Sabrina Sculfort

Pierre Braunstein
email: braunstein@unistra.fr

Dr Marie-Madeleine Rohmer

Intramolecular d 10 -d 10 interactions in Heterometallic Clusters of the Transition Metals †

This review deals with the synthesis and structures of heterometallic transition metal clusters displaying intramolecular, metallophilic d 10 -d 10 interactions.

Introduction

Whereas organometallic chemistry primarily focuses on compounds with metal-carbon bonds, coordination chemistry is concerned with all aspects of the interactions between ligands and metals, and includes the study of the synthesis, physical and chemical properties of the resulting metal complexes. Optimizing their synthesis and understanding their electronic structures, the nature of the bonding between metals and ligands and their reactivity are essential to this chemistry. Recognizing that direct bonding between metal atoms could exist in molecules -and not only in the bulk state -was the start of a "post-Wernerien" chemistry. Cotton coined the term « metal atom cluster » in 1964: "The term cluster seems an appropriate one for a finite group of metal atoms which are held together mainly, or at least to a significant extent, by bonds directly between the metal atoms, even though some nonmetal atoms may also be intimately associated with the cluster". [START_REF] Bertrand | [END_REF][2][3] Metal clusters are now very familiar not only to molecular chemists, i.e. coordination and organometallic chemists, but also to specialists of solid-state and physical chemistry since metal-metal bonding can occur in molecular compounds, inorganic solids (e.g. oxides, chalcogenides, halides) or in the gas phase (e.g. alkali metals). If one extends the original definition to include carbon and the remarkable fullerenes, even organic chemistry becomes concerned with clusters. This is not unreasonable when considering that boron has long been accepted "in the club", in the form of boranes and carboranes whose structures and bonding have helped considerably understand the structural chemistry of transition metal clusters. [4][5][6] Thus, all branches of chemistry, including materials sciences, nanosciences and catalysis, have become concerned, to a variable extent of course and for fundamental or applied reasons, with the study of element-element bonding.

During the last 20 years, several books have illustrated the rapid developments of cluster chemistry, its relevance to the synthesis of new chemical bonds, to a deeper understanding of chemical bonding, to the multisite reactivity and activation of small molecules, to homogeneous, supported and heterogeneous catalysis, and have emphasized its central role in nanosciences and its fruitful interfaces with biology and physics. [START_REF] Shriver | The chemistry of metal cluster complexes[END_REF][START_REF] Mingos | Introduction to Cluster Chemistry[END_REF][START_REF] Gonzalez-Moraga | Cluster Chemistry[END_REF][START_REF] Schmid | Clusters and Colloids. From Theory to Applications[END_REF][START_REF] De Jongh | Physics and Chemistry of Metal Cluster Compounds. Model Systems for Small Metal Particles[END_REF][START_REF] Kreibig | Optical properties of metal clusters[END_REF][START_REF] Kauzlarich | Chemistry, structure, and bonding of Zintl phases and ions[END_REF][START_REF] Mingos | Structural and electronic paradigms in cluster chemistry[END_REF][START_REF] Adams | Catalysis by Di-and Polynuclear Metal Cluster Complexes[END_REF][START_REF] Schmidbaur | Gold: Progress in Chemistry[END_REF][START_REF] Braunstein | Metal Clusters in Chemistry[END_REF][START_REF] Dyson | Transition metal carbonyl cluster chemistry[END_REF][START_REF] Feldheim | Metal Nanoparticles[END_REF][START_REF] Driess | Molecular clusters of the main group elements[END_REF][START_REF] Fehlner | Molecular clusters: a bridge to solidstate chemistry[END_REF] One of the many facets of cluster chemistry that attracted considerable attention from the synthetic chemists was the fascinating possibility to link chemically different metals through direct metal-metal bonding, thus opening a new field where 1540 chemical bonds can be envisaged, just to mention the possibilities offered by 56 transition elements. If many of these heterometallic bonds have now been formed, either in dinuclear complexes or in mixed-metal clusters, many combinations still remain unknown. Associating metals that do not form alloys in the bulk state turned out to be possible at the molecular level, and the relevance of such "molecular alloys" to e.g. bimetallic catalysis attracted considerable attention. [START_REF] Shriver | The chemistry of metal cluster complexes[END_REF][START_REF] Adams | Catalysis by Di-and Polynuclear Metal Cluster Complexes[END_REF][START_REF] Braunstein | Metal Clusters in Chemistry[END_REF] The first examples of heterometallic complexes containing a metal-metal bond between a group 11 metal (Cu, Ag, Au) and another transition metal, such as W, Mn, Fe, Co, were published by Coffey, Lewis and Nyholm in 1964. [START_REF] Coffey | [END_REF] Such complexes allowed the authors to investigate the conditions favouring the formation of covalent metal-metal bonds by taking into account the electronic configuration and the effective electronegativity of the metal atoms. With its d 10 electronic configuration, the coinage metal ion was found to behave like a pseudohalogen (cf. the similarity between e.g. [Mn(CO) 5 Cl] and [Mn(CO) 5 -Au(PPh 3 )]). Later, the similarity between [Au(PPh 3 )] + and H + was emphasized [24][25][26] and further analyzed in terms of the isolobal analogy, a remarkably fruitful concept. [27][28][29] Attempts were made to prepare and isolate a Au-Au complex of the type [(R 3 P)Au-Au(PR 3 )] (L = phosphine) that would be isolobal to dihydrogen, but although a structural report has appeared in the literature for a complex that has not been fully described, 30 stabilisation and full characterization of such a digold unit was achieved when coordinated to a Pt(II) centre in [PtCl(AuPPh 3 ) 2 (PEt 3 ) 2 ] + . 31 With a d 8 Pt(II) centre and a formally neutral digold unit isolobal to H 2 , the two-electron, three-centre bonding involving the metals in this triangular structure is analogous to that in the well-known 2e H interactions) has been rapidly growing in chemistry because they had to involve concepts other than classical covalent or dative bonding. Best evidenced by X-ray diffraction studies, their attractive nature results in inter-or intramolecular distances which are shorter than the sum of van der Waals radii of the atoms concerned. Although van der Waals radii may not be ideal points of comparison when they do not derive from structures of existing systems, 34 they are often used in the literature for benchmarking purpose and they will therefore also be used here in this context. They are more meaningful when they are obtained from intermolecular distances in molecular crystals. [START_REF] Pauling | The Nature of the Chemical Bond, 3rd Ed[END_REF][START_REF] Muñiz | [END_REF] Being particularly significant in the case of gold, although closed-shell metal cations such as Au(I) ([Xe]4f 14 5d 10 ) could be expected to repel each other on the basis of electrostatics, the term of aurophilicity was introduced by Schmidbaur in 1988 to describe interactions between Au(I) centres which could not be explained by conventional concepts of chemical bonding. [START_REF] Schmidbaur | Gold: Progress in Chemistry[END_REF][37][38][39][40][41][42] It should be recalled that attractive interactions between closed-shell Cu(I)-Cu(I) and Ag(I)-Ag(I) ions had already been recognised by Hoffmann in 1978 43 and Jansen in 1980, respectively 44,45 The aurophilicity concept suggests a new type of chemical bonding between gold atoms with the following characteristics: a) Steric effects permetting, the metal-metal distances are significantly shorter than the sum of the van der Waals radii (3.7 Å), b) Several atoms can gather around a gold centre to form polynuclear species, such as triangles, squares, lozenges, etc... of gold atoms, c) The bond energy associated with the interaction is generally small, in the order of magnitude of H-bonding, but larger than the energy of standard van der Waals contacts.

Early theoretical studies on the octahedral ions [{Au

(PH 3 )} 6 X m ] m+ (X 1 = B, X 2 = C, X 3 = N)
used as models for a fascinating series of main-group-element-centred octahedral gold complexes, focused on the role of the gold 5d atomic orbitals in Au-Au bonding. 46 The participation of d-orbitals can be achieved only if the formal d 10 configuration is broken, e.g. through 6s/5d hybridisation. It was thus concluded that there is a prominent contribution of the gold 5d atomic orbitals to the Au-Au bonding within the cluster, via 6s/5dz 2 hybridisation in the MOs of a 1g symmetry. Furthermore, it was argued that the effect has its origin in the relativistic modification of the gold valence atomic orbitals energies, which brings the 5d and 6s orbitals into close energetic proximity. Investigating the origin of the aurophilic attraction, Pyykkö and Li concluded that the effect was primarily due to electron correlation rather than to s-d hybridisation. 47,48 The variation of the relativistic contraction of the 6s shell for the elements ranging from Cs (Z = 55) to Fm (Z = 100) is represented in Figure 1 and a pronounced local minimum is observed for gold. Relativistic contributions to the bonding between gold ions, or between gold(I) and another d 10 ion, will therefore be significant and relevant to the molecular and supramolecular chemistry of this element, including the use of structure-directing building blocks for the design of nanostructured functional materials. 49 Copyright American Chemical Society). A clear minimum is observed for Au, which corresponds to a maximum of the relativistic effects for this element.

"Metallophilic attraction" has been coined to broaden the concept of aurophilicity, 48 and is best described as a correlation-dispersion phenomenon, enhanced by induction. This phenomenon continues to give rise to numerous theoretical investigations and developments. [START_REF] Muñiz | [END_REF]41,[51][52][53][54][55][56][57][58][59][60][61][62][START_REF] Schwerdtfeger | Gold Chemistry: Applications and Future Directions in the Life Sciences[END_REF] Relativistic effects can result in an increase or a decrease of metallophilicity, depending on the system and the level of theory employed. An increase is found for aurophilicity at the MP2 level, but at the higher, coupled-cluster levels a weakening was recently reported. [START_REF] Grady | [END_REF] If aurophilicity manifests itself in a rapidly increasing number of molecules, in colloids and small particles, 49,65 chemists rapidly became curious to see if a similar phenomenon could be observed for other closed shell or pseudo-closed shell elements. Thus for exemple, square-planar d 8 complexes can be regarded as pseudo-closed shell systems owing to the strong ligand field splitting of the d orbitals. 66,67 Remaining within the group 11 metals, it is particularly interesting to compare, experimentally and theoretically, d 10 -d 10 interactions involving copper(I) [68][69][70] or silver(I) [71][72][73] ions with those involving gold(I) ions in closely related complexes. Properties such as luminescence, catalysis, conductivity, magnetism and electrochemistry are strongly influenced by the occurrence of such d interactions, 42,49,60,66,[74][START_REF] Gimeno | Modern Supramolecular Gold Chemistry: Gold-Metal Interactions and Applications[END_REF][START_REF] Laguna | Modern Supramolecular Gold Chemistry: Gold-Metal Interactions and Applications[END_REF][START_REF] Barbieri | [END_REF][78][79][80][81][82][83][84][85][86][87][88][89] we will focus here on intramolecular d 10 -d 10 interactions in heterometallic clusters of the transition metals, which are less represented in other reviews.

Even in this case, it remains almost impossible to evaluate the energetic contribution of the attractive d 10 -d 10 interactions to the global stabilization of the molecule considered since these will generally be supported by ancillary ligands or bridging metal atoms. As indicated above, interactions between metal ions with a closed-shell electronic configuration represent such a broad field that we will restricting ourselves to the relevant heterometallic clusters of the transition metals. We will therefore leave out complexes and clusters, even when heterometallic, involving elements from the groups 13 or 14, which in their low-valent state, may involved d 10 s 2 -d 10 rather than strictly d 10 -d 10 interactions. We will not cover high nuclearity heterometallic clusters containing group 11 metals, which generally form the central core of their metal skeleton. The latter is often compared to the structures of the bulk metals and their complexity prevents a detailed discussion of d 10 -d 10 interactions.

We will focus in particular on the nature of the metallic core and the resulting interactions between the d 10 ions. First, we will consider intramolecular homometallic metallophilic interactions between Cu(I), Ag(I) or Au(I) ions in heterometallic complexes of the transition metals, then heterometallic d [START_REF] Schmid | Clusters and Colloids. From Theory to Applications[END_REF] To achieve a meaningful evaluation of the consequences of intramolecular d 10 -d [START_REF] Schmid | Clusters and Colloids. From Theory to Applications[END_REF] interactions, in terms of structures and/or properties, requires to compare the synthesis, structure, and bonding of molecules as closely related as possible, in which only the nature of the d 10 ion is varied, while keeping the other chemical parameters constant. The availability of sets of isoelectronic systems should provide unique opportunities for studying structure/bonding/reactivity relationships both experimentally and theoretically. In this review, we will not examine the theoretical aspects of the metallophilic interactions. The metal centres in the copper/iron mixed cluster [Cu 3 Fe 3 (CO) 12 ] 3-(3) also form a planar skeleton which consists of a ν 2 -triangle (i.e., in a ν n -polyhedron, there are n+1 equally spaced atoms along each edge) in which the central Cu 3 triangle is inscribed within the larger triangle formed by the three Fe(CO) 4 groups. 92,93 The distances between the copper atoms are found between 2.582(2) and 2.613( 2 In the octanuclear copper-cobalt mixed cluster 5 described by Klüfers, the inner Cu 4 square is inscribed within a Co 4 square and each Cu-Cu edge is bridged by a Co(CO) 4 fragment, thus leading to a ν 2 -square structure. 95 The values of the Cu-Cu distances range from 2.703(4)-2.731(4) Å and the Co-Cu-Co angles from 167.41( 4)° (at Cu( 1)) to 158.07( 6)° (at Cu( 2)), respectively. -with [AuCl(PPh 3 )] in CH 2 Cl 2 . 33 The distance of 2.737(1) Å between the two Au(I) centres is significantly shorter than in gold metal (2.884 Å). This complex further reacts with [AuCl(PPh 3 )] to form a cationic FeAu 3 cluster as the only product (eq. 1) in which the arrangement of the metals was established to be tetrahedral by a Wide Angle X-ray Scattering study. 33 Perspective view of 26. Phenyl groups have been omitted for clarity.

Fe

Au(PPh Å and 2.7598(5)-2.8248(5) Å, respectively. These are shorter than the sum of the van der Waals radii for silver (3.34 Å) 34 and even comparable to the sum of the covalent radii of 2.72 Å proposed by Alvarez. [START_REF] Cordero | [END_REF] The four Au atoms are not exactly coplanar, the Au-Au-Au angles

Intramolecular heterometallic d 10 -d 10 interactions in heterometallic clusters.

The number of mixed-metal complexes and clusters containing at least two chemically different d 10 ions susceptible to interact which each other, in addition to another metal, is still limited but rapidly increasing. It is therefore interesting to examine the consequences of these interactions on the structure and properties of the complexes and compare them with those in which the d 10 ions are the same. Heterobimetallic complexes containing two metals of the group 11 have been recently shown to possess interesting luminescence properties. 85,111,[126][127][128][129][130][131][132][133][134][135][136][137][138] Examples of mixed-metal clusters containing two different group 11 metals and another metal remain rare. We have recently examined ways to prepare such trimetallic complexes in order to compare them with the related bimetallic compounds.

Cu-Ag Interactions

Reaction the first trimetallic cluster with such a ν 2 -square geometry. 119 The Cu-Ag distances in 38 are in the range 2.734(1)-2.814(1) Å and are clearly shorter than the sum of the van der Waals radii for copper and silver (3.12 Å) or to the value of 2.9931 (12) Å found in a dinuclear Ag-Au complex containing bridging ligands. 141 They are even comparable to the sum of the covalent radii of 2.77 Å for copper and silver. [START_REF] Cordero | [END_REF] The Cu-Ag d [START_REF] Schmid | Clusters and Colloids. From Theory to Applications[END_REF] "Wrapping" the [Au 6 Cu 6 (C 2 Ph) 12 ] cluster in a [Au 3 (diphosphine) 3 ] 3+ "belt" was found to increase its stability, particularly under irradiation. Its luminescence is much more intense than that of the corresponding homometallic compounds and was attributed to the presence of the Cu 6 Au 6 cluster core. 143

Cu-Au Interactions

Specific properties may be associated with the presence of heterometallic d 10 -d 10 interactions, as shown recently with a trinuclear, Cu-Au-Cu chain complex which functions as a luminescent vapochromic sensor, 144 or in Cu(I) halide butterfly dimers interacting with a Au(I) cationic dicarbene unit. 128 In the complex [Au 3 (µ 3 -S)Cu(µ-PPh 2 py) 3 ](BF 4 ) 2 (PPh 2 py = diphenylphosphine-2-pyridine), the Cu-Au contacts within the tetrahedral metal core were found in the range 2.9000(13)-2.9871(14) Å. A coordination isomer was also characterized in which two monocationic Au 3 S units bridge two Cu(I) ions through the pyridine groups. There is only one Cu-Au bonding interaction in this compound and its value of 2.7954(9) Å is shorter than in the other isomer. 115 The trimetallic, hexanuclear clusters [MM'Ru 4 (µ 3 -H) 2 {µ-Ph 2 P(CH 2 ) n PPh 2 }(CO

Ag-Au Interactions

In the complex [Au 3 (µ 3 -O)Ag(µ-PPh 2 py) 3 ](BF 4 ) 2 (PPh 2 py = diphenylphosphine-2-pyridine), which exhibits extremely bright luminescence in the solid-state at room-temperature, the Ag-Au contacts within the tetrahedral core are in the range 2.8985(5)-2.9690(5) Å. 114 A square pyramidal Ag 4 Au cluster has been characterized in which the apical Au(I) centre is connected to two basal Ag(I) centres via 3c-2e aryl bridges but this did not affect significantly the Ag-Au distances, which were in the range 2.9019(6)-3.0134(6) Å. 104 A helical coordination polymer containing pyridine-substituted N-heterocyclic carbene ligands as supports for Ag(I)-Au(I) interactions of 2.8359(4) and 2.9042(4) Å has been characterized. 135 (2.8797(8)-3.2841(2) Å). [162][163][164][165][166] This complex also undergoes dynamic behaviour in solution, but this time the motion involves a sliding of the Pd centre along the Fe-Hg-Fe chain.

Changing the reagent opposed to the chain complex [Hg{Fe[Si(OMe) 3 ](CO) The difficulties associated with a detailed theoretical analysis of the metallophilic interactions are that not only must relativistic basis sets be used but the energy surfaces are rather shallow. We are currently trying to assess the role of the metallophilic interactions in assemblies. 66,74,[START_REF] Barbieri | [END_REF][78][79]85,111,[126][127][128][129][130][131][132][133][134][135][136][137][138]143,[181][182][183][184][185][186][187][188]

Figure 1 .

 1 Figure 1. Variation of the ratio between relativistic and non-relativistic 6s shell radii in the atomic ground states of the elements 55-100 as a function of Z (Reprinted with permission from ref. 50.

2. Intramolecular homometallic d 10 -d 10 interactions in heterometallic clusters. 2 . 1 . 1 2

 10211 Cu-Cu interactions. Specific geometries of the metal core of heterometallic clusters appear to facilitate the development of d 10 -d 10 interactions, such as the planar (rhomboidal) raft of six metals found in [Cu 2 Re 4 H 16 (PMe 2 Ph) 8 ] 2+ (1) or the trigonal bipyramidal geometry in complex [Ir 2 Cu 3 H 6 (MeCN) 3 (PMe 2 Ph) 6 ] 3+ (2). 90,91 Like planar clusters, 1 represents an attractive geometric model of a metal surface and the unique arrangement of the metals around the central Cu 2 unit and the steric accessibility of copper suggest possible enhanced reactivity of this dication. In 2, the iridium atoms occupy the apical sites of the trigonal bipyramidal structure. Three phosphine ligands are facially coordinated to the pseudo-octahedral iridium centres and each copper is bonded to an acetonitrile ligand. The Cu-Cu distances are 2.493 Å in 1 and 2.57(2) Å in 2. Only 6 of the 16 hydride ligands were located. Only the metals and the inner coordination The terminal phenyl groups have been omitted sphere donor atoms are shown. The 6 for clarity. bridging hydride ligands between Cu and Ir are not represented.

  ) Å. The Cu-Cu-Cu angles are in the range 59.25(4)-60.44(5)° and the Fe-Cu-Fe angles in the range 174.26(7)-175.23(7)°. The Cu 3 Os 3 core of [Cu 3 Os 3 H 9 (PMe 2 Ph) 9 ] (4) has also a ν 2 -triangular structure and the orientation of the three bridging Os(PMe 2 Ph) 3 groups is such that the molecule possesses a C 3 axis.94 The Cu-Cu distances are equal to 2.626(3) Å and all the Os-Cu-Os angles are within 3.6° of 180°.

5 (

 5 Reproduced with permission from ref. 95. Copyright Wiley-VCH Verlag GmbH & Co. KGaA) The triangular and square geometries observed in these Cu(I) clusters thus appear to favour d 10 -d 10 interactions. It is noteworthy that although the bridging metalloligands [Fe(CO) 4 ] 2- and [Co(CO) 4 ] -are isoelectronic and almost isosteric, they lead to a ν 2 -triangular and a ν 2square core structure in 3 and 5, respectively. The reactions of [Cu(NCMe) 4 ]PF 6 with [MoCp(CO) 3 ] -or [Mo(CpNMe 2 )(CO) 3 ] - (CpNMe 2 = η 5 -C 5 H 4 NMe 2 ) afforded the ν 2 -triangular clusters [Cu 3 {MoCp(CO) 3 } 3 ] (6) and [Cu 3 {Mo(CpNMe 2 )(CO) 3 } 3 ] (7), respectively, which like 3 and 4, adopt a 2-D raft-type arrangement. 62,96 The central Cu(I) core forms an almost equilateral metal triangle inscribed within a molybdenum triangle. The mean Cu-Cu distances are similar in 6 and 7, 2.627(8) Å and 2.617(1) Å, respectively, and much shorter than the sum of the van der Waals radii (2.80 Å). These two clusters are very similar, except for the orientation of the metalloligands with respect to the Cu-Cu edge they bridge. The Mo(1)-Cu(1)-Mo(2), Mo(2)-Cu(2)-Mo(3) and Mo(3)-Cu(3)-Mo(1) angles are 175.92(5)°, 177.87(5)°, 171.25(5)° for 6 and 175.50(2)°, 173.41(2)°, 176.85(2)° for 7, respectively.

6 7 A 97 8

 6797 similar reaction between [CuCl(tmed)] (tmed = N,N,N',N'-tetramethylethylendiamine) and Na[MCp(CO) 3 ] (M = Cr, Mo, W) afforded the dinuclear complex [(tmed)Cu{MCp(CO) 3 }] (8) in which the tmed ligand remained chelated to copper, thus preventing cluster formation.In clusters 1-7, the Cu-Cu distances are close to the sum of the metal atom radii (2.556 Å) but shorter than the sum of the van der Waals radii (2.80 Å),34,[START_REF] Wiberg | Lehrbuch der Anorganischen Chemie[END_REF] which suggests attractive metallophilic interactions between the Cu centres. These distances are even comparable to the sum of the covalent radii of 2.64 Å proposed by Alvarez.[START_REF] Cordero | [END_REF] Notably shorter Cu-Cu distances are found in the ν 2 -triangular compared to the ν 2 -square structures. Interestingly, heterobimetallic clusters of Cu(I) with core geometries related to the square arrangement found in 5 have been observed which do not display d 10 -d 10 interactions, as in e.g. the octanuclear [Et 4 N] 4 [M 4 Cu 4 S 12 O 4 ] (M = Mo, W) (9) and the dodecanuclear clusters [M 4 Cu 4 S 12 O 4 {Cu(tmen)} 4 ] (M = Mo, W; tmen = N,N,N,N-tetramethyl ethylenediamine) (10). 100 The eight metal atoms in the former cluster form a distorted ν 2square arrangement, with sulphur atoms doubly or triply bridging the metals. The derived dodecanuclear cluster 10 can be considered as formed of a [M 4 Cu 4 S 12 O 4 ] 4-unit linked to four Cu(tmen) + groups at each M atom. These Cu(tmen) + groups are positioned alternately on one and the other side of the Cu 4 M 4 square. The Cu-Cu separations, in the range 5.415(1)-5.515(1) Å for 9 and 5.236(1)-5.354(1) Å for 10, are much too large to allow direct interactions between the copper centres. These two examples serve to illustrate that if some structural core arrangements appear favorable for the occurrence of metallophilic interactions, other parameters such as the nature and position of the ligands play an important role.

9 10 C

 910 atoms are omitted for clarity It is interesting to compare the Cu-Cu distances found in the heterometallic clusters of type {M[m]} n (M= Cu, n = 3 or 4) mentioned above with those in e.g. the homometallic copper square [Cu 4 (µ-C 4 H 3 S) 4 ] (11) 101 or the copper-cobalt cluster [Cu 3 {µ 2 -(CCHCo 2 (CO) 6 )} 3 ] (12). The latter contains a triangular Cu(I) core surrounded by CCHCo 2 (CO) 6 moieties, each copper being bonded to two cobalt atoms and one carbon, with an average value for the short Cu•••Co non-bonding distance of 2.584 Å. 102 The Cu-Cu distances in 11 and 12, in the range 2.453(3)-2.507(3) Å and 2.494(2)-2.508(2) Å, respectively, are short enough to allow cuprophilic interactions.

2 .

 2 Ag-Ag InteractionsThe metal core of the Pt(II)-Ag(I) cluster [Ag 2 {PtMe 2 (N,N)} 2 ](OTf) 2 (N,N is a diimine ligand) forms a Ag 2 Pt 2 parallelogram whose Ag-Ag diagonal has a length of 2.6972(2) Å.103 When Cu(I) was used as a precursor in place of Ag(I), a dinuclear Pt-Cu complex was obtained instead, consistent with argentophilic interactions being stronger than cuprophilic interactions. An unusual square pyramidal Ag 4 Au cluster has been characterized in which the distances between the four basal Ag(I) centres were in the range 2.8424(8)-2.9396(7) Å.104 In 1985, the cationic cluster [Ag 3 Rh 3 H 9 (tripod) 3 ] 3+ (13) was obtained by reaction of a methanolic solution of [RhCl 3 (tripod)] with 4 equiv. of AgCF 3 SO 3 .105 Its almost planar metal core forms an equilateral ν 2 -triangle in which the Ag 3 core is bridged on all three edges by rhodium atoms. All the Ag-Rh bonds are hydrogen-bridged and there is an alternation of single and double bridges around the Rh triangle. This structural assignment was supported by the alternation of the Ag-Rh distances, which show shorter contacts (2.795(4)-2.807(4) Å) where a double hydrogen bridge has been postulated and longer contacts (2.884(4)-2.933(4) Å) for the single bridges. The Ag-Ag distances are in the range 2.968(4)-2.998(4) Å and much shorter than the sum of the van der Waals radii for silver (3.44 Å).

13

  The terminal phenyl groups have been omitted for clarity. The hydride positions are not indicated.105 The first structurally characterized carbonyl clusters of the type [AgM(CO) 4 (dmpe)] 3 (M = Nb, Ta; dmpe = 1,2 bis(dimethylphosphino)ethane)(14) were reported in 1989.106 These clusters were isolated from the reaction of NEt 4 [M(CO) 4 (dpme)] with AgNO 2 or AgBF 4 in THF at room temperature in a 1:1 molar ratio. The solid state structures of 14a (M = Nb) and 14b (M = Ta) are isotypic. In these ν 2 -triangular raft-clusters, the Ag 3 core is bridged on all three edges by niobium or tantalum atoms. The almost identical Ag-Ag distances of 2.8424(5) Å are in the range of those found in the literature (median value from CCSD Nov. 2008: 3.016 Å, σ = 0.20 Å for 3343 samples).14 (Reproduced with permission from ref. 106. Copyright Wiley-VCH Verlag GmbH & Co. KGaA) The first octanuclear heterometallic cluster Ag 4 M' 4 analogous to the Cu 4 M' 4 cluster 5 was described by Klüfers. 107 The centrosymmetric silver-cobalt cluster 15 contains a planar metal core and each Ag-Ag edge of the central square is bridged by a Co(CO) 4 fragment in such a way that this square is inscribed within a slightly distorted cobalt square (ν 2 -square structure). The Ag-Ag distances are in the range 3.01(1)-3.03(2) Å, and the inward bending of the Co(1)-Ag-Co(2) angle (161.25(4)°) is consistent with attractive argentophilic interactions. 15 The related anionic cluster [Ag 4 {Fe(CO) 4 } 4 ] 4-(16) was reported in 1994 by Longoni et al. who also provided a bonding analysis. 108 It was selectively obtained by reaction of AgBF 4 or AgNO 3 with Na 2 [Fe(CO) 4 ]•xTHF in tetrahydrofuran and/or acetonitrile solution. Its structure contains an idealized Ag 4 square (average Ag-Ag distance 3.149 Å) surrounded by four edgebridging Fe(CO) 4 groups. These fragments adopt a C 2v conformation and 16 has idealized D 4h symmetry. The inward bending of the Fe-Ag-Fe linear sequences (average angle 165.4°) suggests, like in the case of 15, the occurrence of Ag-Ag attractive interactions. 16 The ν 2 -square clusters [Ag 4 {MoCp(CO) 3 } 4 ] (17) 62 and [Ag 4 {Mo(CpNMe 2 )(CO) 3 } 4 ] (18) 96 were recently characterized and are interestingly related to 15 and 16. The mean d 10 -d 10 distances within the silver square of 17 (2.8703(1) Å) and 18 (2.8687(1) Å) are similar, they are much shorter than the sum of the van der Waals radii for silver (3.44 Å) and even comparable to the sum of the covalent radii of 2.90 Å. 99 The main difference between clusters 17 and 18 is to be found in the orientation of the metalloligands with respect to the Ag-Ag edge they bridge. The Mo-Ag-Mo angles are 147.80(3)° for Ag(1) and 163.56(3)° for Ag(2) in 17 and 149.81(4)° for Ag(1) and 157.55(3)° for Ag(2) in 18 and the inward bending of these bonds is again consistent with attractive argentophillic interactions.

17 18 24 2. 3 .

 1718243 The structures of [Ag 8 Fe 4 (CO)16 (dppm) 2 ] (19) and [Ag 4 Au 4 Fe 4 (CO) 16 (dppe) 2 ] (20) 109 (dppm = bis(diphenylphosphine)methane, dppe = bis(diphenylphosphine)ethane) are organized around a central silver square whose edges are spanned by Fe(CO) 4 groups which are placed alternately above and below the Ag 4 plane. This arrangement is similar to that found in 16. The opposite vertices defining the long diagonal of the central square of 19 and 20 are bonded to Ag 2 (dppm) and Au 2 (dppe) fragments, respectively. The d 10 -d 10 distances in the central silver square are short, with an average of 2.799 Å for 19 and 2.755 Å for 20. Cluster 19 represents a rare example of a neutral Ag-Fe cluster containing phosphines as ancillary ligands for silver. 19 The terminal phenyl groups have been omitted for clarity. 20 The terminal phenyl groups have been omitted for clarity. An octanuclear cluster [(η 5 -C 5 Me 5 )WS 3 Ag] 4 (21) and an unusual helical polymer {[(η 5 -C 5 Me 5 )WS 3 ] 2 Ag 3 (CN)} ∞ (22) have been synthesised and structurally characterized. 110 The authors reacted (PPh 4 )[(η 5 -C 5 Me 5 )WS 3 ] with AgCN with the objective to introduce metal cyanide moieties into a cluster in view of the ability of the cyanide ligand to bridge between metal atoms, and obtained 21 and 22. However, the minor octanuclear cluster 21 did not contain cyanide ligand and was better synthesized by reaction of (PPh 4 )[(η 5 -C 5 Me 5 )WS 3 ] with 1 equiv. of [Ag(NCMe) 4 ]PF 6 in MeCN (85% yield). The Ag-Ag separations in this cluster were too long to correspond to significant bonding between the d 10 ions. 21 Structure of the repeat unit of 22 In contrast, the Ag-Ag distances in the nearly symmetrical Ag 3 cluster [Ag 3 (CH 3 im(CH 2 py)) 2 (NCMe) 2 ](BF 4 ) 3 (23), which was obtained from the N-heterocyclic carbene precursor 1-methyl-3-(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate, 111 and in the square cluster [Ag 4 (L) 2 ](PF 6 ) 4 •Et 2 O•MeCN (24) (H 2 L = 2,6-bis(N-(pyridylmethyl) imidazoliumyl)pyridine) 73 are much shorter and similar to those in the heterometallic complexes of the type {Ag[m]} n [m = Nb(CO) 4 , Ta(CO) 4 , n = 3 (14); m = Co(CO) 4 , Fe(CO) 4 , MoCp(CO) 3 , n = 4 (15-18)] seen above. They are in the range 2.7598(8)-2.7832(8) Å for 23 and 2.784(1)-2.820(1) Å for 24. 23 Au-Au Interactions In 1981, the unusual mixed-metal cluster [(Ph 3 PAu) 3 V(CO) 5 ] (25) was synthesized by treatment of the sodium or cesium salt of the pentacarbonylvanadate(3-) ion, [V(CO) 5 ] 3-, with [AuCl(PPh 3 )] in THF. 112 Its tetrahedral metal core contains three gold atoms and a eightcoordinated vanadium centre. The Au-Au distances are in the range 2.768(1)-2.855(1) Å. 25 ORTEP of 25. The phenyl groups have been omitted for clarity. (Reprinted with permission from ref. 112. Copyright American Chemical Society) An Au 2 Mo 4 phosphinidene complex with an unusual H-shaped planar metal core has been characterized which exhibits an Au-Au separation of 3.022(1) Å. 113 The first heterometallic borole complexes of Fe and Au were reported in 1998 and include the Au 2 Fe cluster [(OC) 2 {η 5 -(1-phenylborole)}Fe{Au(PPh 3 )} 2 ] (26) which was obtained by reaction of the metalate [HFe{η 5 -(1-phenylborole)}(CO) 2 ]

  I) triangle is also present in the cations [Au 3 (µ 3 -O)M(µ-PPh 2 py) 3 ] 2+ (M = Cu(I), Ag(I), PPh 2 py = diphenylphosphine-2-pyridine), which exhibits extremely bright luminescence in the solid-state at room-temperature.114,115 The Au-Au contacts within the tetrahedral metal core were found in the range from 2.9801(6) to 3.0000(6) Å for M = Cu and from 3.0661(3) to 3.2096(3) Å for M = Ag. The reaction of [(C 5 H 4 SiMe 3 ) 2 NbH 3 ] with [Au{N(SiMe 3 ) 2 }(PPh 3 )] produced the raft cluster [NbAuH 2 (C 5 H 4 SiMe 3 ) 2 ] 3 (27) in high yield. 116 Its structure consists of a gold triangle surrounded by three Cp' 2 Nb groups (Cp' = C 5 H 4 SiMe 3 ) and six bridging hydrides [Au-Au 2.764(2), 2.757(2) and 2.780(3) Å]. Molecular orbital calculations indicated a striking electronic stabilization of this geometry, which may be of importance in other gold hydrides. 27 The 6 bridging hydride ligands between Au and Nb are not represented. The dinuclear complex [CpMo(CO) 3 (AuPPh 3 )], obtained by reaction of Li[MoCp(CO) 3 ] with [AuCl(PPh 3 )] at -95 °C in dichloromethane, reacted under UV irradiation with excess [AuN 3 (PPh 3 )] to afford the cationic cluster [CpMo(CO) 2 (AuPPh 3 ) 4 ] + (28). 117 The five metal atoms form a trigonal bipyramid with the Mo atom occupying an equatorial position. The Au-Au distances in this cluster are in the range 2.812(1)-2.856(2) Å. 28 Phenyl groups have been omitted for clarity.The neutral, "square-in-a-square" or ν 2 -square clusters [Au 4 {MoCp(CO) 3 } 4 ] (29) and [Au 4 {Mo(CpNMe 2 )(CO) 3 } 4 ] (30) display Au-Au distances in the range 2.7417(8)-2.8030(9)

  are around 90° and the Mo-Au-Mo angles between 150.99(3)° and 154.29(3)° for 29 and between 149.88(3)° and 162.07(3)° for 30. The inward bending of the Mo-Au-Mo units is consistent with attractive aurophilic interactions.

29 30 Cluster

 2930 29 could be synthesized directly by a 1:1 reaction between [AuBr 2 ] -118 and Na[MoCp(CO) 3 ] 62 or by reaction of [AuBr 2 ] -with the linear heterotrinuclear complex [n-Bu 4 N][Au{MoCp(CO) 3 } 2 ], 119 itself obtained by the reaction of [n-Bu 4 N][AuBr 2 ] with 2 equiv. of Na[MoCp(CO) 3 ]. These clusters form an interesting series of oligomers of the type [M{MoCp(CO) 3 }] n n-(n = 3 for M = Cu; n = 4 for M = Ag, Au) in which the group 11 metals are always in the M(I) oxidation state. Furthermore, the structural similarity between 29 or 30 and the cluster Na 2 [Pd 4 {MoCp(CO) 3 } 4 ]•2THF (31) is noteworthy, 120 although the electronic configuration of the Pd centres is different from d 10 . This Pd 4 Mo 4 anionic octanuclear cluster was the first example of a ν 2 -square cluster where Pd has a formal oxidation state of (+½). 120 The redox reaction between [Pd 4 (OAc) 4 (CO) 4 ] (which contains a rectangular metal core) and a large excess of Na[MoCp(CO) 3 ] in THF which afforded 31 also yielded the dinuclear complex [CpMo(CO) 3 ] 2 and air-sensitive palladium-molybdenum complexes.

31 AScheme 2 .

 312 Scheme 2. Synthesis of 34 by condensation reaction.123

35 36

 35 Phenyl groups in 36 have been omitted for clarity.

  of the salt [N(PPh 3 ) 2 ] 2 [Ru 4 (µ-H) 2 (CO) 12 ] with 1 equiv. of [CuCl(PPh 3 )] and 1 equiv. of [AgI(PPh 3 )] in dichloromethane, in the presence of TIPF 6 , afforded red microcrystals of the trimetallic cluster [CuAgRu 4 (µ 3 -H) 2 (CO) 12 (PPh 3 ) 2 ] (37) in ca. 35-45% yield. 139,140 The metal skeleton of 37 consists of a tetrahedron of ruthenium atoms with the Ru(1)Ru(3)Ru(4) face capped by a Cu(PPh 3 ) moiety and the Cu(1)Ru(3)Ru(4) face of the CuRu 3 tetrahedron thus formed further capped by a Ag(PPh 3 ) unit to give a capped trigonal bipyramidal metal core geometry. A comparison of the interatomic distances between 37 and the analogous bimetallic copper and silver complexes shows that the Cu-Ag separation

[ 2 .

 2 764(1) Å] is intermediate between the Cu-Cu [2.699(2) Å] and the Ag-Ag separation [2.857(1) Å]. 37 The phenyl groups have been omitted for clarity. The structure of [CuAg 3 {MoCp(CO 3 } 4 ] (38) contains a central square metal core formed by three silver(I) and one copper(I) ions, inscribed within a molybdenum square since each edge of the coinage metal square is bridged by a MoCp(CO) 3 fragment. This molecule is

  -d 10 distances are intermediate between those in the analogous copper [Cu 3 {MoCp(CO 3 } 3 ] (mean value of Cu-Cu: 2.627(8) Å) and silver complexes [Ag 4 {MoCp(CO 3 } 4 ] (mean value of Ag-Ag: 2.8699(9) Å). The Ag-Ag distances in 38, in the range 2.9093(8)-2.9351(11) Å, are longer than the Ag-Ag distances in [Ag 4 {MoCp(CO 3 } 4 ]. The Ag1-Cu1-Ag3, Cu1-Ag3-Ag2, Ag3-Ag2-Ag1 and the Ag2-Ag1-Cu1 angles of 83.94(3)°, 97.22(2)°, 78.83(2)° and 99.68(3)°, respectively, are indicative of distorsions of the metal core away from planarity.

38

  Reaction of RNHC(S)PPh2 NPPh 2 C(S)NR (HRSNS; R = Me, Et) with Cu(I), Ag(I) or Au(I) salts followed by deprotonation afforded zwitterionic complexes of general formula [M(RSNS)] (M = Cu, Ag, Au). 142 The complexes [Cu(RSNS)] and [Ag(RSNS)] were used as building blocks for the assembly of dicationic, pentanuclear multi-zwitterionic Cu 5 , Cu 3 Ag 2 (39) and Ag 5 clusters of the general formula [M' 2 {M(RSNS)} 3 ] 2+ (M = Cu, M' = Cu, Ag; M = M'= Ag). 142 The reaction of [Ag(EtSNS)] with [Cu(NCMe) 4 ]PF 6 in a 3:2 ratio did not afford the expected complex [Cu 2 {Ag(EtSNS)} 3 ][PF 6 ] 2 but rather a mixture of species, in which [Ag 2 {Cu(EtSNS)} 3 ] 2+ was the major product. In 39, the average Ag-Cu and Ag-Ag bond lengths are 2.899(6) Å and 3.149(7) Å, and the average Ag-Cu-Ag and Cu-Ag-Cu angles are 65.8(2)° and 94.8(1)°, respectively. 39 Phenyl groups have been omitted for clarity.

  ) 12 ] [M = Cu, M' = Ag or Au; M = Ag, M' = Au] were synthesized for comparison with the analogous bimetallic clusters. 145 The molecular structure of [AuCuRu 4 (µ 3 -H) 2 {µ-Ph 2 P(CH 2 ) 2 PPh 2 }(CO) 12 ] (40) consists of a tetrahedron of ruthenium atoms of which the Ru(1)Ru(3)Ru(4) face is capped by a copper atom. One of the CuRu 3 faces of the tetrahedron thus formed is further capped by a gold atom, to give an overall capped trigonal bipyramidal metal core geometry. The Cu-Au distance in 40 of 2.614(2) Å suggests the occurrence of direct metal-metal interactions. Similarly, the mixed metal cluster [MM'Ru 4 H 2 (µdppf)(CO) 12 ] [M = Cu, M' = Au; dppf = Fe(η 5 -C 5 H 4 PPh 2 ) 2 ] (41) 146 was compared to the analogous bimetallic clusters [MM'Ru 4 H 2 (µ-dppf)(CO) 12 ] [M = M' = Cu, Ag or Au]. The metal skeleton of 41 is similar to that of 40. The Cu-Au distance in 41 [2.641(1) Å] is intermediate between the Cu-Cu and Au-Au distances in the bimetallic copper and gold analogues [2.528(2) Å and 2.901(1) Å, respectively].

40 41 99 43 44 (

 40419944 The cluster [Cu 2 Au 6 {Fe(CO) 4 } 4 (dppe) 2 ] (42) contains a mixed Cu 2 Au 2 square.123 The average distance between copper and gold is 2.558 Å, which is shorter than the Au-Au distances in the gold analogue 34 (range 2.737(2)-2.772(2) Å). 12342 In [CuAu 3 {MoCp(CO 3 } 4 ] (43), which was one of the first trimetallic cluster with 38 to have a ν 2 -square structure, the central metal square is formed by two Au(I) centres and the other two opposite positions contain a mixture of gold and copper. The best fit for the structure refinement was obtained when assuming a 80:20 Cu/Au occupation for one of these two sites, 20:80 Cu/Au for the other. Each edge of the square is by a MoCp(CO) 3 metalloligand. The molybdenum atoms form a distorted Mo 4 square and are not coplanar with the gold and copper atoms, as a result of a slight tetrahedral distortion. The distances between the group 11 metals are in the range 2.6657(15)-2.9037(9) Å and are much shorter than the sum of the van der Waals radii (3.06 Å). They are even comparable to the sum of the covalent radii proposed for Cu and Au (2.68 Å). The reaction of [(RC 2 Au)PPh 2 C 6 H 4 PPh 2 (AuC 2 R)] (R = Fc or C 6 H 4 Fc) with [Cu(NCMe) 4 ]PF 6 afforded the heterometallic complexes [{Au 3 Cu 2 (C 2 Ph) 6 }Au 3 (PR 2 C 6 H 4 PR 2 ) 3 ][PF 6 ] 2 (44). 130 The average Cu-Au distance of 2.853 Å is in the range found for the other gold-copper complexes with direct Au-Cu bonding. Reprinted with permission from ref. 130.Copyright American Chemical Society)A luminescent alkynyl Cu 2 Au 4 cluster of C 2 symmetry was obtained by reaction of an alkynyldigold(I) complex with [Cu(NCMe) 4 ] + and its crystal structure revealed that the weak Au•••Au interaction present in the precursor complex was replaced by two Cu•••Au contacts of 2.8524(16) Å.147 In the heterobimetallic Cu(I)-Au(I) alkynyl cluster(45),148 the intramolecular Cu•••Au distances are in the range 2.743(1)-2.980(1) Å, which indicates the possible occurrence of Cu•••Au interactions. The homometallic Cu••••Cu and Au••••Au separations of 4.153 Å and 3.451-3.476 Å are two long to represent significant coppercopper or gold-gold interactions. The complex was prepared by a modification of previous literature procedures. 149 45 The phenyl groups have been omitted for clarity.

99 The Ag 2 4 . Intramolecular metallophilic interactions in heterometallic clusters involving metals other than from group 11 .

 992411 A complex analogous to 44 containing Ag(I) in place of Cu(I) displays Ag-Au separations ranging from 2.9194(13) to 3.0121(13) Å.150 Trimetallic clusters containing silver, gold and another metal are rare. An example was encountered with 20 in which the Ag-Au separations range 2.767(2)-2.793(2) Å.109 Two pairs of diastereomeric tetrahedral clusters [Re 2 (MPPh 3 )(M'PPh 3 )(µ-PCy 2 )(CO) 7 ] with mixed coinage metals (M = Au, M' = Cu(46) or Ag(47)) have been synthesised.151 The Cu-Au and Ag-Au distances are 2.584(2) Å and 2.7026(18) Å, respectively. M' = Cu(46), Ag(47) The terminal phenyl and cyclohexyl groups have been omitted for clarity. silver(I) and -gold(I) phenylethynyl complexes can be made.152 The structure of the anionic cluster in [Ag(PMe 3 ) 2 ] + [Ag 2 Au 3 (C≡CPh) 6 ] -(48) has an idealized D 3h symmetry with the gold atoms of three collinear PhC≡CAuC≡CPh anions forming an equilateral triangle. The silver atoms are also in close contact with the gold atoms, with distances ranging from 2.854(2) to 3.039(1) Å, which clearly represent metallophilic interactions and are comparable with the sum of the covalent radii of 2.81 Å for Ag and Au. Au 3 trigonal-bipyramidal metal core has six axial-equatorial Ag-Au bonds, but the equatorial-equatorial Au-Au contacts (3.95 Å average) are too long to represent significant metallophilic interactions. This type of pentanuclear anion has been characterised previously by Abu-Salah who prepared several complexes of formula [M 2 M' 3 (C≡CR) 6 ] -(M, M'= Cu, Ag, Au). 149,153 Because of their relevance and structural analogy with some of the complexes discussed above, we will briefly examine the situation in heterometallic complexes involving closed shell atoms outside group 11 elements. The heterotrinuclear metal-metal bonded chain complex [Hg{Fe[Si(OMe) 3 ](CO) 3 (dppm-P)} 2 ] (dppm = Ph 2 PCH 2 PPh 2 ) 155-157 behaves like a trimetallodiphosphine and chelates a d 10 Cu(I) centre through its P donors. The resulting complex [Hg{Fe[Si(OMe) 3 ](CO) 3 (µ-dppm)} 2 Cu] + (50) has a T-shape geometry and the existence of a d 10 -d 10 interaction between Cu(I) and Hg(II) is supported by the distance of 2.689(2) Å between them and by theoretical calculations. 158 An intramolecular dynamic behaviour was evidenced by VT-NMR spectroscopy and explained by an oscillation of the P-Cu-P unit about the Cu(I)-Hg(II) axis. Although the resulting complexes were not cluster compounds, it is interesting to note that metallomacrocycles containing basic nitrogen atoms and Lewis acidic mercury centres have been used to produce complexes featuring Cu(I)-Hg(II) interactions (2.921 and 2.919 Å) 159 and Hg(II)-Pd(II) (3.1020(3) and 3.2337(3) Å) interactions. 160 Metallophilic interactions between Au(I) and Hg(II) have been shown to quench the fluorescence of Au nanoclusters and this has been applied to the highly selective and sensitive detection of mercuric ions. 161 Reaction of the Fe-Hg-Fe chain complex [Hg{Fe[Si(OMe) 3 ](CO) 3 (dppm-P)} 2 ] mentioned above with a d 10 Pd(0) precursor afforded the related complex [Hg{Fe[Si(OMe) 3 ](CO) 3 (µ-dppm)} 2 Pd] (51) in which a Pd-Hg bond has formed (2.6915(8) Å). It is no longer orthogonal to the Fe-Hg-Fe axis, as in the case of the related Cu-HgFe 2 complex, because of a favourable interaction of the Pd centre with a Fe-bound carbonyl ligand. 156 The Pd-Hg bond length is shorter than is complexes showing Pd-Hg d 8 -d 10 contacts

53 54 (

 5354 Reproduced with permission from ref. 172. Copyright Wiley-VCH Verlag GmbH & Co. KGaA) The ν 2 -triangular cluster complex [Ir 3 Pt 3 (µ-CO) 3 (CO) 3 (η-C 5 Me 5 ) 3 ] (55) was obtained in quantitative yield by reaction of [Ir(CO) 2 (η-C 5 Me 5 ) 3 ] with the Pt(0) precursor [Pt(C 2 H 4 ) 3 ] in diethylether at 0 °C. 176 It contains a nearly planar array of metal atoms, with a central triangle of platinum atoms (mean Pt-Pt distance of 2.703(3) Å), whose edges are bridged by an iridium fragment. The iridium atoms each carry a η-C 5 Me 5 ligand and are coordinated by two CO ligands. The Ir 3 Pt 3 (µ-CO) unit has approximate C 3 -symmetry. The centroids of the η-C 5 Me 5 ligands are well out of the Ir 3 Pt 3 plane, one above and two below this plane. Although the Ir-Pt interactions are not of the d 10 -d 10 type, the ν 2 -triangular structure of this cluster is strikingly similar to those of clusters involving d 10 ions of the group 11 metals discussed above.

55 Conclusions

 55 The number of metal complexes and clusters in which metallophilic interactions contribute significantly to their structure and properties is now very large and continues to grow. Even when considering only interactions involving the d 10 ions from the group 11 metals, in order to facilitate comparisons, it is clear that their occurrence is possible in a number of structural types, although some arrangements of the metal cores seem to occur more frequently. We have focused here on heterometallic clusters of the transition metals, whose diversity is of course much larger than that of the homometallic ones, which allows an even larger scope for metallophilic interactions. Whereas unsupported d 10 -d 10 interactions may only represent ca. kcal/mol, recognizing their importance allows for a better understanding of the structural and physical properties of complex molecules. An evaluation of the energetics involved is difficult in metal clusters where bridging metal atoms and ligands enforce certain structural geometries and this prevents an easy separation of the various energetic contributions. Qualitative structural elements are often suggestive of the occurrence of metallophilic interactions, such as metal-metal distance and/or bond angles. Thus for example, attractive d 10 -d 10 interactions appear responsible for the inward bending of the Mo-Ag-Mo or Mo-Au-Mo sequences in the "square-in-a-square" octanuclear clusters (ν 2 -square structures) 17, 18, 29 and 30. The values of the separations between the d 10 ions involved represents a major indicator of the occurrence of attractive metallophilic interactions, and these values can be compared to the sum of the van der Waals radii, and are sometimes even shorter than the sum of the covalent radii.

  the family of clusters [M{MoCp(CO) 3 }] n n-(n = 3 for M = Cu; n = 4 for M = Ag, Au) where the Cu 3 Mo 3 clusters 6 and 7 have a ν 2 -triangular structure, in contrast to the ν 2 -square geometry of the Ag 4 Mo 4 and Au 4 Mo 4 clusters 17, 18 and 29, 30, respectively. In related clusters, square metal cores have been encountered in this review with Cu(I) and triangular arrays with Ag(I) and Au(I)! Of course, steric factors should not be overlooked, and are often difficult to quantify. An additional challenge is to prepare and characterize polymetallic clusters containing two or more chemically different d 10 metal ions. Will their geometry and properties be directly related to that of clusters containing only one type of d 10 ion or not and if yes, which of the d 10 ion will influence most the structure and properties. In complexes containing mixed d 10 ions interacting with each other, a shortened separation compared to analogous complexes containing only one type of d 10 ion has often been noted. It has been attributed to the introduction of attractive Coulomb interactions between the dissimilar metals which increase the dispersion forces. 178,179 Such interactions have also been invoked to explain the stability and shortened intermetallic separations in the chain complexes Pd(0)-Tl(I)-Au(I) and Pt(0)-Tl(I)-Au(I) which were the first to involve three different closed-shell metals. 180 Over the last few years, it has been increasingly observed that heterometallic d 10 -d 10 interactions can bring about new photophysical properties and more examples will certainly emerge of unique behaviour for such polymetallic

  In the ν 2 -square cluster [(η 5 -CH 3 C 5 H 4 )Mn(CO)2 Hg] (53), the Hg-Hg distances of 2.888(2) Å and the non-linearity of the Mn-Hg-Mn unit (157.2°) are indicative of Hg-Hg interactions.172 For comparison, a value of 1.75(7) Å has been calculated for the van der Waals radius of Hg(II), 173 in good agreement with the value of 1.73 Å obtained experimentally for Hg(II) in [HgR 2 ] 2 .174 The structure of [CdFe(CO) 4 ] 4 •2acetone (54) is also that of a ν 2 -square cluster and consists of a nearly planar, centrosymmetric, eight-membered ring of alternating Cd and cis-Fe(CO) 4 units.175 Its core geometry is strongly distorted because of the coordination of acetone molecules to two Cd atoms but no Cd-Cd interactions were observed, the distance between these atoms being in the range 3.55-3.60 Å. These examples show that although this type of ν 2 -square geometry is favourable for metal-metal interactions within the central square, these do not always take place.

	3 (dppm-P)} 2 ] for
	another d 10 fragment, this time a Pt(0) centre, afforded with 52 yet another type of
	complex. 157 Although in 52 a triangular Fe-Hg-Pt unit is present, with a Pt-Hg distance of

2.824(1) Å, a intramolecular redox reaction has taken place and a trimethoxysilyl group originally bound to Fe has migrated to platinum, a rare event.

[167][168][169][170][171]

The possibility of reversible isomerization involving the Pt analog of 51 has been discussed.

157

Table 1 .

 1 Table 1 provides structural informations from the literature for benchmarking purposes. Covalent, atomic metal and van der Waals radii for the group 11 metals (in Å)

	M = Cu	M = Ag	M = Au

Acknowledgment. We are grateful to the referees for their comments, to Dr. Roberto Pattacini for assistance, and to the CNRS, the Ministère de l'Enseignement Supérieur et de la Recherche, the DFH/UFA, the DFG International Research Training Group GRK532 and the Agence Nationale de la Recherche (ANR-06-BLAN-410) for support of our own research in this field.

48

The terminal phenyl groups have been omitted for clarity.

The crystal structure of the complex [AgAu 4 (CH 2 SiMe 3 ) 4 (µ-dppm) 2 ]SO 3 CF 3 (49) 154 shows the presence of a silver(I) centre solely bonded to four gold atoms in a distorted tetrahedral environment. The silver-gold and the silver-silver distances are in the range 2.7179(13)-2.7822(13), 3.2170(9)-3.2773(12) Å, respectively.