*Ir(PMe3)] and ethylene will certainly be formed, and, given their very low binding energies, will be able to dissociate and isomerize very readily. Triplet [CpIr(PH3)] also forms two covalently bound adducts with ethylene, in which the latter is either  1 -or  2 -coordinated. Formation of these adducts involves surmounting small activation barriers. The  1 adduct is formed first, via what is essentially a radical addition process. Indeed, the  1 -ethylene complex of triplet [CpIr(PH3)] or [Cp*Ir(PMe3)] is in a certain sense a radical adduct of the triplet 16-electron complex to ethylene, with one remaining unpaired electron on iridium, a roughly sp 3 carbon atom, and an sp 2 carbon radical, see Figure 8. The adduct lies at -1.5 and -3.1 kcal mol -1 for the real and model systems, respectively, and is separated from reagents by a barrier lying at +0.8 kcal mol -1 for the model compound. The ring is essentially symmetric ( 5 ) in these species, and the configuration is planar at the metal. That is, the metal sits approximately in the plane defined by the P atom, the ethylene C atom which is bonded to it, and the center of gravity of the cyclopentadienyl ligand. As can be appreciated from Figure 8, the "radical-type" C atom has a planar configuration. It is interesting to note the different orientation of the  1 -C 2 H 4 ligand, which appears to prefer a "vertical" position in the less hindered model compound, whereas it is more "horizontal" in the real system. Although the barrier to rotation between the Ir and the sp 3 -hybridized C atom is obviously rather small, the much greater steric bulk of the ligands in the real system does appear to somewhat destabilize this adduct, which is 3.1 kcal mol -1 below reagents for the model, but only 1.5 kcal mol -1 below for the real system. In a second step, this species forms a second Ir-C bond to afford an  2 ethylene complex, see Figure 9, with this being the lowest energy species on the triplet surface. As in the model system  1 adduct, but unlike the singlet ethylene complex, the ethylene is bound in a "vertical" configuration, with the C-C bond orthogonal to the C5R5 plane. Also, the cyclopentadienyl undergoes ring-slipping and is only  3 -coordinated to iridium, presumably because an  5 configuration leads to an 18 electron species, which is not possible for a triplet. This conformational change may contribute to the small computed barrier for the rearrangement process, which lies 0.3 kcal mol -1 below the separated triplet [CpIr(PH3)] and ethylene. The configuration at the metal is again planar, like for the  1 -ethylene triplet species. Unlike the  1 adduct, the ethylene ligand in this  2 complex is "vertical" in the real system as well as in the model, indicating that the electronic preference for this orientation is much stronger in Thus, the "vertically" coordinated ethylene ligand is very close to the methyl groups of the PMe3 and Cp* groups, with the shortest H-H contacts being of respectively 2.54 and 2.46 Å. In this case, the use of the model compound is therefore somewhat misleading. With the increasing computing power available nowadays, many such cases are being observed where the use of model compounds leads to incorrect conclusions as here. [25,[START_REF] Maseras | Steric effects with large, realistic ligands can also be studied computationally using QM/MM methods. For a review[END_REF] Overall, there seems to be an increasing amount of steric yield as one goes from the van der Waals triplet complex to the  1 then  2 -complexes. Although we have not located the two corresponding transition states for the real system, steric hindrance will probably act on them too, so that they will lie somewhat higher in energy than in the model [CpIr(PH3)] system. This means that the  1 and especially  2 adducts may be formed less readily than may be assumed upon looking at the potential energy surface of Figure 6: crossover to the singlet surface (see below) can occur from both the van der Waals and  1 adducts, and this process will compete with barrier crossing on the triplet surface.

Triplet Insertion Chemistry.

Given that [Cp*Ir(PMe3)] is expected to be formed in its triplet state, one should also consider the possibility that C-H insertion to form the vinyl hydride product can occur upon the triplet potential energy surface. This is indeed possible, but as shown in Fig. 6, the triplet vinyl hydride is less stable than the reagents, lying at +9.5 kcal mol-1, and the corresponding transition state is extremely high, at 22.8 kcal mol -1 . Clearly, this route is not compatible with the experimental observations.

Overall, the experimental observations of Bergman et al. cannot be explained by considering the triplet surface alone, just as they could not be reconciled with the features of the singlet surface.

Instead, one must consider both surfaces together, and the regions of configurational space where the two lie close in energy, as discussed below.

Singlet-Triplet Surface Crossings -MECPs

For compounds containing transition metals, especially those from the third row such as iridium, potential energy surfaces corresponding to wavefunctions of defined electronic spin are often not a good description of the system, and are often not particularly useful even as zero-order representations. This is because the corresponding wavefunctions are diabatic states, i.e. they are not eigenfunctions of the full Hamiltonian of the system, which includes spin-orbit coupling, and has adiabatic eigenstates for which the electronic spin is not well defined. In the present system, the high spin-orbit coupling due to the iridium atom will lead to substantial mixing between the singlet and triplet states for many of the geometries discussed here. Therefore, the reactions in this system may well occur in a completely adiabatic manner, passing smoothly from regions where the wavefunction is mostly singlet in nature to regions where it is mostly triplet, and then back to singlet in the product region. Although this requires passing from the singlet diabatic surface to the triplet surface, this spin-forbidden character may not impede the reaction in any significant way. †

The most appropriate ab initio method for the study of the mechanism of such processes would be † Obviously, the situation may be different for systems containing only second-or expecially first-row transition metal atoms, as discussed elsewhere (ref.

[9c]).

A Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3) p. 21/30 one which included spin-orbit coupling in the corresponding Hamiltonian, instead of just treating the kinetic energy and Coulomb terms.

However, such methods are not available, especially when one takes into account additional difficulties such as the large number of atoms in the system, scalar relativistic effects, correlation, etc. Therefore, one must make do with computational studies of the individual spin states. For systems such as the present one, where the crossing from one surface to another occurs in a mechanistically important region of the global potential energy surface, it is not enough to locate stationary points (minima, transition states) on each surface separately, as is usually done. One must also find the relevant minimum energy crossing points (MECPs) between surfaces, which represent the energy barriers the system needs to cross in the zero-order representation of the potential energy surfaces. Of course, strong spin-orbit coupling will mix the surfaces to a considerable extent around the MECPs, but even so, their geometries and relative energies provide insight, at least at the semi-quantitative level, into the features of the adiabatic spin-coupled potential energy surface.

We discuss below four regions where the singlet and triplet surfaces cross, and how this may affect reactivity. Most of these MECP calculations have only been carried out on the less expensive Cp-PH3 model system. However, one of the MECPs was studied for the real system, and this led to a very similar result to that obtained with the model. We note that the present procedure to characterize surface crossings, explicit optimization of the MECP without geometry restrictions, was shown in ref.

XX to be faster and more accurate than previously used "partial optimization" techniques leading to rough lower and upper boundaries on the energy of the MECP.

MECP at [CpIr(PH3)(CH4)] or [CpIr(PH3)].

The first significant crossing of the singlet and triplet surfaces occurs in the vicinity of the [CpIr(PH3)] complex with methane. We find a crossing lying just above the singlet dissociation asymptote. The geometry of this crossing, shown in Figure 10, has the CH4 moiety far from the transition metal center, so that it does not really play a significant role in mediating the crossing of the two surfaces. In fact, the [CpIr(PH3)] system itself at the geometry of this MECP, but with the methane atoms omitted, is very close to an MECP as judged from the energy splitting and effective gradient. The Cp adopts a  5 coordination mode but is slightly asymmetric, the three C atoms directly opposite to the PH3 being significantly farther as a MECP near [CpIr(PH3)] + ethylene van der Waals adduct. This structure is shown in Figure 11, and lies at +2.3 kcal mol -1 . It can be seen to be rather similar in geometry to the transition state converting the singlet vinyl hydride and the ethylene adduct (Figure 5). In fact, geometry optimization on the singlet potential energy surface starting at this MECP leads to the vinyl hydride compound, suggesting that if crossover occurs in this region of configurational space, formation of the vinyl hydride product will be favored. Unlike the TS in Figure 5, however, the MECP in Figure 11 shows a more symmetric  5 coordination mode for the Cp ligand. In addition, MECP near [CpIr(PH3)( 1 -H2C=CH2)]. This MECP, shown in Figure 12, lies at -2.5 kcal mol -1 and is similar in geometry to the  1 triplet adduct (Figure 7). ‡ No clear indication of an Ir … H interaction is shown in this structure, since the Ir is symmetrically disposed over the sp 3 hybridized C atom (Ir-C-C = 109.87°). On the other hand, the Ir … C separation to the second ethylene C atom is shorter than in the triplet  1 -C 2 H 4 adduct of Figure 7 (2.972 vs. 3.088 Å), indicating an incipient ‡ The triplet  1 adduct of ethylene with [Ir] has one unpaired electron on iridium, and one on the distal carbon atom of the ethylene group. Spin-coupling between these two electrons should be fairly weak, so that there should be an open-shell singlet state very close in energy. Using restricted B3LYP, the closed-shell singlet lies 15.7 kcal mol -1 above the triplet at its minimum. Using unrestricted B3LYP, and the method of Noodleman et al. [ref. 27], the open-shell singlet can be estimated to lie just 4.0 kcal mol -1 higher than the triplet. For most other species discussed here, however, the lowest energy singlet should be closed-shell in nature. For example, UB3LYP computations predict open-shell 1 [Ir] to lie 12.4 kcal mol -1 above the triplet -4.0 kcal mol -1 higher than the closed-shell singlet. Nevertheless, the fact that we use RB3LYP to compute singlet states does mean that some regions of the potential energy surface may not be quantitatively accurate. However, this is expected to play a far less important role than spin-orbit coupling, and should have no bearing on our qualitative results.

A Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3) p. 24/30 interaction. Indeed, optimization from this point leads to the -ethylene adduct, suggesting that that product would be formed if crossover occurs in that region. To reach this MECPs, it is necessary to first cross the low barrier leading to the  1 ethylene adduct. The geometry of this MECP was also optimized for the real [Cp*Ir(PMe3)] system. The geometry is similar to that found for the model.

The relative energy is very slightly higher, in line with the slight destabilization of in the former, and "vertically" in the latter. This might suggest that the triplet complex, which lies well above the singlet in energy, may not be able to relax very efficiently to the singlet. In fact, the singlet state is very close in energy to the triplet at the triplet minimum geometry, and there is an MECP lying close in geometry (shown in Figure 13, cf. with Figures 9a) and energy (see Table 2).

Likewise, the MECP should be close in energy and geometry to the corresponding triplet minimum for the "real" system. Therefore, if the triplet [Cp*Ir(PMe3)( 2 -H2C=CH2)] complex is formed, it should be readily able to cross over to the singlet surface, leading to the global minimum, singlet Discussion. There are many regions of the potential energy surface in the [CpIr(PH3)] + ethylene region where the singlet and triplet states are near degenerate, and we have located three MECPs.

From two of these, the steepest descent route leads to the singlet ethylene compound, and from the third, one reaches the vinyl hydride compound. Since this third MECP is the highest in energy, and in fact lies somewhat above the energy of the reagents, one could conclude that our calculations predict the sole formation of singlet [CpIr(PH3)( 2 -H2C=CH2)], which would disagree with experiment.

In fact, this is not the case for several reasons. First of all, the highest-lying MECP, from which the steepest-descent path leads to the vinyl hydride product, is the only one which can be reached directly from the triplet ethylene van der Waals complex. The other two MECPs are only reached after crossing one or two transition states. Although these transition states are low-lying for the model system studied here, they may lie somewhat higher for the real system. In particular, the MECP which is close to the triplet  2 -ethylene complex, and which from a first glance would be the easiest way in which to reach the singlet surface, may not be significantly involved in the reaction. This is because the transition state leading to the triplet  2 -ethylene complex, and this complex itself, are subject to quite severe steric hindrance from the methyl groups on Cp* and PMe3 in the "real" [Cp*Ir(PMe3)] system. In this case, therefore, the use of a model compound to derive the reaction profile is slightly misleading, although the effect is less dramatic than has sometimes been

observed [ref GGG].
Secondly, although the two other MECPs differ in that the direct downhill route from them leads to different products, they are not entirely dissimilar in geometry, and both are fairly similar to the singlet transition state separating the two products. Also, they both lie higher in energy than that transition state. This means that the seam of crossing between the two surfaces must lie at fairly low energies across quite a broad region of the potential energy surface and that the system may be able to cross in many places, not restricted to the immediate vicinity of the two MECPs we have located.

It also means that once the system does cross onto the singlet surface, how it partitions between the vinyl hydride and the ethylene complex may be determined to a large extent by the dynamics, rather than by the nature of the steepest descent path. As discussed above, this dynamical question would require much more work to investigate.

Finally, one has to realize that a product ratio of 2:1 corresponds to a very small difference in free energy between the two pathways, and that the computational difficulties in a system such as the present one are simply too challenging for one to be able to predict the outcome quantitatively.

Given these provisos, the general picture emerging from our calculations (see Figure 14) is in agreement with the experimental observations. Thus, our calculations predict the following, based on the features of the singlet and triplet surfaces, and of their intersections:

* Heating [Cp*Ir(PMe3)H(C6H11)] will lead to loss of cyclohexane, and formation of [Cp*Ir(PMe3)] in its ground, triplet state. This is due to the fact that a crossing between singlet and triplet surfaces occurs very close in energy to the singlet state.

* The triplet will interact with ethylene to form a weakly bound van der Waals adduct. Given the low interaction energy in this intermediate, it will be able to interconvert readily, in agreement with the isotope effect data. It is worth to mention here that the calculated geometry of this intermediate is as predicted in the experimental study, [18] except for the spin state.

* Addition to form triplet  1 -and  2 -ethylene adduct may also occur, although there are barriers for formation of these species, especially the latter one. In the model system, where we have optimized the corresponding transition states, these barriers are very low, but they are predicted to be somewhat higher in the real system.

* The singlet surface intersects the triplet surface at multiple points, all lying at relatively low energy, so that crossover to the singlet state should be relatively facile. However, the surface crossing will lead to small barriers, and thus to a finite lifetime for the triplet intermediates. It is hard to predict the ratio of products on the singlet surface. In particular, because the transition state which separates [Cp*Ir(PMe3)( 2 -C2H4)] and [Cp*Ir(PMe3)(CH=CH2)H] is lower in energy than the relevant surface crossings, dynamical factors will affect to which side of this partitioning barrier the system will fall. Thus, although our results are at first sight suggestive of the ethylene complex being preferentially formed, upon close consideration, especially concerning the uncertainties involved, the results are compatible with the observed 2:1 ratio in favor on the vinyl hydride.

A

  Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3) p. 5/30

  Scheme 2. The two mechanistic possibilities A and B which account for the existence of an intermediate having the stoichiometry [Cp*Ir(PMe3)(C2H4)].

Figure 2 .

 2 Figure 2. Singlet potential energy surface for the process leading from [CpIr(PH3)(CH3)H] to the ethylene oxidative addition and -addition products. Energy values (in brackets) are at the B3LYP/TVZ* level in kcal mol -1 units.

  Hydride to  2 -Alkene. The optimized structures of [CpIr(PH3)-(CH=CH2)H] and [CpIr(PH3)(C2H4)], and of the corresponding [Cp*-PMe3] analogues, together with the most relevant bond distances, are shown in Figures 3 and 4, respectively. The structures of the vinyl hydride systems compare favourably with the experimentally determined structure of [Cp*Ir(PMe3)(CH=CH2)H], except that the optimized vinyl C=C distance is significantly longer

Figure 3 .

 3 Figure 3. B3LYP/TZV* optimized structures of (a) [CpIr(PH3)(CH=CH2)H] and (b) [CpIr(PH3)(C2H4)] with selected bond distances in Å.

Figure 4 .

 4 Figure 4. B3LYP/TZV* optimized structures of (a) [Cp*Ir(PMe3)(CH=CH2)H] and [Cp*Ir(PMe3)(C2H4)] with selected bond distances in Å.

Smith, Poli and Harvey A

 Harvey Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3) p. 15/30 respectively, relative to the corresponding triplet iridium species. The very similar results on the Cp-PH3 and Cp*-PMe3 systems supports use of simplified models for this aspect of the problem. The transition state for the isomerization process was only optimized for the model system. The geometry and relevant distances are shown in Figure 5. The distance between Ir and the five individual Cp carbon atoms indicate a significant slipping of the ring toward the  3 configuration.

[

  CpIr(PH3)] and it is likely placed on the barrierless reaction pathway leading from singlet [CpIr(PH3)] and C2H4 to either of the compounds separated by the barrier discussed here: [CpIr(PR3)H(C2H3)] or [CpIr(PR3)(C2H4)] (see Figure 2).

Figure 5 .

 5 Figure 5. B3LYP/TZV* optimized transition state between [CpIr(PH3)(CH=CH2)H] and [CpIr(PH3)(C2H4)] with selected bond distances in Å.

Figure 6 .

 6 Figure 6. Triplet potential energy surface for the interaction between [CpIr(PH3)] and ethylene.Energy values are at the B3LYP/TVZ* level in kcal mol -1 units.

Figure 7 .

 7 Figure 7. Two views of the B3LYP/TZV* optimized structure of the van der Waals complex obtained by the addition of C2H4 to triplet [CpIr(PH3)].

Figure 8 .

 8 Figure 8. B3LYP/TZV* optimized structures of (a) [CpIr(PH3)( 1 -C2H4)] and (b) [Cp*Ir(PMe3)( 1 -C2H4)] in the triplet state, with selected bond distances in Å.

Figure 9 .

 9 Figure 9. B3LYP/TZV* optimized structures of (a) [CpIr(PH3)( 2 -C2H4)] and (b) [Cp*Ir(PMe3)( 2 -C2H4)] in the triplet state, with selected bond distances in Å.

  this case. As a result, the relative energy of this intermediate is quite strongly affected by the increased steric bulk of the [Cp*Ir(PMe3)] system. Thus, [Cp*Ir(PMe3)( 2 -C2H4)] lies only 5.1 kcal mol -1 below [Cp*Ir(PMe3)], compared to 10.8 in the [CpIr(PH3)] model system. The reason for this substantial destabilization can be readily understood upon considering the optimized structure.

  the second ethylene carbon is further away from the Ir atom (3.143 Å) and the Ir-C-C angle is much more open (119.50°) relative to the TS in Figure5. Equally, the ethylene structure at this MECP is scarcely changed with respect to its optimal geometry, so there should be no additional barrier separating this MECP from separated [CpIr(PH3)] and ethylene.

Figure 11 .

 11 Figure 11. B3LYP/TZV* optimized MECP in the triplet [CpIr(PH3) . (C2H4)] region, leading to singlet [CpIr(PH3)(CH=CH2)H].

[

  Cp*Ir(PMe3)( 1 -C 2 H 4 )] compared to [CpIr(PH3)( 1 -C 2 H 4 )].

Figure 12 .

 12 Figure 12. B3LYP/TZV* optimized MECP leading from triplet [CpIr(PH3)( 1 -C2H4)] to singlet [CpIr(PH3)( 2 -C2H4)].

[

  Cp*Ir(PMe3)( 2 -H2C=CH2)].

Figure 13 .

 13 Figure 13. B3LYP/TZV* optimized MECP leading from triplet [CpIr(PH3)( 2 -C2H4)] to singlet [CpIr(PH3)( 2 -C2H4)].

Figure 14 .

 14 Figure 14. Schematic drawing showing the four MECPs between singlet and triplet potential energy surfaces, and thereby the two-state pathway leading from singlet [CpIr(PH3)H(CH3)] to singlet [CpIr(PH3)H(CH=CH2)] or singlet [CpIr(PH3)(  -CH=CH2)] via triplet [CpIr(PH3)]. Relative energies in kcal mol -1 .

Table 1 .

 1 Computed relative energies of singlet and triplet [(C5H5)Ir(PH3)] from this work and

	previous literature reports.
	∆ES-T a)	Method	ref.
	N.R.(<0) b)	X	[5a]
	33	ROHF	[6a]
	24	MP2(UHF)	[6a]
	26	CI	[6a]

A Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3)

p. 10/30

a) E(singlet)-E(triplet) in kcal mol -1 . b) Not reported. The singlet is stated to be of lower energy than the singlet (1-6 kcal mol -1 ).

Table 2 .

 2 Energies (in kcal mol -1 ) of all calculated species relative to triplet [CpIr(PH3)] or

	[Cp*Ir(PMe3)] and CH4 or C2H4.
	Species	[Ir] = [(C5H5)Ir(PH3)]	[Ir] =
				(C5Me5)Ir(PMe3)
		B3LYP/LanL2DZ B3LYP/TZV*	B3LYP/TZV*
	1 [Ir]H(CH3)	-28.1	-32.3	-31.3
	1 [Ir]…CH4 TS	1.6	(a)	/
	1 [Ir] . CH4	1.1	(a)	/
	MECP [Ir] . CH4	6.7	10.7	/
	1 [Ir]	6.4	8.4	6.8
	3 [Ir] (Erel = 0) (b)	-306.42541	-641.35249	-955.84525
	1 [Ir]H(C2H3)	-37.2	-41.2	-42.8
	1 [Ir]( 2 -C2H4)	-46.4	-51.8	-49.3
	1 [Ir](H-C2H3) TS	-5.0	-7.7	/
	3 [Ir].C2H4	-1.2	-1.1	/
	Add. TS to 3

Table 2 .

 2 For the intermediate, our results are in qualitative agreement with

	Smith, Poli and Harvey	A Computational Study of Ethylene C-H Bond Activation by Cp*Ir(PR3)	p. 11/30
	previous calculations (see

Table 1 )

 1 ; in particular, the optimized geometries are very close to those previously reported by others and no detailed analysis thereof is warranted here. The Cartesian coordinates of these structures, as well as all other optimized structures reported in this contribution, are deposited as Supporting Information. The triplet is found to be the ground state, but the singlet

	is fairly low-lying, with excitation energies of 6.4 and 8.4 kcal mol -1 for [CpIr(PH3)] at the
	B3LYP/LanL2DZ * and B3LYP/TZV* levels, respectively; and of 6.8 kcal mol -1 for [Cp*Ir(PMe3)]

Acknowledgement. R.P. and JNH thank the Région Bourgogne and the EPSRC (Grant M92089), respectively, for supporting this research; and K.M.S. thanks the European Commission for a TMR Marie Curie Postdoctoral Fellowship.

The energy of the crossing found here is slightly above that of singlet [CpIr(PH3)]. This means that surface crossing will not substantially accelerate alkane dissociation with respect to the spin-allowed reaction, unlike the situation found in other cases. [12] However, if one takes into account the strong iridium spin-orbit coupling, and the low energy of the MECP relative to the singlet minimum, it is clear that [CpIr(PH3)] will have a very short lifetime (or none at all if crossing occurs during the late stages of the dissociation) before relaxing to the triplet state. Thus, the singlet intermediate will not have time to react with other species, e.g. ethylene. In practical terms, pyrolysis of the methyl or cyclohexyl hydride will directly lead to [CpIr(PH3)] (or [Cp*Ir(PMe3)]) in its triplet ground state.

The next three MECPs are in the vicinity of the three [CpIr(PH3)] + ethylene triplet minima discussed above. Indeed, unlike methane, which does not significantly alter the position of the MECP between singlet and triplet [CpIr(PH3)], ethylene leads to qualitatively new MECPs in which the ethylene moiety is very close to the iridium atom.

Supporting Information Cartesian coordinates of all calculated complexes.

A The unusual pattern of reactivity in addition of ethylene to [Cp*Ir(PMe3)] led Bergman to label it a mysterious reaction. Our DFT study of the singlet and triplet hypersurfaces, including the spin crossover points (one of which is shown here), shows that the experimental results can only be rationalized by a Two State Reaction pathway involving triplet intermediates.