Rinaldo Poli

Elsje Alessandra Quadrellib

Aboratoire De Synthe

Experimental and computational studies of the stability and reactivity of a half-sandwich 16-electron spin triplet MoII complex containing a terminal hydroxide ligand¤

Compound CpMo(OH)(PMe 3

) 2 , 1, a stable monomeric 16-electron organometallic complex with a spin triplet ground state and a terminal hydroxide ligand, is obtained from the reaction of CpMoCl(PMe 3 ) 3 with KOH. Density functional (B3LYP) geometry optimizations for a CpMo(OH)(PH ) model system a †ord a spin triplet ground state with a bent endo hydroxide ligand. An excited singlet state with 3 2 a bent exo OH ligand is found 2.27

kcal mol~1 higher in energy. Analogous calculations on other related CpMoX(PH ) systems provide a spin triplet ground state for X \ Cl and H, whereas the compound with X \ PH has a 3 2 singlet ground state. k 2 Addition of PH leads to a calculated bond energy of 20.9, 14.3, 7.2 and 6.8 cal mol~1 relative to the l 3 16-electron sing et state for H, Cl, PH and OH, respectively. These values are consistent with qualitative expectations based on the strength of 2 the MowX p interaction and steric strain in the CpMo(PH 3 ) 3 and CpMo(PH 3 ) 2 fragments. Crude estimates of the thermodynamic strength of the MowX p interactions have been derived. The calculations also show that an a-H elimination from the hydroxide ligand to a †ord CpMoH(O)(PH )plus PH should be thermodynamically favored but suggest that it is kinetically difficult. Understanding the relationship between structure (both molecular and electronic) and reactivity has always constituted a challenge for chemistry in general and organometallic chemistry in particular. The relevance of such factors as orbital overlap, p Ðlled-Ðlled repulsion,1 push-pull interactions1 and ligandÈligand repulsive (steric) e †ects,2,3 are commonly discussed. The inÑuence of the spin state on the structure, stability and reactivity is appreciated for Wernertype coordination compounds, but application to organometallic chemistry has been neglected due to the relative paucity of high-spin compounds.4 Some of our recent contributions have been directed toward Ðlling this gap. In particular, we have shown by both experimental and theoretical studies that an unsaturated intermediate may be energetically stabilized by the release of pairing energy by adopting a higher spin state,5h15 with the immediate e †ect of weakening metalÈligand interactions and accelerating ligand dissociation processes for more saturated, lower spin state compounds.

A particular system of interest is the 16-electron CpMoXL , where X is a one-electron ligand and L a twoelectron 2 ligand, according to the Green classiÐcation. 16 While most combinations of X and L give extremely reactive, short-¤ Dedicated to Roald Ho †mann in recognition of the inÑuence of his teaching on our thinking. Non-SI units employed : cal B 4.18 J ; hartree B 2.63 ] 106 J mol~1. lived intermediates, a few stable and diamagnetic examples containing strongly p-donating X groups (e.g., phosphido, arsenido) have been described.17h19 We have recently reported the Ðrst examples of this class having a spin triplet ground state, i.e. complexes Cp*MoClL (L \ PMe , PMe Ph ; L \ dppe),20,21 and studied the inÑuence 2 of the 3 spin 2 state 2 change on the kinetics of the ligand addition to a †ord 18-electron diamagnetic adducts. 12 We have proposed that the relative stability of these compounds toward ligand addition or oxidative addition processes can be attributed in part to the release of pairing energy by adopting the spin triplet ground state, alongside the steric protection o †ered by the bulky Cp* ligand. The importance of steric factors is shown by the equilibrium favoring the 16-electron species for PMe addition to ) not dissociate e 3 to any measurable extent. 21 The 3 retical MP2 calculations estimated a 10.9 kcal mol~1 stabilization of triplet CpMoCl(PH 3 ) 2 relative to the corresponding singlet.12 u An important q estion is whether steric protection is always necessary in order to stabilize this class of compounds. We thus turned our attention to less bulky Cp derivatives containing the hydroxide ligand. This ligand can be foreseen to provide additional stability to an unsaturated system by virtue of both its expected greater p-donating power and the possibility of providing greater pairing energies, i.e. greater energetic stabilization for the triplet state, because of a greater electronegativity of the oxygen atom. Our investigations, which are reported here, have allowed us not only to prepare the desired hydroxo complex, but also to gather useful experimental and theoretical information on the relationship between the molecular and electronic structures and the reactivity. A preliminary account of part of this work has appeared. 22 Experimental General All operations were carried out under an atmosphere of dinitrogen with standard Schlenk-line techniques. Solvents were puriÐed by conventional methods and distilled under argon prior to use (

Syntheses and reactions

Preparation of 1. Solid KOH (ca. 300 CpMo(OH)(PMe 3 ) 2 , mg, ca. 5.3 mmol) was added to a blue solution of (215 mg, 0.507 mmol) in 6 mL of THF. After CpMoCl(PMe 3 ) 3 stirring overnight at room temperature, complete evaporation of the solvent from the yellow-brown solution yielded an oily residue, which was extracted into heptane (

Ferrocenium oxidation of 1 in the presence of

A solu-PMe 3 . tion containing equimolar amounts of 1 and was pre-PMe 3 pared in situ as described above by reacting CpMoCl(PMe 3 ) 3 (222 mg, 0.522 mmol) and an excess of dry KOH for 8 h in THF (10 mL), followed by Ðltration through Celite. To the resulting solution was added dropwise a solution of (140 mg, 0.512 mmol) in THF (20 mL). The solu-Cp 2 FeBF 4 tion was evaporated to dryness at reduced pressure and the residue was washed with diethyl ether until the washings were colorless (3 ] 10 mL). The light-orange solid was dried under vacuum and recovered as a powder (99 mg). Crystals suitable for an X-ray analysis Myellow needles of

Theoretical calculations

All electronic structure and geometry optimization calculations were performed using GAUSSIAN 9426 on the SGI Power Challenge at the Universite de Bourgogne. The LanL2DZ set was employed to perform complete geometry optimization with a density functional theory (DFT) approach. The three-parameter form of the Becke, Lee, Yang and Parr functional (B3LYP) [START_REF] Becke | [END_REF] 1), as expected for a Curie paramagnet. The OH resonance could not be observed in the 1H NMR spectrum. Warming to a higher temperature for an extended period of time resulted in complete decomposition of 1 to give a mixture of Cp complexes that we have not further investigated. The IR spectrum of 1 shows a weak absorbance at 3092 cm~1 in assigned to the OH stretching vibration. This band is C 6 D 6 , replaced by a vibration at 2388 cm~1 for the deuterated analogue [1-OD], which was synthesized CpMo(OD)(PMe 3 ) 2 according to eq. 1 by using anhydrous NaOD.

As implied by the synthetic procedure, ) complex, however, 1 reacts with CO, yielding a mixture of diamagnetic products as shown by the complexity of the 1H NMR and IR spectra (see Experimental). We assume that this reaction is initiated by addition of CO to 1 to generate the 18-electron compound followed by CpMo(OH)(PMe 3) 2 (CO), subsequent transformations that we have not further investigated.

Compound 1 is formulated as a monomeric species with a spin triplet ground state. The alternative formulation as a bis(hydroxo)-bridged dimer is excluded because it would involve electronically saturated (18-electron) metal centers, leading to diamagnetism. Electronically saturated organometallic complexes have never been found to be paramagnetic for transition metals beyond the 3d series, and there are arguments against the occupation of metal-ligand antibonding orbitals for such complexes.4 An example of a diamagnetic, dinuclear compound with the same stoichiometry as ) 2 , data. In addition, such a formulation should lead to a diamagnetic ground state by virtue of its 18-electron conÐguration. The computational work [see section (F) below] further supports this conclusion.

1 is [CpMo(l-Br)(

(B) Oxidation of 1 in the presence of PMe 3

As compound 1 could not be crystallized and analyzed, its formulation has been further conÐrmed by a derivatization study. Treatment of an equimolar solution of 1 and PMe 3 , formed in situ from the parent chloride according to eq. and3). The presence of a terminal hydroxo ligand in compound 2 was previously conÐrmed by X-ray crystallography, including the direct location of the hydroxide H atom. 22 The stoichiometry of eq. 3 requires the formation of protons, which are probably trapped by the phosphine ligand that is concomitantly liberated by the reaction. Further studies of this oxidation process will be reported elsewhere.33 ). The nature of all the decomposition products remains unchanged, but the 4 : 5 ratio is signiÐcantly greater in the presence of these catalysts (ca. 10 : 1). A GC-MS investigation of the solution resulting from the water-catalyzed decomposition also reveals the presence of pentadeuterophenol,

CpMo(OH)(PMe 3 ) 2 ] PMe 3 ] Fc`] [CpMo(OH)(PMe 3 ) 3 ]`] Fc (2) [CpMo(OH)(PMe 3 ) 3 ]`] Fc`] [CpMo(O)(PMe 3 ) 2 ]`] PHMe 3 `] Fc (3) (
The addition of ferrocenium does not a †ect the C 6 D 5 OH. decomposition rate, ruling out the intervention of an electrontransfer-catalysis (ETC) mechanism.34 As shown in the previous section, ferrocenium is a sufficiently strong oxidizing agent to oxidize 1 in the presence of Carrying out the PMe 3 . decomposition reaction under also has no e †ect on the H 2 rate and the nature of the products.

In order to establish whether the hydrido ligand in product 4 originates from the hydroxo proton of 1, an analogous study was carried out on the deuteroxo complex 1-OD. From the integration of a 1H NMR spectrum, the majority of product 4 that is obtained during the initial stages of this experiment ( \ 10% decomposition) contains a hydrido ligand. A 31PM1HN NMR experiment shows that the main resonance for 4 is a singlet. Flanking resonances that could be assigned to a 1 : 1 : 1 triplet for 4-D are estimated as \ 30% of the total intensity. This and all the other results are identical with those of the decomposition of 1, except for a higher relative proportion of the product of H/D exchange [see Fig. The half-life of this PMe 3 -d0. process is approximately 8 h at room temperature under the conditions described in the Experimental section. This experiment also shows the small but signiÐcant growth of a 31P resonance due to free (d0 : d8 : d9 ca. 10 : 1 : 100 after PMe 3 -d8 35 min at ambient temperature). No other iso-PMe 3 -dn topomers are formed in observable amounts during this process.

CpMo(OH)[P(CH 3 ) 3 ] 2 ] 2P(CD 3 ) 3 ] CpMo(OH)[P(CD 3 ) 3 ] 2 ] 2P(CH 3 ) (4)
An exchange of also occurs for 4, as shown by the PMe 3 analogous 1H and 31P monitoring of the reac-4-d0/PMe 3 -d9 tion (eq. 5). This reaction, however, is slower with respect to the corresponding exchange on compound 1 (half-life ca. 1 day with a threefold excess of and no free PMe 3 -d9), PMe 3 isotopomers other than are observable in the 31P PMe 3 -d0 NMR spectrum. ) error, to that expected for 4, indicating that there is little or no deuterium incorporation into the hydride position. The same result was obtained by carrying out the reaction in THF-d8. Thus, the only possible source of the hydride ligand is another ligand of the precursor complex This result CpMoCl(PMe 3 ) 3 . is fully consistent with the mechanistic interpretation of the decomposition of 1 (see Discussion).

CpMoH[P(CH 3 ) 3 ] 3 ] 3P(CD 3 ) 3 ] CpMoH[P(CD 3 ) 3 ] 3 ] 3P(CH 3 ) 3 (5)

(F ) Computational studies

The gradient-corrected density functional (B3LYP) computational approach has been shown to be quite reliable for both geometries and energies. 35 Previous studies have shown that nonlocal DFT methods yield results within 5 kcal mol~1 of the experimental values. 36 Comparative studies of B3LYP and other computational approaches (DFT-BLYP and MP2 with various di †erent basis sets) have been recently carried out on systems analogous to those reported here, where there is also a variability of spin state. 37,38 We are therefore conÐdent that the computational approach chosen here provides us with sig-niÐcant results, especially in terms of trends.

Geometry optimizations have been carried out on both spin triplet and spin singlet and on spin singlet

CpMoX(PH 3 ) 2 (X \ H, Cl, OH, Optimized geometrical CpMoX(PH 3 ) 3 PH 2
). parameters and total energies are collected in Table 1, while views of the optimized geometries and relative energy diagrams are shown in Fig. 3. All systems could in principle be calculated in two symmetric limiting forms, where the X ligand is either eclipsed with a Cp carbon atom (e.g., I) or projecting onto the middle of a Cp CwC bond (staggered, II). Previous detailed studies on systems (M \ Cr, CpMCl 2 (PH 3 ) Mo)37 have shown that the two structures di †er in energy by All calculations on the H and Cl systems were carried out by imposing symmetry. The 16-electron chloro system, C s was previously examined at the MP2 level.12 CpMoCl(PH 3 ) 2 , The B3LYP optimized parameters reported here are quite close to the corresponding MP2 parameters and to the experimental values. Since no structural model is available for the hydroxide system in either the 16-electron bis(phosphine) or the 18-electron tris(phosphine) conÐguration, a linear MowOwH starting conÐguration was arbitrarily chosen as the initial geometry for the hydroxide ligand in symmetry. C s For the 16-electron systems, this evolved to a bent endo geometry (MowOwH angle \ 124.05¡) for the spin triplet structure and to a bent exo geometry (MowOwH angle \ 124.62¡) for the spin singlet structure. Both systems have also been calculated without any symmetry constraint and with a starting geometry having a Cp(center)È MowOwH dihedral angle of 90¡, leading to a singlet local minimum [dihedral Cp(center)wMowOwH \ 87.3¡] 9.8 kcal mol~1 higher than the absolute minimum, or 12.1 kcal mol~1 higher than the triplet minimum [see Fig. 4(a)]. The metalÈ ligand distances are essentially identical with those of the absolute minimum. The major di †erences are observed for selected angle : the CNTwMowO angle has decreased substantially to 108.16¡, and the MowOwH angle has decreased to 118.16¡. This local minimum, therefore, is probably related to the existence of a barrier against rotation of the OH ligand around the MowO bond (see Discussion). The asymmetric spin triplet calculation, on the other hand, converged to the same geometry and energy as for the system. The Optimized metalÈligand distances are typically longer in the higher spin state for each 16-electron system, as has been found for other calculations4 and attributed to di †erences in electron density distribution.38 Addition of causes the PH 3 MowX distance to lengthen relative to the singlet state (by 0.021, 0.182, 0.126 and 0.381 for X \ H, Cl, OH and Ó PH 2 , respectively). While the small lengthening of the MowH bond may be attributed to small r e †ects caused by the introduction of the ligand on the MowH bond, the much PH 3 greater lengthening in the other cases is probably due to a combination of a r e †ect and the rupture of the MowX p interaction (see Discussion). As for the distances in MowPH 3 the 18-electron systems, these are on the average greater in the order probably as a result of steric

(X \ H, Cl, OH, n \ 2, 3) Ó CpMoX(PH 3 ) n PH 2 ; X \ H X \ Cl X \ OH X \ PH 2 n \ 2 n \ 2 n \ 2 n \ 2 S \ 1 S \ 0 n \ 3 S \ 1 S \ 0 n \ 3 S \ 1 S \ 0 n \ 3 S \ 1 S \ 0 n \ 3 MowCNTa 2.
H \ PH 2 \ OH B Cl, interactions (see Discussion).
The overall geometry of the 16-electron systems, with the notable exception of the hydride structures, is based on the "" three-legged piano stool ÏÏ in both spin states, i.e. on a pseudo-octahedral geometry with the Cp ring replacing three mutually facial ligands. In this geometry, the angles between bonds to two monodentate "" leg ÏÏ ligands should ideally be 90¡, and the angles between any of these bonds and the CNTwMo bond should be 125.26¡. These angles are in the 83È97¡ and 122È131¡ ranges, respectively, for all systems (see Table 1) except for the systems, which can be CpMoH(PH 3 ) 2 better described as "" four-legged piano stools ÏÏ where the leg trans to the hydrido ligand is missing (see Fig. 3). The distortion is more pronounced for the singlet system. This dramatic di †erence is attributable to the small size of the hydrido ligand and to the lack of p interactions for the hydrido system (see Discussion). ) 2 which factors determine the relative stability of these unsaturated complexes with respect to ligand adducts. In principle, steric e †ects, the participation of ligand lone pairs and a spin state change could contribute to the relative energetic stabilization of the 16-electron species relative to a ligand adduct. A separation of these factors is impossible from the experimental point of view. In addition, assessment of a metalÈligand p bond strength (even for systems where there are no spin state complications) is subject to experimental difficulties.1 A kinetic approach was examined by Bryndza et 2 ) perfect accord with the experiment. The choice between two spin states always depends, in monoelectronic terms, on two factors : a pairing energy and an orbital gap. 4 The nature of X inÑuences the pairing energy by virtue of changes in MowX bond polarity, with the more electronegative ligands resulting in an increase of pairing energy via contraction of the metal orbitals. This e †ect was shown to determine the choice of spin state in the (X \ Cl, systems41 and trans-TiX

2 (dmpe) 2 CH 3 ) (M \ Cr, Mo ; X \ Cl,
On the other CpM(NO)X 2 CH 3 ).38c hand, X can also have a profound e †ect on the orbital gap. As qualitatively discussed previously,20 the two metal-based orbitals of interest are nonbonding relative to the MowX r interaction, but they can both establish MowX p interactions if X has lone pairs with a suitable orientation.

The H ligand cannot establish any p interaction and the DFT calculations for this hydrido system show that the orbital gap (0.30 eV between the two semioccupied orbitals in the triplet state) is smaller than the pairing energy, giving a marginal preference to the triplet state. The major contribution to the lower-energy orbital is from the Mo orbital, d xy while the higher-energy orbital is mostly Mo the z axis d z 2 , coinciding with the MowCp(center) vector. This picture corresponds to the MO analysis of complexes, Ðrst CpML 4 -type reported by Ho †mann and co-workers. 42 The Cl ligand is a double-faced p donor. Its lone pairs can interact with both HOMO and LUMO. The metal electrons will occupy the dpÈpp out-of-phase (antibonding) combinations, qualitatively shown in III and IV. To a Ðrst approximation, the chlorine lone pairs are equally good at transferring p-electron density to the metal in both spin states. In the singlet state, there is a positive two-electron donation to one metal orbital and a Ðlled-Ðlled repulsion with the other, while in the triplet state there are two individual two-center-three-electron interactions, totaling six electrons in two p-orbital interactions for both spin states. While the orbital gap is raised in the singlet state (2.01 eV, relative to 0.27 eV for the triplet), the lack of a large di †erence in p stabilization plays in favor of the triplet by virtue of a reduced electron pairing. The larger tripletÈ singlet gap for the Cl relative to the H system, therefore, should mostly be attributed to a pairing energy di †erence, given the expected greater e †ective positive charge on the metal for the chloride complex.

The variations of angular preference for the OH and PH 2 ligands in the spin triplet and singlet conÐgurations (see Fig. 3) hint at the importance of MowX p e †ects for these two systems. For the hydroxo derivative, an important question is how many p interactions the OH lone pairs can establish with the Mo orbitals. The conventional view is that a linear (sphybridized) MowOH conÐguration allows the p interaction of both oxygen lone pairs, whereas for a bent geometry (approaching the sp2 hybridization) the hybrid lone pair is rendered less available. However, the B3LYP optimization for the spin triplet shows that the OH ligand CpMo(OH)(PH 3 ) 2 establishes both p interactions in spite of a preferred bent geometry (endo conformation relative to the Cp ring), as shown in the qualitative interaction diagram of Fig. 5. This is rendered possible by the angular relationship between the on the other hand, the single two-electron p interaction occurs, as predictable, between the highest energy O lone pair (an essentially pure p orbital) and the lowest energy metal orbital to result in the observed exo conÐguration (see (d xy ) Fig. 5). The B3LYP calculation gives a 0.57 eV gap between the two singly occupied orbitals of the triplet and a 2.39 eV HOMOÈLUMO gap for the singlet.

As attested by the similar optimized MowO distances in both spin states (see Table 1), the hydroxide ligand has a similar p donor strength in both spin states, formally providing two p electrons from one lone pair in the singlet and one p electron from each lone pair (in two distinct 2cÈ3e interactions) in the triplet. Thus, the OH system qualitatively behaves like the chloro system discussed above. However, the presence of the OwH bond in place of a Cl lone pair reduces the unfavorable Ðlled-Ðlled repulsion in the singlet state. Sig-niÐcantly, the bent exo OH ligand in the optimized singlet structure maximizes the distance between the O lone pair and the Mo orbital. This rationalizes why the calculated d z 2 tripletÈsinglet gap for the hydroxo system is smaller than for the Cl system, in spite of an expected greater pairing energy for the system with the more electronegative OH group. It is interesting to note that the singlet local CpMo(OH)(PH 3 ) 2 minimum for a parallel (relative to the Cp ring) MowOwH plane [Fig. 4(a)] is destabilized by 9.8 kcal mol~1 relative to the absolute singlet minimum. This destabilization can be attributed to a less favorable p donation from the higher energy oxygen p lone pair to the metal orbital (i.e., the d z 2 in-phase combination corresponding to IV) and to a stronger OH(sp2 ÐlledÈÐlled repulsion. hybrid)wMo(d xy ) The electronic situation drastically changes in the phosphido system, because the ligand is a single-faced p PH 2 donor. The singlet state involves only one proÐtable p interaction between the p lone pair of the perpendicular (relative to the Cp plane) ligand and the metal orbital (see III) PH 2 d xy and no Ðlled-Ðlled repulsion, a situation analogous to that of the exo hydroxo complex. On the other hand, the triplet state can only beneÐt from a single 2cÈ3e interaction and, consequently, loses considerable p stabilization. Somewhat unexpectedly, the lowest energy optimized structure does not involve a p interaction with the energetically more favorable metal orbital, but rather with the orbital, providing a d xy d z 2 pyramidalized group with endo H atoms, similar to MoPH 2 the endo triplet OH structure. It is likely that the choice of the metal orbital for the p interaction is not dictated by the d z 2 strength of the three-electron p interaction, but MowPH 2 rather by the greater stability of the pyramidalized P atom. Thus, when can only donate one p electron (triplet state), PH 2 the pyramidalization of P is the energetically dominating factor and the P lone pair (close to a sp3 hybrid according to the bond angles) overlaps best with the energetically less favorable metal orbital. ]p roves that there is sufficient room for coordination of a third ligand to 1. PMe 3 We have sought to understand to what extent various factors (steric and electronic in nature) regulate this di †erent binding ability. For this purpose, we have chosen to compare all the di †erent systems with respect to the addition PH 3 process [eq. ( 6)] in our computational studies. The results shown in Fig. 3 ). be considered that real systems may exhibit a slightly di †erent ranking of strengths for steric reasons. Furthermore, the PH 3 process is necessarily accompanied by a decrease in binding entropy. However, the consideration of steric and entropic factors qualitatively reconciles the result of the calculations with the experimental results.

CpMoX(PH 3 ) 2 ] PH 3 EF CpMoX(PH 3 ) 3 (6)
We will now attempt to provide a more quantitative assessment of the factors regulating the strength of the MowPH 3 interaction in the di †erent systems. The di †erent bond dissociation energies (relative to the spin singlet 16-electron species) in Fig. 3 ) 2 (vide supra), except for X \ H. Part of the energy gained in the formation of the bond is therefore spent in break-MowPH 3 ing the MowX p interaction. If this were the only factor at the source of the di †erences in bond strengths, we MowPH 3 could easily calculate the strength of the MowX bond in the singlet 16-electron compounds by normalization relative to the p-neutral hydride complex : X \ Cl, 1.1 ; OH, 12.3 ; PH 2 , 14.6 kcal mol~1. These values are in agreement with the conventional view that hydroxide and phosphido ligands are better p donors than chloride. It has to be borne in mind, however, that there are also changes in the r-bonding network beyond those taken into account within the "" relaxation energy ÏÏ correction factor, and perhaps also in the p and MowCp d bonds, accompanying the coordi-MowPH 3 nation of the additional ligand. Therefore, the above PH 3 numbers can only be taken as indicative values. The most important r e †ect is perhaps the trans inÑuence of X on the newly formed bond. We note, however, that the MowPH 3 hydride is the strongest trans-labilizing ligand in the series examined here (cf.

relative to bond MowP trans MowP cis lengths for each 18-electron compound in Table 1). Therefore, the e †ective p strength for the various MowX bonds may be greater than the numbers derived above. The order found for this system may not be valid in general, because a di †erent trend was found for the bond dissociation energy in RuwNH 3 as a function of X.45 RuHX(CO)(PH 3 )(NH 3 ) On the basis of all the computational results, it seems that the p stabilization is of paramount importance in providing a stable (i.e. isolable) 16-electron system with the CpMoXL 2 stoichiometry. The release of pairing energy on going from singlet to triplet state apparently provides only a secondary stabilizing factor : 16-electron systems can be stabilized and isolated with strong p donors in a singlet state [e.g. but there are as yet no known Cp*Mo(PPh 2 )(PMe 3

) 2 ],19 stable systems with a spin triplet conÐguration and p-neutral X ligands. However, the concept that a spin state change may help thermodynamically stabilize an unsaturated reaction intermediate,4 thereby providing kinetic help to a reaction, appears justiÐed.

In summary, the computational studies on the addi-PH 3 tion process to suggest that coordination CpMo(OH)(PH 3 ) 2 of a third ligand to 1 is energetically discouraged by PMe 3

two factors : the promotion to the singlet state and the disruption of the MowO p bonding, the latter being the most important factor. Analogous considerations can be applied to the hypothetical dimerization process to a †ord a di-l-hydroxo structure, [CpMo(l-OH) where each metal has (PMe

3 ) 2 ]
2 , achieved a saturated electronic conÐguration. An additional factor against dimerization is the Ðlled-Ðlled repulsion between the metal lone pairs. It is to be remarked that it is unusual to Ðnd hydroxide ligands in a terminal binding mode for organometallic compounds. The availability of lone pairs on the oxygen atom frequently leads to ligand loss and formation of doubly-or triply-bridged oligomers, such as for instance ( (C ) Can a-H elimination from the hydroxo ligand occur ?

The theoretical calculations on the model system suggest PH 3 that complex 1 is stabilized by MowO p bonding and by adopting a spin triplet ground state, but they also indicate that it could gain even greater stability by dissociating a phosphine ligand and undergoing an a-H elimination process from the OH ligand to a †ord a diamagnetic oxo-hydrido derivative of MoIV (see Fig. 4). There are precedents for the formation of stable oxo-hydrido compounds by a-H elimination from hydroxide ligands, e.g. This computational Cp* 2 Ta(O)H.58 result has prompted us to investigate the thermal stability of 1. As detailed in the Results section, no a-H elimination process occurs up to 70 ¡C, while decomposition takes place under more forcing conditions, but no diamagnetic hydridecontaining products are observed. While it cannot be excluded that the decomposition occurs via initial a-H elimination from the OH ligand, the experimental observations clearly establish that the activation barrier for this process is high.

We observe that the process of a-H elimination from the OH ligand to a †ord the more stable (according to the calculation) oxo-hydrido product must involve, in whichever order, a 90¡ rotation of the OH ligand around the MowO bond, a reduction of the MowOwH angle leading to a MoÉÉÉH interaction and the a-elimination process, the dissociation of a ligand and a spin state change. The calcu-PMe 3 lations show that rotation of the OH ligand involves loss of considerable stabilization (Fig. 4). In addition, it can be easily imagined that the transition state leading to the a-H elimination process and to the dissociation requires addi-PH 3 tional energy. Even though a detailed analysis of hypothetical reaction coordinates was not carried out, the calculations on the model systems suggest that a smooth, low-activation PH 3 pathway for the a-H elimination process is not available. ) raise a number of questions, all of which are satisfactorily answered, we believe, by the mechanism illustrated in Scheme 1. The decomposition is initiated by the oxidative addition of a solvent CwD bond. Deuterated benzene adds by far the fastest amongst all solvents tried, in agreement with trends established for the oxidative addition to other 16electron intermediates. 59 While process (i) is irreversible (vide supra), processes (ii) and (iii) must necessarily be followed by the corresponding reverse process (either to return to the starting complex or to go forward to the H/D-exchanged complex). The greater amount of deuterium in the NMR solvent relative to hydrogen on the ligands statistically sets the thermodynamics PMe x \ k t /(k t ] k c ) molecules II-dn,m exits the cycle, while the remaining fraction (1 [ x) will undertake an additional cycle. It is easy to predict that if an extremely small fraction of II exited the cycle after each exchange event (very small x), the isotopic distribution would tend to favor the fully deuterated phosphine, PMe 3 -d9, as the sole product. On the other hand, if a large fraction of II exited the cycle, the distribution should be in favor of PMe 3 -A qualitative examination of Fig. 2(a) indicates that the d1. value of x must be small. The irreversibility of process (i), together with the observed prevalent formation of 4-dn,m over 4-D-dn,m, is an indication that the steady-state concentration of II-dn,m is greater than that of II-D-dn,m for each set of n and m values. Reductive elimination processes involving CwH(D) bonds are predicted to show a signiÐcant inverse isotope e †ect (that is, k ~4H/k ~4D and should be signiÐcantly smaller than 1). Prek ~5H/k ~5D viously reported values of kH/kD, for instance, are 0.51(1) and 0.5 (1) (D.3) Simulation of the deuterated phosphine distribution. The program implementation for the simulation, which aims to reproduce the distribution in the early stages of PMe 3 -dn the reaction, is based on the mechanism outlined in the previous section. The details of the program, along with the assumptions and approximations it relies on, are available from the authors. The optimized parameters are x, the frac-tion of molecules trapped after each H(D)/D exchange cycle, and the isotopic factor y, deÐned as

The output of k 4 H/k 4 D. the simulation program for given values of the parameters x and y is a set of relative populations of the free mol-PMe 3 -dn ecules, which is then plotted against the 31P chemical shift and compared with the experimental spectrum. The process was repeated with di †erent values for the parameters x and y until the best agreement between the experimental and the simulated spectra was obtained. The population distribution proved to be weakly sensitive to changes in the isotopic factor y ; outputs obtained with y in the reasonable 1.1È1.5 range for the same value of x were very similar. E †ects from changes of y, observable only when outrageous values for y were chosen (e.g. 4), showed that they were tantamount to changes of x at constant y. In light of these results, a value of y \ 1.3 was arbitrarily imposed and only x was further optimized. The best simulated spectrum [Fig. 2(c Now, a mechanism must be found for interchanging the deuteroxide D atom and the hydride ligand in VIII. In principle, two pathways could render these two atoms equivalent : the Ðrst one is oxidative addition of the OwD bond to yield a Mo(H)(D)(O) moiety for an extremely unlikely 20-electron MoVI intermediate. The second, more likely, mechanism is reductive elimination of the hydrido and deuteroxide ligands to a †ord a Mo(OHD) moiety in the MoII intermediate IX.

Reversal of this process by oxidatively adding the OwD bond, and subsequent reverse ortho-metallation, generates compound 1-d1, which can then reenter the H/D exchange cycle as discussed in the previous section. However, the PMe 3 isotopic distribution would not be changed if all of the 1-d1 dn immediately entered the H/D exchange cycle at this point. A fraction of the ligand must be exchanged with the PMe 3 -d1 available free before complex 1 enters the cycle. A PMe 3 -d0 control experiment (eq. 5) conÐrms that the exchange PMe 3 process on complex 1 takes place relatively rapidly and it is logical to suppose that this process takes place via an associative mechanism in view of the computational results (vide supra). In excellent agreement with this proposed mechanism, treatment of needs to be rationalized is the catalytic e †ect of protic substances such as water and methanol. That this is a real catalysis is shown by the formation of identical decomposition products. The ratio between products 4 and 5 changes from 4 : 1 in the absence of catalyst to 10 : 1 in the presence of either water or methanol. This phenomenon could be rationalized by a reversibility of the HDO elimination step but more (k 2

), detailed studies need to be carried out to further clarify this point. It is also important to notice that this catalytic e †ect is not related to the coordinating ability of water or methanol : no acceleration is observed in THF (in fact, the decomposition in neat THF is slower than in neat As a corollary of C 6 D 6

). the established catalytic e †ect, the formation of water during the decomposition of 1 (see Schemes 1 and 2) should make this reaction autocatalytic. The gathered qualitative experimental evidence (see Results) is consistent with this conclusion.

A relatively simple rationalization of this catalytic e †ect comes from an understanding of the electronic and molecular structure of 1. We have learned that the 16-electron hydride intermediate II, as well as the deuteride analogue II-D, is extremely reactive and activates the CwH(D) bonds of the solvent or those of its own ligands at a much faster rate PMe 3 than the already favorable coordination. An oxidative PMe 3 addition process relies on the availability, at appropriate energies, of a metal-based empty orbital to accept the electrons of the bond being activated, plus a metal-based Ðlled orbital to interact with the CwH(D) r* orbital. This situation is realized in a spin singlet 16-electron fragment. Although the model is calculated to have a triplet ground CpMoH(PH 3 ) 2 state at the B3LYP level (vide supra), the tripletÈsinglet gap is within thermal energy and a spin-forbidden barrier is probably nonexistent.4,15,70 A geometry reorganization-related barrier is also likely to be rather small, because the optimized singlet and triplet structures have very similar geometries (see Results). The same situation, on the other hand, is not realized for derivatives (of any electron count) where all the available metal orbitals heavily engage in p interactions with Ðlled ligand orbitals. Such is the case for the spin singlet where the single phosphido lone pair Cp*Mo(PPh 2 )(PMe 3 ) 2 , locks the only available metal orbital, or for the spin triplet or for compound 1. The X ligand lone Cp*MoCl(PMe 3 ) 2 , pair(s) would have to be "" pushed back ÏÏ before the metal orbital could be used for any ligand addition or oxidative addition process.1

Since water and methanol share the property of having ionizable protons, and since the hydroxide lone pairs can easily engage in hydrogen bonding with sources of ionizable protons, it is relatively straightforward to imagine and propose that the action of water or methanol is simply to remove the OH lone pairs from the active p interaction with the metal orbitals and weaken or remove the HOwMo p donation. Thus, the electronic structure of the complex, and consequently also its reactivity, would become similar to those of the hydride system and a CwH oxidative addition process may more easily take place.

In summary, the catalytic e †ect of water and methanol on the decomposition of 1 establishes an interesting new relationship between structure (molecular and electronic) and reactivity. To the best of our knowledge, the BrÔnsted acid-catalyzed activation of an unsaturated complex toward the oxidative addition of a CwH (or any other) bond had not been previously established.

Conclusion

We have examined in some detail the structure (both molecular and electronic)Èreactivity relationship for the 16-electron system and arrived at the following speciÐc CpMoX(PMe 3 ) 2 conclusions :

(i) When X is the electronegative and p donor hydroxide system, the compound is stabilized relative to products of dimerization, ligand additions or oxidative addition reactions by adoption of a spin triplet conÐguration and by the MowO p interactions, the latter being the most important factor.

(ii) Theoretical calculations on a model system with PH 3 ligands suggest that both oxygen lone pairs engage in p interactions, even for a bent MowOwH moiety. A reduced Ðlled-Ðlled repulsion in the exo singlet state determines the lower tripletÈsinglet gap relative to the Cl system.

(iii) An energetically favored (according to the calculations) a-H elimination with phosphine ligand dissociation to a †ord the MoIV system is kinetically inaccessible CpMoH(O)(PMe 3 ) and is not experimentally observed.

(iv) When X, on the other hand, is the less electronegative and p-neutral hydride, the calculations on the 16-electron system yield a geometry that can promote facile ligand addition or oxidative addition processes. The induction of multiple H/D exchanges between the coordinated ligands and PMe 3 during the decomposition of 1 is proposed to take place C 6 D 6 via the involvement of this very intermediate.

(v) The reactivity of the hydroxide complex is enhanced by proton sources that can establish hydrogen-bonding interactions with the OH lone pairs and thereby disrupt the MowO p interactions. The metal orbital that is freed up in this manner becomes available for CwH (or D) oxidative addition chemistry. There is, we believe, no previous example of the promotion of CwH activation by acids via BrÔnsted the diversion of stabilizing MwX p interactions into hydrogen bonding interactions.

In more general terms, we have examined in some detail the relationship between molecular structure, electronic structures and reactivity for a class of open-shell compounds of formula (X \ H, Cl, OH, phosphido), including an CpMoX(PMe 3 ) 2

estimate of the strength of the MOwX p interaction. The results obtained and the considerations made should be relevant to a greater number of open-shell systems that constitute intermediates of stoichiometric or catalytic reactions.

3 3 3 , 4 ,C 6 D 6

 33466 by a process that is initiated by the Compound 1 slowly decomposes in C D in the presence of PMe u 3 activation of a solvent CwD bond. This 6 6 process produces a mixt re of CpMoH(PMe 3 ) CpMo(g2-5, and (n \ 1È9) molecules. The relative isotopic distribution of the free CH 2 revealed a rapid exchange of the deuteroxide D atom and a PMe ligand H atom. The decomposition of 1 in the presence of PMe in 3 e is catalyzed by water or methanol. Th molecules are believed to engage in hydrogen i 3 bonding interact ons with the OH lone pairs, thereby modifying the electronic structure of compound 1 and enhancing the reactivity toward the oxidative addition of the solvent CwD bonds.

( 3 does 3 PM 2

 2 Cp*MoCl(PMe ) , whereas compound CpMoCl PMe 3 o

3 ) 3 D 6 .

 336 THF (2 mL) and the suspension was left to stir overnight. The yellow-brown mother liquor was Ðltered into a new Schlenk tube and evaporated to dryness ; the residue was dissolved in An aliquot of the resulting solution was investigated by C 6 1H NMR, conÐrming the presence of 1. The atmosphere in the reaction vessel was changed to CO ; an immediate color change from brown to red ensued. An aliquot of the solution was investigated by 1H NMR spectroscopy, showing the complete disappearance of the resonances of 1. A complex mixture of diamagnetic Cp-containing non-hydridic complexes had formed. A major product, whose intensity accounted for more than 50% of the peaks, was identiÐable. 1H NMR : (s, Cp), 1.28 (m, Me). A second aliquot was investigated by IR spectroscopy in the CO-stretching vibration region. IR (KBr, 2000È1700 cm~1) : 1933(s), 1891(w), 1849(vs), 1780(s).

Fig. 1

 1 Fig. 1 Temperature dependence of the contact-shifted resonances for compound 1. Solvent \ C 6 D 6

D 6 [

 6 C ) Decomposition of 1 in C 6 D 6 solutions of slowly decompose (75% in 30 C 6 D 6 1ÈPMe 3 days) upon standing at room temperature in a sealed NMR tube, to a †ord the previously reported23 complexes 4, and CpMo(g2-a ca. 4 : 1 ratio. The rate of this decomposition is highly dependent on the solvent, the relative rate of transformation being The stability in C 6 THF [ C 6 D 12 . THF was checked by periodically withdrawing aliquots of the solution and recording the NMR spectra in acetone-d6. After 5 days, 30% decomposition is seen in while only traces C 6 D 6 , of 4 are seen in THF and none in While the com-C 6 D 12 . pound leads to observable amounts of 5 (but no 4 ! ! !) in C 6 D 12 after 10 days, its THF solutions are unstable over long periods of time and lead to insoluble powders, which were not further investigated.A more detailed study in with 1H and 31P NMR C the isotopic distribution favoring the fully deuterated product since the initial stages [see Fig.2(a)]. The isotopic distribution changes over time, with the heaviest phosphines becoming even more prevalent. The appearance of free occurs on a longer time scale relative to the dis-PMe 3 -dn appearance of 1 and the appearance of This is due to C 6 D 5 H. the slow release of the deuterated ligand from the coordination sphere in complex 4 (see Discussion). On the other hand, no D incorporation into free is observed when PMe 3 the experiment is carried out in In addition, the NMR C 6 D 12 . studies show that the presence of water or methanol greatly increases the decomposition rate (only a few hours at room temperature for the complete disappearance of 1 and PMe 3

4 .

 4 Fig. 2 31P NMR spectra in the resonance region of free for PMe 3 the reaction at room temperature. (a) Reaction of 1 ] PMe 3 ] C 6 D 6 compound 1 ; t \ 120 days. (b) Reaction of 1-OD ; t \ 90 days. (c) Simulation of the experimental isotopic distribution shown in (a)

  (E) Attempted synthesis of 4-D Since (4-D) is a possible product of the CpMoD(PMe 3 ) 3 decomposition of 1 in (see Discussion), we have attempt-C 6 D 6 ed to synthesize it by adapting the synthetic strategy previously employed for the synthesis of 4, namely the metathesis reaction of the chloride precursor with CpMoCl(PMe 3 ) 3 in THF. Spectroscopic inspection of the product by LiEt 3 BD 1H NMR, however, shows the typical quartet resonance of the hydride ligand of 4. The intensity of this resonance (relative to those of Cp and corresponds, within experimental PMe 3

Fig. 3

 3 Fig. 3 B3LYP-optimized geometries and relative energies for the (left : singlet ; center ; triplet) and CpMoX(PH 3 ) 2 ] PH 3 (right) systems. X \ (a) H, (b) Cl, (c) OH, (d) The CpMoX(PH 3 ) 3 PH 2 . B3LYP total energy of is [8.269 867 hartrees PH 3

Fig. 4 PH 3 Fig. 4 (

 434 Fig. 4 B3LYP energies for singlet and triplet (left), trans-CpMoH(O) (center) and cis-CpMoH(O) (right). A CpMo(OH)(PH 3 ) 2 (PH 3 ) 2 (PH 3 ) 2 qualitative illustration of the optimized geometry is also given for each calculation. The dashed lines show correspondence between systems with the same spin state The spin triplet geometry is calculated as more stable in all cases except for (see Fig. 3), in full agreement with X \ PH 2 the available experimental Ðndings. While compounds 1 and or have spin triplet Cp*MoClL 2 (L \ PMe 3 L 2 \ dppe)20,21 ground states, is a stable diamagnetic Cp*Mo(PPh 2 )(PMe 3 ) 2 compound.19 Binding the ligand to the spin singlet PH 3 as is shown in Fig. 3, results in a stabilizing CpMoX(PH 3 ) 2 , e †ect for all systems, the binding energy relative to MowPH 3 the singlet being in the order H [ Cl [ PH 2 [ OH. To further discount the possible alternative formulation of 1 as an oxo-hydrido derivative of MoIV rather than as a hydroxo derivative of MoII (vide supra), calculations were also carried out on singlet and triplet conÐgurations for fourlegged piano stool geometries, with the CpMoH(O)(PH 3 ) 2 ligands both in cis and trans relative conÐgurations. The PH 3 results of these calculations are shown in Fig. 4. The calculations for the cis-CpMoH(O) (no imposed symmetry) (PH 3 ) 2 lead to the expulsion of one ligand from the coordination PH 3 sphere and the generation of a three-legged piano stool in both the singlet and the triplet spin state CpMoH(O)(PH 3 ), [Fig. 4(b)]. The triplet system has a high energy, ] 10.0 kcal mol~1 relative to the triplet while the CpMo(OH)(PH 3 ) 2 , singlet is stabilized by 13.8 kcal mol~1. The Cp ring is distorted in both spin states with the carbon atom(s) opposite to the oxo ligand being farthest from the metal center. This feature can be attributed to the trans labilizing e †ect of the oxo ligand and is also experimentally observed by X-ray crystallography for the isoelectronic complexes and To the best of [CpMo(O)(PMe 3 ) 2 ]`Cp*ReOCl 2 .33,39 our knowledge, no stable complex having the stoichiometry has been reported to date. CpMoH(O)(PR 3 ) The isoelectronic complex22,24 has been [CpMo(O)(PMe 3 ) 2 ]s hown to resist coordination of a third ligand.33 PMe 3 The trans geometry [Fig. 4(c)] was calculated with imposed symmetry with the unique Cp carbon eclipsed with either C s the oxo or the hydrido ligand. Only the lowest energy structure is shown for each spin system. The triplet converged to a regular four-legged piano stool structure with a high energy value, 12.4 kcal mol~1 higher than spin triplet The singlet conÐguration a †orded a CpMo(OH)(PH 3 ) 2 . system with a severely distorted Cp ring at a higher energy relative to the singlet plus system [see CpMoH(O)(PH 3 ) PH 3 Fig. 4(c)].Given that the oxo-hydrido MoIV formulations lead (as expected from the ability of the oxo ligand as a r ] 2p donor) to strong preferences for singlet structures, the calculations provide further support to the assignment of the hydroxo MoII formulation to the paramagnetic compound 1. The lack of a spontaneous transformation of 1 into a diamagnetic oxohydrido MoIV derivative with loss of will be addressed PMe 3 in the Discussion section.

Fig. 5

 5 Fig. 5 Qualitative correlation diagram for the interaction between the Mo and orbitals and the oxygen lone pairs in d z 2 d xy CpMo(OH)(PH 3 ) 2

/C 6 D 6 D 6

 66 (D) Mechanism of the decomposition of 1 in and C 6 D 6 promotion of the H/D exchange PMe 3 The multitude of phenomena that are observed during the slow room temperature decomposition of 1 in (see C 6 Results

6 D 5 H

 65 the solvent and coordinated This is shown by the com-PMe 3 . bination of two facts : (i) no is formed from 1 in the PMe 3 -dn presence of free before the formation of 4 (in fact, the PMe 3 release of free is much slower than the conversion of PMe 3 -dn 1 to 4) and (ii) 4 is not a catalyst for the H/D exchange process : a genuine sample of 4 does not lead to the formation of any observable (n [ 0) over a period of several PMe 3 -dn months upon standing in in the presence of a small C 6 D 6 amount of free This observation also demonstrates PMe 3 . that the trapping reaction of intermediate II in Scheme 1 to generate 4 must be irreversible. The irreversibility of this step is also indirectly suggested by another experiment : 4 does not react at room temperature with CO (P \ 1 atm) over several days, whereas treatment of with CO results CpMoCl(PMe 3 ) 3 in the immediate replacement of one ligand to a †ord PMe 3 that the pathways of H/D exchange between a coordinated phosphine ligand and the solvent must have the C 6 D 6 possibility of occurring repeatedly before the active species is trapped to a †ord 4. Provision for this result is easily made by invoking an oxidative addition process of a molecule at C 6 D 6 the level of the 16-electron intermediate II-d0,1 to gener-(k 5 D) ate a MoIV hydrido-deuterido species VI-d0,1, followed by reductive elimination of the observed product C ( k ~5H) to generate a deuteride intermediate II-D-d0,1. By repeating the oxidative addition/reductive elimination process, the latter complex leads to either II-d0,2, II-d1,1 or again II-d0,1, depending on whether oxidative addition occurs at a CwH bond of either or or at the CwD bond of PMe 3 PMe 3 -d1, This cycle may be repeated a number of times with PMe 3 -d1. multiple incorporation of D atoms into the coordinated phos-phine ligands. At each stage of the multiple H/D exchange cycle, competitive trapping of the 16-electron intermediates II-dn,m and II-D-dn,m by coordination is important to consider that both intermediates II and II-D can equally undertake three di †erent processes : (i) addition of free (ii) ortho-metallation of a ligand PMe 3 , PMe 3 and (iii) oxidative addition of the solvent. The rate con-C 6 D 6 stant for each of these processes should not strongly depend on whether the metal is bonded to a hydride or a deuteride ligand. On the other hand, while intramolecular oxidative addition of a phosphine CwH bond is productive when it occurs on intermediate II-D, the oxidative addition of C 6 D 6 leads to a useful result only when it takes place on II.

  kinetic control. We may deÐne as an e †ective turnk c over rate, intending for this the rate at which one ligand CwH bond (or a CwD bond already present) is replaced by a CwD bond. The value of is not a simple function of the forward k c rate constants of the cycle but must also take into account the reversibility of all steps, which reduces (and favors trapping k c of II by over continued H/D exchange). The rate with PMe 3 which II-dn,m is trapped with formation of 4-dn,m is k t ( \ k 6 [PMe 3 -d0]). The relative probability for II-dn,m to either continue the H/D exchange process or be trapped is therefore given by the ratio More speciÐcally, a fraction of k c /k t .

( 3 3

 33 Scheme 3 fore, the formation of 4 (and not 4-D) as the predominant product from and Ðts well with the CpMoCl(PMe 3 ) 3 LiEt 3 BD mechanistic proposal for the decomposition of 1 and with the results of the computational work.

  No change in the 1H NMR properties was observed upon warming to 70 ¡C over a period of two hours, except for the expected temperature shift of the contact-shifted resonances of 1 (see Results). Upon further warming to 95 ¡C for 18 h, the resonances of 1 disappeared and were not replaced by any new resonance attributable to diamagnetic Cpcontaining compounds.

	Thermal stability of 1. Compound 1 (45 mg, 0.11 mmol) was
	dissolved in sealed. Preparation of C 6 D 6 CpMo(OD)(PMe in an NMR tube, which was then Ñame-1-OD. Solid NaOD 3 , ) 2 was added to a solution of (40 mg, 0.09 CpMoCl(PMe 3 ) 3 mmol) in (1 mL) and left to stir overnight. An aliquot C 6 D 6 was taken for the NMR analysis. The formation of compound
	1-OD, and free the 1H resonances at d 21.8 and 20.3 for the former, and by was indicated by CpMo(OD)(PMe 3 ) 2 , PMe 3
	the 1H resonance at d 0.79 and the 31P resonance at d [61.9
	for the latter. The remaining solution was used for an IR
	investigation. IR 2388(w), 2280(s), 2266(s), 1617(m), 1332(s), 1261(s), 1101(m), cm~1) : 2959(s), 2895(s), 2638(m), (C 6 D , 6
	924(s), 817(m).
	Reaction of 1 with CO. Solid KOH (ca. 10 mg) was added
	to a blue solution of	CpMoCl(PMe	(30 mg, 0.07 mmol) in
	3 ] 5 mL). The
	leftover white precipitate was Ðltered o † and dried, and subse-
	quently redissolved in water, giving a positive for Cl~ions. An aliquot of the yellow-brown heptane solution test Ag`/HNO 3
	was dried under reduced pressure and the residue was investi-
	gated by 1H NMR and IR in (br, 18H, Hz, Me), 20.3 (br, 5H, 1H NMR C 6 D 6 . w 1@2 \ 90 w 1@2 ( C 6 \ 75 D 6 IR cm~1) : 3092(w), 2960(s), 2897(s), 2801(w), 2280(s), d) : 21.8 , Hz, Cp). (C 6 D , 6 2266(s), 1785(m), 1750(m), 1419(s), 1270(s), 1101(m), 930(s),
	820(m). The mother liquor, concentrated to 1 mL, did not
	separate a solid even at [80 ¡C.
	An analogous reaction was carried out in tored by 1H NMR, showing the concomitant formation of 1 and moni-C 6 D 6
	and one equivalent of free (d, Hz). 31PM1HN NMR PMe 3 . J PH \ 2.9 tinued 1H NMR monitoring of this solution showed a slow 1H NMR d) : 0.791 ( C 6 D , 6 d) : [61.8 (s). Con-(C 6 D , 6
	decrease in the intensity of the resonances of 1 and the growth
	of new resonances attributed to 5, by comparison with the lit-4, and CpMoH(PMe 3 , ) 3 CpMo(PMe 3 ) 2 (g2-CH 2 PMe ), 2 erature spectra.23 Compound 4 : 1H NMR d) : 4.48 (s, (C 6 D , 6 5H, Cp), 1.29 (vt, Hz, 27H, [8.35 (q, J HP \ 9 PMe 3 ), J \ 53 HP Hz, 1H, H). 31PM1HN NMR d) : 21.7 (s, Com-(C 6 D 6 , PMe ). 3 pound 5 : 1H NMR d) : 4.32 (s, 5H, Cp), 1.25 (d, (C 6 D 6 , J \ HP Hz, 6H, 1.2 (br), [0.38 (br, 2H, 31PM1HN 9 PMe 2 ), CH ). 2 NMR d) : 26.9 (d, 2P, Hz), [40.6 (t, (C 6 D 6 , PMe 3 \ 40 , J PP 1P, Hz). The ratio of 4 and 5 is ca. 4 : 1 PMe 2 CH 2 \ 40 , J PP from 1H NMR integration. Ca. 70% decomposition had
	occurred after 10 days at room temperature.

  1H and 31P NMR investigation of the red prisms conÐrmed their identity as 3, by comparison with the values previously reported.22,24 The relative amount of products 2 and 3 could not be established, as compound 2 is NMR-silent. ) was transferred to an NMR tube and the tube was Ñame-sealed. The 1H and 31P NMR spectra show the resonances of 1, 4, 5 and unidentiÐed Cp-containing byproducts in an approximate 75 : 18 : 5 : 2 ratio. The evolution of these spectra is described in the Results section. They revealed the formation, inter alia, of , residual from the synthesis of 1. After a PMe 3 few days there is no noticeable change. After one month, more than 50% of 1 is still in solution and some red precipitate has separated. Resonances attributed to 5 are present. Their intensity accounts for 50% of the decomposed starting material.

	2, 3N were obtained from an acetone and red prisms of , solution upon slow di †usion of a heptane layer. A unit cell [CpMo(OH)(PMe 3 ) 3 ]BF 4 [CpMo(O)(PMe 3 ) 2 ]BF 4 , determination conÐrmed the nature of the yellow needles as 2 by comparison with the literature,22 while a NMR monitoring of the reaction of 1 and in A PMe 3 C 6 D 6 . solution of 1 and was prepared by stirring dry KOH PMe 3 (ca. 70 mg) and (84 mg, 0.198 mmol) in CpMoCl(PMe 3 ) 3 (3 mL). After 18 h, an aliquot of the brown suspension C 6 (b) Methanol. A solution of 1 was prepared in situ as described above from (84 mg, 0.198 mmol) CpMoCl(PMe 3 ) 3 and an excess of dry KOH in (1 mL). The yellow-brown C 6 D 6 supernatant was split into two equal parts, which were trans-ferred into two NMR tubes. MeOH (10 lL) was added to one of the tubes and then both were monitored by 1H and 31P NMR spectrometry. 1 was completely consumed within 2 h in the MeOH-containing tube (4 : 5 ratio \ 10 : 1), while it was still present after 24 h in the other tube (4 : 5 ratio \ 4 : 1). No other Cp-containing products were observed. Oxidation of 1 and in A solution of fer-PMe 3 C 6 D 6 . rocenium (Fc`) hexaÑuorophosphate (8.8 mg, 26 lmol) in acetone-d6 (0.70 mL, 38 mM) was prepared. A solution of 1 was prepared in situ as described above by reacting (142 mg, 0.334 mmol) and an excess of dry CpMoCl(PMe 3 ) 3 KOH for 8 h in (0.9 mL, 0.37 M). An aliquot of the C 6 D 6 yellow-brown supernatant (0.35 mL, 0.130 mmol) was trans-ferred to an NMR tube containing neat acetone-d6 (0.30 mL), and a second aliquot was transferred to an NMR tube containing the ferrocenium solution (0.30 mL, 11 lmol, Fc`/Mo \ 0.08). NMR monitoring showed no signiÐcant dif-ference between the two reactions after 48 h. NMR monitoring of the reaction of 4 and in PMe 3 -d9 C 6 D 6 . To a solution of 4 (0.03 mmol) in (0.5 mL) containing C 6 D 6 free (0.1 mmol), was added in threefold PMe 3 -d0 PMe 3 -d9 excess. The ratio increased within hours PMe 3 -d0 : PMe 3 -d9 and complete equilibration was reached in three days (equilibrium ratio \ ca. 1 : 1 from 31P NMR). No PMe 3 -dn (1 O n O 8) was detected. Reaction of with CO. A toluene solution CpMoCl(PMe 3 ) 3 (5 mL) of (30 mg, 70 lmol) was exposed to CpMoCl(PMe 3 ) 3 an atmosphere of CO ; the blue solution immediately turned yellow. An aliquot of the solution was transferred into another Schlenk tube and evaporated to dryness, followed by D (ca. 0.5 mLIn the 31P PMe 3 -dn. NMR spectrum each isotopomer generates a singlet shifted upÐeld with respect to free The shift is proportional to PMe 3 . the deuterium content and consists in 0.305 ppm per deute-rium atom. The ppm separation between peaks is constant upon change in the spectrometer magnet strength. NMR monitoring of the reaction of 1-OD and in PMe 3 (100 mg, 0.235 mmol) and an excess C 6 D 6 . CpMoCl(PMe 3 ) 3 of dry NaOD were introduced in an NMR tube containing (0.5 mL). The tube was Ñame-sealed and the reaction C 6 D 6 was monitored by 1H and 31P NMR spectrometry. The for-mation of compound 1-OD, and free CpMo(OD)(PMe 3 ) 2 , was conÐrmed by 1H and 31P NMR (vide supra). The PMe 3 evolution of this spectrum is analogous to that of the experi-ment described in the previous paragraph, with a notable exception for the resonance of (see Results). PMe 3 -d1 NMR monitoring of the reaction of 1 and in PMe 3 C 6 D 12 . A solution of 1 in (ca. 0.2 mmol in 0.5 mL) was trans-C 6 D 12 ferred to an NMR tube and added as an internal stan-Cp 2 Fe dard (21 mg, 0.112 mmol). The tube was sealed and the reaction monitored over time. 1H NMR d) : 22.2 (br, (C 6 D 12 , 18H, Hz, Me), 20.8 (br, 5H, Hz, Cp). Free w 1@2 \ 80 w 1@2 \ 65 is also presentNo activation of free is noticed. PMe 3 the water-free tube after 48 h, the water-containing tube showed the complete replacement of the resonances of 1 with those of 4 and 5 (4 : 5 ratio \ 10 : 1). No other Cp-containing products were observed. An aliquot of the water-containing sample was diluted in air with ca. 50 parts of and an CH 2 Cl 2 , aliquot of the resulting solution was subsequently analyzed by GC-MS (50È300 ¡C, 17¡ min~1 ; EI, scan m/z 50È500). was identiÐed as a by-product (retention C 6 D 5 OH time \ 5.79 ^0.04 min, M`\ 99) by comparison with a genuine sample of under the same conditions C 6 H 5 OH (retention time \ 5.77 ^0.04 min, M`\ 94). NMR monitoring of the reaction of 1 and in PMe 3 -d9 C 6 D 6 . Compound 1 was prepared in situ as described above from (200 mg, 0.470 mmol) and dry NaOH in CpMoCl(PMe 3 redissolution of the residue in The observed resonances C 6 D . 6 at d 4.67 (s, Cp) and 1.30 (d, Hz) in the 1H PMe 3 \ 9 , J HP NMR and at d 21.0 in the 31P NMR spectra, and the CO ) 3 (2 mL). An aliquot (1 mL) was transferred to an NMR C 6 D stretching frequency at 1789 cm~1 in the IR spectrum, are 6 tube and evaporated to dryness under vacuum to remove the free phosphine. To the residue was added (0.5 mL) and C 6 D assigned to by comparison with the lit-CpMoCl(CO)(PMe 3 ) 2 erature values.25 6 (0.1 mL, 9.7 mmol). The reaction was monitored by PMe -d9 3 1H and 31P NMR spectrometry (see Results). Addition of a acid to the reaction of 1 and in BrÊnsted Reaction of with in THF-d8. A CpMoCl(PMe 3 ) 3 BD LiEt 3 1 M solution of in THF (68 lL, 68 lmol) was evapo-BD LiEt 3 rated to dryness under vacuum and the residue was dissolved PMe 3 (a) W ater. A solution of 1 was prepared in situ as C 6 D 6 . described above from (46 mg, 0.11 mmol) CpMoCl(PMe 3 in THF-d8 (0.5 mL). Solid (27 mg, 64 lmol) CpMoCl(PMe 3 ) 3 was added and the solution was stirred for 1 h at room tem-) 3 and an excess of dry KOH in (1 mL). The yellow-brown C 6 perature. The solution was then transferred to an NMR tube D 6 for a 1H NMR investigation. The spectrum showed the supernatant was split into two equal parts, which were trans-ferred to two NMR tubes. (5 lL) was added to one of H 2 expected resonances for 4, including the hydridic P-coupled O the tubes and then both were monitored by 1H and 31P NMR spectrometry. While 1 had undergone \5% transformation in quartet at [8.35 ppm Hz) with an approximately (J \ 53 HP correct intensity relative to the Cp resonance (ca. 1 : 5).

paramagnetically shifted resonances at 21.8 and 20.3 ppm in a 18 : 5 ratio. These resonances shift linearly with the inverse temperature in the 30È75 ¡C temperature range (see Fig.

	and C and the relativistic electron core potential (ECP) sets of
	Hay and Wadt for the heavy atoms.29h31 Electrons outside
	the core were all those of the H, C and O atoms, the 3s, 3p
	electrons in Cl and P, and the 4s, 4p, 4d and 5s electrons in
	Mo. A CpMoX(PH C s 3 symmetry arrangement was imposed for the systems (X \ OH, Cl, H, n \ 2, 3) and ) n PH ; 2 for the trans-CpMoH(O) system. No constraints were (PH 3 ) 2 imposed for the systems. C -symmetric 1 The mean value of the spin of the Ðrst-order electronic
	wavefunction, which is not an exact eigenstate of S2 for unre-
	stricted calculations on open-shell systems, was considered
	suitable to identify the spin state unambiguously. Spin con-
	tamination was carefully monitored and the energies shown in
	the Results section correspond to unrestricted B3LYP
	(UB3LYP) calculations. The value of SS2T for the UB3LYP
	calculations on open-shell systems at convergence were :
	2.0123 ; 2.0138 ; 2 ) , CpMo(PH CpMoCl(PH 3 2.0143 ; trans-2.0128 ; ) , 2 2 )(PH 3 ) , 2 2.0167 and cis-CpMoH(O) 2.0134 (PH 3 CpMo(OH)(PH 3 CpMoH(PH 3 ) 2 , CpMoH(O) ) 2 , (PH 3 , ) 2 for the spin triplet systems ; 0.7556 and CpMo(PH 3 ) , 3 0.7558 for the spin doublet systems, indicating CpMo(PH 3 , ) 2 minor spin contamination.
	Results
	(A) Synthesis and characterization of	CpMo(OH)(PMe 3	) 2	,	1
	reacts with dry KOH in THF or yield insoluble KCl and CpMoCl(PMe 3 ) 3 1, along with to C 6 D 6 CpMo(OH)(PMe 3 ) , 2 free trimethylphosphine (see eq. 1). If the KOH is not dry, the
	reaction products rapidly engage in a decomposition reaction,
	which is discussed in detail in section (C) below. NMR moni-
	toring of the reaction in the formation of one equivalent of allowed us to observe directly C 6 D 6 concomitantly with PMe 3 1. The reaction is selective, no other products being detected
	by 1H NMR spectroscopy under dry conditions (see Fig. 1 of
	ref. 22).
	CpMoCl(PMe 3	) 3	] KOH ]
					CpMo(OH)(PMe 3	) 2	] KCl ] PMe 3	(1)
					1
	Compound 1 is extremely soluble in all common organic
	solvents, including saturated hydrocarbons at [80 ¡C and
	Me 3 two	SiOSiMe 3	.	The 1H NMR spectrum of 1 in	C 6 D 6	displays
							was employed. The LanL2DZ
							basis set includes both Dunning and HayÏs28 D95 sets for H

  1H NMR resonances of Cp and phosphine a-H atoms in 1 are comparable with those reported for other 16-electron half-sandwich MoII complexes, namely Cp*MoClL

	g4-C 4 for the 2 The contact shifts observed H 6 )] .32 2 (L \ or and A PMe 3 , PMe 2 Ph L 2 \ dppe) CpMoCl(PMe 2 .21 Ph) 2 possible alternative formulation as an oxo-hydrido MoIV
	derivative,	CpMoH(O)(PMe 3	is not consistent with the IR

Table 1

 1 B3LYP-optimized geometric parameters (distances in and angles in ¡) and energy (in Hartrees) for

  atoms bending toward the Cp ligand (endo conformation, see Fig.3). The MowPwH and HwPwH bond angles in the triplet (103.26¡ and 95.01¡, respectively) are indicative of a high contribution ([75%) of the phosphorus p orbitals in the bond hybrids and a smaller p contribution in the lone pair.

	18-structure, meanwhile, was found C s to have a lower total energy with a bent asymmetric OH electron CpMo(OH)(PH 3 ) 3
	ligand [Cp(center)wMowOwH dihedral angle of 100.72¡].
	Analogously, no structural models are available for the
	phosphido compounds. Thus, the 16-electron were calculated starting from various symmetric and asym-systems PH 2
	metric energy when PH 2 symmetry), while the corresponding triplet prefers a pyr-conformations. The singlet geometry has lower is planar and perpendicular to the Cp ring PH 2 (C s amidalized arrangement, again symmetric, with the H C s
	The optimized 18-electron asymmetric, pyramidalized equal to 297¡), with one PwH bond roughly parallel and model has an CpMo(PH 2 )(PH 3 ) 3 group (sum of angles at P PH 2
	another roughly perpendicular (exo) to the Cp plane.

  In summary, the calculations show that the hydroxide ligand behaves, even though bent, as a double-faced p donor. This provides p stabilization and results in a low energy gap between the two highest energy metal-based orbitals. The latter feature, in combination with a high pairing energy, enforces the triplet ground state for compound 1. A reduced Ðlled-Ðlled repulsion for the exo OH lone pair in the singlet, however, is responsible for a lower tripletÈsinglet gap relative to the chloro system.

	two p electrons (singlet state), the PwMo p interaction
	becomes the energetically dominating factor and sacriÐces the
	P pyramidalization. In this case, the P lone pair (p orbital)
	chooses the more favorable has a low group electronegativity, thus a lower pairing energy metal orbital. The ligand d xy PH 2
	could also contribute to the overall greater stability of the
	singlet state for CpMo(PH 2	)(PH 3 ) 2	.
	(B) Energetics of the	PMe 3	addition
	It is known from experiment that not react with PMe 3 , N 2 Cp*Mo(PPh 2 or CO,19,43 whereas does )(PMe 3 ) 2 reacts with all three, albeit quantitatively Cp*MoCl(PMe 3 ) 2 only with CO.12,20 As we have shown here, 1 does not react
	with completely into a complex mixture of CO-, Cp-and PMe and However, it does bind CO to transform N 2 PMe . 3 -3 products (see Experimental). Furthermore, our containing
	mechanistic interpretation of the decomposition of 1 (see
	below) indicates that the hypothetical irreversibly. Thus, it appears that the binding ability of binds CpMoH(PMe 3 ) 2 PMe 3 di †erent systems towards neutral two-CpMoX(PMe 3 ) 2 \ OH \ electron donors goes qualitatively in the order PR 2 Steric factors deÐnitely play an important role : while Cl \ H.
	does not tend to dissociate any 16-electron derivative is formed for some bulkier analogues the CpMoCl(PMe 3 ) 3 PMe , 3
	e.g. other hand, all the trends observed along the CpMoX(PMe or On the CpMoCl(PMe 2 Ph) 3 Cp*MoCl(PMe 3 .21,23 ) 3 3 ) 3 families cannot be fully characterized only with steric con-
	siderations. In fact, while Cl has a larger van der Waals radius
	than O (1.70È1.90 vs. 1.50 Ó dissociate the third phosphine, while 1 does not coordinate does not Ó ),44 CpMoCl(PMe 3 ) 3
	PMe 3	,	even if the existence22 of	[CpMo(OH)(PMe 3 ) 3
					d z 2	Conversely, when	PH 2	can donate

  CwH bond to either II or II-D should be slightly favored over the ortho-metallation of a CwD bond to give steady-state concentrations of III-H 2 [ III-HD [ III-This would introduce an additional factor in favor of a D 2 . greater steady-state concentration of II relative to II-D. It is not possible to establish whether the resting state of the cycle is III or VI (in other words, whether the limiting rate for

	for the reductive elimination of RH from
	(R \ 3,5-C 6 H 3 respectively).66,67 This fact alone can rationalize the greater and Cp*Rh(PMe 3 )(R)(H) Me 2 CH 2 , CH 3
	steady-state concentration of II relative to II-D. In addition,
	there should be a direct kinetic isotope e †ect for the process of
	CwH oxidative addition (i.e. and II-D. Previously reported kH/kD values for oxidative addi-and to intermediates II k 4 ) k 5
	tions are slightly greater than 1, for instance 1.38 for the reac-
	tion of ortho-metallation of a the cycle is Cp*Ir(PMe 3 ) or k ~4 k ~5). provided by the decomposition of 1 in with C 6 H 12 /C 6 This information could potentially be Thus, the D .68 12 in the absence of C 6 D 6 free In principle, the most stable species within the PMe . 3 H/D exchange cycle should be the observed Ðnal product
	under these conditions. However, 4 was still obtained in sig-
	niÐcant amounts, in addition to other Cp-containing products
	that do not exhibit any hydride resonances. This indicates that
	the resting state of the H/D exchange cycle is unstable relative
	to further decomposition involving loss of becomes available to trap intermediate II. The same Cp-which then PMe , 3
	containing by-products are also observed, albeit in much
	smaller relative amounts, during the decomposition of 1 in the
	presence of PMe 3 Fig. 2 shows that the D-enriched . tually freed-up in solution. This process cannot take place by ligands are even-PMe 3
	dissociation from 4-dn,m because, as mentioned above, PMe 3 coordination to II is irreversible. The alternative is an associa-
	tive exchange mechanism, which probably occurs via a
	slipped-ring intermediate VII. pounds are precedented in the literature.69 A control study ring-slipped com--rich PMe 3
	(see Results) has conÐrmed that 4 does take place (eq. 4).	PMe 3	exchange on compound
	It is also interesting to note that the presence of not a †ect the course of the decomposition of 1 in C 6 D 6 H 2 in the does presence of In particular, there was no trace of the PMe . 3 formation of the known compound CpMoH 3 (PMe 3 ) ,23 2 which could be envisaged as the product of oxidative addition
	of datively adds the CwH(D) bonds of coordinated to intermediate II. This means that intermediate II oxi-H 2 or the PMe 3 CwD bonds of the solvent, or adds faster than C 6 D 6 , PMe 3 it oxidatively adds H 2 .

  ) leads to a new intermediate (VIII), which is identical to intermediate III except for the presence of a deuteroxide ligand in place of a deuteride ligand.

		)] was obtained for x \ 0.040
	(corresponding to II is trapped by PMe k t /k 3	That is, 4.0% of intermediate after each H/D exchange cycle. \ 0.042). c
	-d1 from 1-OD. The observation of a signiÐcant di †erence in the (D.4) Formation mechanism for the excess of PMe 3
	isotopic distribution of free of 1 and 1-OD [cf. Fig. 2(a) and 2(b)], and notably the greater from the decompositions -dn PMe 3
	proportion of 1-OD, can be rationalized by invoking an additional process obtained from the decomposition of -d1 PMe 3
	that allows selective incorporation of the deuteroxide D atom
	into a cesses examined in the previous sections. Given that, accord-ligand before the occurrence of the other pro-PMe 3
	ing to Scheme 1, competition before solvent oxidative addition may take place at the level of inter-metallation and PMe 3
	mediates II and IV, a similar competition could be invoked at
	the level of compound 1. Solvent addition pathway previously discussed, whereas PMe 3 (k 1 ) metallation leads to the , (k 8 see Scheme 2

Acknowledgements

are grateful to the NSF (grant CHE-9508521) and the We DOE (grant DEFG059ER14230) for support of this work and to Prof. Odile Eisenstein (Montpellier, France) for helpful discussions.


Brett Pleune

Rinaldo Poli

James C Fettinger

Synthesis and Structure of the Stable Paramagnetic Cyclopentadienyl Polyhydride Complexes [Cp*MH 3 (dppe)] + (M ) Mo, W): Stronger M-H Bonds upon Oxidation

Transition-metal polyhydride derivatives have been at the focus of much experimental 2 and theoretical 3 work, especially dealing with their high fluxionality 4 and their structure type (i.e. classical vs nonclassical). 5 There is scarce information on how these properties change upon oxidation to the 17-electron configuration because of the paucity of stable complexes. Only a few monohydride complexes have have been isolated, 6 as facile decomposition via deprotonation or disproportionation pathways usually occurs. 7 Isolable paramagnetic polyhydrides are even less common. We are only aware of TaCl 2 H 2 L 4 (L ) PMe 3 or L 2 ) dmpe) 6g and [WCl 2 H 2 (PMe 3 ) 4 ] + BF 4 -, 6h as well characterized, unambiguous examples. 8 Chemical reactivity studies of such species are also rare. 9 When these compounds are obtained by one-electron oxidation of neutral precursors, H 2 reductive elimination is facilitated by the decreased metal p basicity 10 and adds to the array of decomposition pathways available. 11 We wish to report here the new complexes

[Cp*MH 3 (dppe)]- [PF 6 ] (M ) Mo, ([1]PF 6 ), W ([2]PF 6
)), which are accessible by 1-electron oxidation of the parent complexes 1 and 2 with FcPF 6 . [START_REF]To a solution of 2 (115 mg, 0.160 mmol) in 4 mL of CH2Cl2 was added FcPF6 (53 mg[END_REF] Cyclovoltammetric studies show a reversible oxidation for both 1 and 2 in THF (-0.75 and -0.88 V vs Fc/Fc + , respectively). Complex [1] + exhibits a triplet of quartets in the EPR spectrum (g ) 1.989, a P ) 28.9 G, a H ) 11.8 G; see Figure 1) consistent with coupling to three equivalent H and two equivalent P ligands. Chemical oxidation of Cp*MoD 3 (dppe) leads to the formation of [1] + -d 3 , which is characterized by an EPR broad triplet (g ) 1.991, a P ) 28.9 G; see Figure 1). IR investigations 13 show the expected isotope shift upon deuteration, and a 10-20 cm -1 blue shift upon oxidation. From simple theory, the vibrational frequency correlates directly with the bond energy. [START_REF] Berry | Physical Chemistry[END_REF] Thus, the IR data indicate that the M-H/D bonds are stronger in the oxidized materials, consistent with expectations on the basis of a M δ+ -H δ-bond polarity. Previous IR studies on Cp*FeH(dppe) and WH 2 Cl(PMe 3 ) 4 indicated either no change or a slight red shift upon oxidation, 6h,i,15 and a lowering of M-H BDE's was indicated for lower-valent carbonyl complexes by thermodynamic cycles involving acidity and electrochemical measurements. 16 Despite the supposedly greater Mo-H bond energy relative to the neutral precursor, complex [1] + decomposes rapidly (t 1/2 ) 2 min at 25 °C in either THF or CH 2 Cl 2 ) with gas evolution, to afford a new signal consisting of a doublet of triplets (g ) 1.950, a P ) 16.5 G, a H ) 24.0 G; solvent independent), consistent with the formation of Cp*MoH(dppe)(PF 6 ), 3. [1] + -d 3 correspondingly decomposes to yield a triplet of 1:1:1 triplets (a D ) 4.0 G). This indicates that [1] + decomposes by reductive elimination of H 2 . The similar decomposition rate in THF and CH 2 Cl 2 suggests that H 2 elimination may occur without solvent precoordination. Solvent-independent R-R reductive eliminations from dialkyl complexes have previously been reported. 17 No solvent is coordinated to solid 3, 18 which is therefore formulated as either the salt of a rather uncommon 19 15-electron cation or a 17-electron complex with a coordinated FPF 5 ligand. Solvent coordination cannot be excluded in solution. Additional studies on this compound are in progress and will be reported later.

Compound [2] + exhibits a broad triplet resonance in the EPR spectrum (g ) 2.017; see Figure 1), which decays slowly at room temperature in THF (t 1/2 ) 3 h). The nature of the decomposition products is under current investigation. Cooling to -80 °C does not affect the line shape of the signal. Oxidation of Cp*WD 3 -(dppe) leads to [2] + -d 3 , which better reveals the phosphorus coupling (g ) 2.022, a P ) 27.6 G; see Figure 1). A single crystal of [2]PF 6 could be investigated by X-ray diffraction, 20 which permitted the location and refinement of the three hydride ligands (see Figure 2).

The cation and anion are well separated, the closest contacts being between F atoms and Cp* and dppe C atoms (>3.0 Å). Neglecting the hydrogen positions, the geometry of [2] + is very close to that of [Cp*WH 4 (dppe)] + , previously determined as the BF 4 -salt. 21 The major difference consists of a small displacement (0.025(8) Å, as opposed to 0.002(9) Å in the tetrahydrido complex) of the W atom from the plane defined by CNT, P(1), and P( 2) toward the side of the molecule occupied by H2 and H3 (see Figure 2). The W-P and W-Cp* bond distances are also very similar for [2] + and [Cp*WH 4 (dppe)] + , 21a suggesting that the replacement of a W-H bond with a singly occupied W orbital does not greatly affect the effective metal charge (i.e. the W-H bond is highly covalent). Although the H positions are not determined with high precision, the relatively long H'''H separations (H1-H2 2.81 Å; H2-H3 2.11 Å) suggest a classical formulation for [2] + . The geometry of [2] + is intermediate between I (a pseudotrigonal prism) and II (a pseudooctahedron), i.e., the ideal geometries adopted by the isoelectronic d 2 complexes 1 and [Cp*MoH(dppe)(MeCN) 2 ] 2+ , respectively. 21a,22 The equivalence of the P and H hyperfine couplings in the EPR spectrum of [1] + clearly indicates fluxionality. A scrambling mechanism involving a pseudo-Bailar twist between I and II, as previously proposed for the precursor complexes 1 and 2, could also be adopted by [1] + and [2] + . 21a Our future work will attempt to define the chemical reactivity of this new class of 17-electron polyhydride complexes and their decomposition products.

Acknowledgment. We are grateful to the DOE Office of Energy Research (Grant No. DEFG059ER14230) for support of this work and to a reviewer for useful suggestions.

Supporting Information Available: Tables of crystal and refinement parameters, fractional coordinates, bond distances and angles, and anisotropic displacement parameters (10 pages, print/PDF). See any current masthead page for ordering information and Web access instructions. (16) (a) Tilset, M. J. Am. Chem. Soc. 1992, 114, 2740-2741. (b) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077-5083. (17) (a) Pedersen, A.; Tilset, M. Organometallics 1993, 12, 56-64. (b) Fooladi, E.; Tilset, M. Inorg. Chem. 1997, 36, 6021-6027. (18) 12) Å, β ) 100.424(6)°, V ) 3541.4(5) Å 3 , Z ) 4, Dx ) 1.623 g cm -3 , λ(Mo KR) ) 0.71073 Å, µ(Mo KR) ) 3.454 mm -1 , F(000) ) 1724, T ) 153(2) K, R(F) ) 7.13%, R(wF 2 ) ) 8.61% for all 6226 independent reflections [R(F) ) 4.12%, wR(F 2 ) ) 7.67% for 4577 data with Fo > 4σ(Fo)]. Structure solution by direct methods located the W and P atoms and several F atoms. All other non-hydrogen atoms were located by alternating least-squares refinement cycles and difference Fourier maps (SHELXTL) and refined anisotropically. All hydrogen atoms except the three hydrides were placed in calculated positions (AFIX). The three hydride atoms were located from the highest peaks near the W atom with W-peak distances ranging from 1.4 to 1.7 Å. Their refinement was assisted by restraining the W-H distances to be near 1.70 Å (ADFIX with esd of 0.03 Å). Thermal parameters were allowed to refine freely.

Figure 1 .

 1 Figure 1. EPR spectra of complexes [Cp*MX3(dppe)] + (M ) Mo, W; X ) H, D). Solvent ) THF. The starred peak in the spectrum of [2-d 3 ] + is due to an impurity.

( 15 )

 15 Roger, C.; Hamon, P.; Toupet, L.; Rabaa ˆ, H.; Saillard, J.-Y.; Hamon, J.-R.; Lapinte, C. Organometallics 1991, 10, 1045-1054.

  (21) (a) Pleune, B.; Poli, R.; Fettinger, J. C. Organometallics 1997, 16, 1581-1594. (b) Crystals of [2] + PF6 -are orange, while those of [Cp*WH4-(dppe)] + BF4 -are colorless. Colorless crystals of [Cp*WH4(dppe)] + PF6 -have a significantly different unit cell from that of [2] + PF6 -: a ) 14.439(4) Å, b ) 16.575(4) Å, c ) 15.080(4) Å, β ) 100.29(2)°, V ) 3551.0(14) Å 3 .

Figure 2 .

 2 Figure 2. A view of the cation in compound [2] + PF6 -. Selected bond distances (Å) and angles (deg):W1-P1, 2.474(2); W1-P2, 2.506(2); W1-CNT, 1.999(15); W1-H1, 1.71(2); W1-H2, 1.67(3); W1-H3, 1.69(3); CNT-W1-P1, 162.0(2); CNT-W1-P2, 119.1(2); CNT-W1-H1, 109(2); CNT-W1-H2; 103(2); CNT-W1-H3, 113(2); P1-W1-P2, 78.85(5).


Matthieu Bender-Gresse

Edmond Collange

Rinaldo Poli

Sundeep Mattamana

published or not. The documents may come