The interaction of [NbCp(2)H(3)] with fluorinated alcohols to give dihydrogen-bonded complexes was studied by a combination of IR, NMR and DFT methods. IR spectra were examined in the range from 200-295 K, affording a clear picture of dihydrogen-bond formation when [NbCp(2)H(3)]/HOR(f) mixtures (HOR(f) = hexafluoroisopropanol (HFIP) or perfluoro-tert-butanol (PFTB)) were quickly cooled to 200 K. Through examination of the OH region, the dihydrogen-bond energetics were determined to be 4.5+/-0.3 kcal mol(-1) for TFE (TFE = trifluoroethanol) and 5.7+/-0.3 kcal mol(-1) for HFIP. (1)H NMR studies of solutions of [NbCp(2)H(2)(B)H(A)] and HFIP in [D(8)]toluene revealed high-field shifts of the hydrides H(A) and H(B), characteristic of dihydrogen-bond formation, upon addition of alcohol. The magnitude of signal shifts and T(1) relaxation time measurements show preferential coordination of the alcohol to the central hydride H(A), but are also consistent with a bifurcated character of the dihydrogen bonding. Estimations of hydride-proton distances based on T(1) data are in good accord with the results of DFT calculations. DFT calculations for the interaction of [NbCp(2)H(3)] with a series of non-fluorinated (MeOH, CH(3)COOH) and fluorinated (CF(3)OH, TFE, HFIP, PFTB and CF(3)COOH) proton donors of different strengths showed dihydrogen-bond formation, with binding energies ranging from -5.7 to -12.3 kcal mol(-1), depending on the proton donor strength. Coordination of proton donors occurs both to the central and to the lateral hydrides of [NbCp(2)H(3)], the former interaction being of bifurcated type and energetically slightly more favourable. In the case of the strong acid H(3)O(+), the proton transfer occurs without any barrier, and no dihydrogen-bonded intermediates are found. Proton transfer to [NbCp(2)H(3)] gives bis(dihydrogen) [NbCp(2)(eta(2)-H(2))(2)](+) and dihydride(dihydrogen) complexes [NbCp(2)(H)(2)(eta(2)-H(2))](+) (with lateral hydrides and central dihydrogen), the former product being slightly more stable. When two molecules of TFA were included in the calculations, in addition to the dihydrogen-bonded adduct, an ionic pair formed by the cationic bis(dihydrogen) complex [NbCp(2)(eta(2)-H(2))(2)](+) and the homoconjugated anion pair (CF(3)COO...H...OOCCF(3))(-) was found as a minimum. It is very likely that these ionic pairs may be intermediates in the H/D exchange between the hydride ligands and the OD group observed with the more acidic alcohols in the NMR studies.

Complex OsH2Cl2((PPr3)-Pr-i)(2) (1) reacts with 2.0 equiv of HSnPh3 to give the tetrahydridestannyl derivative OsH4Cl(SnPh3)((PPr3)-Pr-i)(2) (2) and ClSnPh3. The structure of 2 has been determined by X-ray diffraction analysis. In the solid state and in solution at temperatures lower than 298 K, the coordination geometry around the osmium atom can be rationalized as derived from a distorted dodecahedron. In the presence of diphenylacetylene, complex 2 gives OsH3(SnClPh2){eta(2)-CH2=C(CH3)(PPr2)-Pr-i}((PPr3)-Pr-i) (3), cis-stilbene, and benzene. In the solid state, the structure of 3 determined by X-ray diffraction analysis can be described as a very distorted pentagonal bipyramid, with the phosphorus atom of the triisopropylphosphine ligand and the midpoint of the olefinic bond of the isopropenyl group of the dehydrogenated phosphine occupying axial positions. In solution, at temperatures higher than 233 K, the coordinated olefin is released. Complex 3 reacts with molecular hydrogen to afford the pentahydride OsH5(SnClPh2)((PPr3)-Pr-i)(2) (4), as a result of the hydrogenation of the coordinated olefinic bond and the d(4)-d(2) oxidative addition of hydrogen. The structure of 4 in the solid state also has been determined by X-ray diffraction. The coordination geometry around the osmium atom can be rationalized as a distorted dodecahedron. In solution, complex 4 does not have a rigid structure even at 193 K. DFT calculations in model systems of 2, 3, and 4, in which the bulky ligands have been replaced by small models, followed by QM/MM optimizations with the real ligands have allowed the complete determination of the hydride positions and of the role played by steric effects in the experimental structures.

Reaction of GeH4 and GeH3Ph with the agostic complex Mo(CO)(dppe)(2) (dppe = Ph2PC2H4PPh2) provides germane sigma complexes Mo(CO)(eta(2) -GeH4-nPhn)(dppe)(2) (n = 0, 1). The coordination in these complexes has been assigned as (eta(2) -Ge-H) on the basis of NMR and IR spectroscopy and by comparison to the analogous complexes of silanes. When the more electron-rich phosphine depe (depe = Et2PC2H4PEt2) is used, oxidative addition (OA) products MoH(GeH3)(CO)(depe)(2) and MoH(GeH2Ph)(CO)(depe)(2) are isolated (NMR and X-ray evidence). However, when the secondary organogermane GeH2Ph2 is used in the depe system, the eta(2)-complex Mo(CO)(eta(2)-GeH2Ph2)(depe)(2) is obtained. This complex was characterized by X-ray crystallography and NMR and IR spectroscopy. The Mo(CO)(eta(2)-GeH3Ph)(dppe)(2) and MO(CO)(eta(2)-GeH2Ph2)(depe)(2) complexes were found to be in tautomeric equilibrium with their OA products in solution. Structure and bonding comparisons are made to the analogous silane complexes, e.g., Mo(CO)(eta(2) -SiH2Ph2)(depe)(2), the X-ray structure for which is also reported. The Ge-H bonds undergo OA much more easily than Si-H, and to obtain further insight into the activation processes, ab initio DFT calculations have been performed on Mo(CO)(EH(4-n)vin(n))(dhpe)(2) model complexes (E = Si, Ge; n = 0-3; dhpe = H2PCH2CH2PH2; vin = CH=CH2) and also the analogous H-2 complex. Because the ease of the whole OA process is a balance between the E-H bonding energy and Mo-E bonding energy, it can be concluded that the factor that makes OA of the Ge-H bond easier than that for Si-H is the relative weakness of the Ge-H bond, despite the fact that the Mo-Ge bond is also weaker. This competition between both factors is also seen for OA of H-2, for which although the Mo-H bonding energy is much higher than Mo-Si and Mo-Ge bonding energies, the H-H bond is also significantly stronger than the Si-H and Ge-H bonds. In general, the ease of OA of molecular hydrogen is between that of germanes and silanes. Calculations show that for alkanes the OA is much more difficult because the loss of the high C-H bond energy (comparable to or greater than that for H-H) is not as well compensated for by the energy of formation of the Mo-C bond due to the weakness of the Mo-C bond.

The hexahydrido complex OsH6((PPr3)-Pr-i)(2) (1) reacts with PHPh2 to give molecular hydrogen and the tetrahydride OsH4(PHPh2)((PPr3)-Pr-i)(2) (2). However, the formation of OsH2(PHPh2)(2)((PPr3)-Pr-i)(2) (3) as a consequence of the substitution of a second hydrogen molecule from 2 by PHPh2 does not occur. The treatment of 2 with 1.0 equiv of PHPh2 in toluene at 80 degreesC leads after 3 days to OsH2(PHPh2)(3)((PPr3)-Pr-i) (4). The preparation of 3 requires the previous acidolysis of 2 with HBF4, which gives [OsH5(PHPh2)((PPr3)-Pr-i)(2)BF4 (5). In contrast to 2, the addition of PHPh2 to 5 affords [OsH3(PHPh2)(2)((PPr3)-Pr-i)(2)]BF4 (6) and molecular hydrogen. Deprotonation of 6 with Et3N yields 3. The skeleton of the cation of 5 has been determined by X-ray diffraction. The configuration is consistent with a Y-shaped OsP3 disposition with the osmium atom in the common vertex. Complex 5 also reacts with methanol and water to give [OsH5- {P(OMe)Ph-2}((PPr3)-Pr-i)(2)] BF4 (7) and [OsH5{P(OH)Ph-2}((PPr3)-Pr-i)(2)]BF4 (8), respectively. The addition of Et3N to 7 affords OsH4{P(OMe)Ph-2}((PPr3)-Pr-i)(2) (9). A theoretical study on the OsH5(PH3)(3)(+) model complex reveals that although a static description is fully consistent with a classical pentahydride assignment, the formation of a dihydrogen is a very low energy costing process, on both thermodynamic and kinetic grounds. Thus, these polyhydride systems might be better described as possessing delocalized hydrogen atoms. A further QM/MM IMOMM study on the actual OsH5(PR3)(+) systems indicates that the inclusion of bulky phosphine substituents plays a role against the stability of dihydrogen forms, because of the higher steric congestion of lower coordination number complexes arising from repulsions between bulky phosphines. Although IMOMM calculations improve significantly the agreement with experimental structures, they do not change the validity of the aforementioned statement concerning delocalization.

The hexahydrido complex OsH6(PiPr3)2 (1) reacts with PHPh2 to give molecular hydrogen and the tetrahydride OsH4(PHPh2)(PiPr3)2 (2). However, the formation of OsH2(PHPh2)2(PiPr3)2 (3) as a consequence of the substitution of a second hydrogen molecule from 2 by PHPh2 does not occur. The treatment of 2 with 1.0 equiv of PHPh2 in toluene at 80 °C leads after 3 days to OsH2(PHPh2)3(PiPr3) (4). The preparation of 3 requires the previous acidolysis of 2 with HBF4, which gives [OsH5(PHPh2)(PiPr3)2]BF4 (5). In contrast to 2, the addition of PHPh2 to 5 affords [OsH3(PHPh2)2(PiPr3)2]BF4 (6) and molecular hydrogen. Deprotonation of 6 with Et3N yields 3. The skeleton of the cation of 5 has been determined by X-ray diffraction. The configuration is consistent with a Y-shaped OsP3 disposition with the osmium atom in the common vertex. Complex 5 also reacts with methanol and water to give [OsH5{P(OMe)Ph2}(PiPr3)2]BF4 (7) and [OsH5{P(OH)Ph2}(PiPr3)2]BF4 (8), respectively. The addition of Et3N to 7 affords OsH4{P(OMe)Ph2}(...

The complexes [OsHCl2(=N=CR2)(PiPr(3))(2)] [CR2 = CMe2 (1), C-a(CH2)(4)(CH2)-H-b(C-a-C-b) (3)] react with Ag(CF3SO3) in the presence of ligands L [H2O, P(OMe)(3), CO]. The reactions in the presence of water lead to [OsHCl(=N=CR2)(H2O)(PiPr(3))(2)][CF3SO3] [CR2 = CMe2 (2), Ca(CH2)(4)(CH2)-H-b(C-a-C-b) (4)], which exist as 1:1 equilibrium mixtures of the isomers 2a/4a (hydride trans to Cl) and 2c/4c (hydride trans to water) in the solid state and in solution. The structure of 2c has been determined by an X-ray diffraction study. The geometry around the metal center can be described as a distorted octahedron with trans phosphane ligands at opposite sites of an ideal coordination plane defined by the other four ligands. The reactions in the presence of P(OMe)(3) afford [OsHCl(=N= CR2)JP(OMe)(3)}(PiPr(3))(2)][CF3SO3] [CR2 = CMe2 (5a), C-a(CH2)(4)(CH2)-H-b(C-a-C-b) (6a)], with the hydride and chlorine ligands trans disposed. Complexes 5a and 6a can be also obtained starting from the equilibrium mixtures of 2a and 2c or 4a and 4c, respectively, and phosphite. Compounds 5c and 6c, with P(OMe)(3) trans to hydride, are formed initially, and subsequently isomerize to 5a and 6a. Reactions under carbon monoxide give [OsHCl(=N=CR2)(CO)(PiPr(3))(2)][CF3SO3] [CR2 = CMe2 (7a), C-a(CH2)(4)(CH2)-H-b(C-a-C-b) (8a)], which also contain the chlorine trans to the hydride ligand. A theoretical study on [OsHCl(=N=CH2)L(PH3)(2)](+) [L = H2O, P(OH)(3) and CO] model complexes shows that these stereochemical preferences arise from the properties of the ligands in the plane perpendicular to the P-Os-P axis. Although the structure with the H and Cl trans disposed is not the most favorable when the related pentacoordinate [OsHCl(=N=CR2)(PiPr(3))(2)](+) model complex is considered, it maximizes the interaction energy between the pentacoordinate complex and L ligand, and thus it is the most stable as a whole.

Treatment of OsH6((PPr3)-Pr-i)(2) (1) with benzophenone and acetophenone in toluene under reflux affords OsH3{C6H4C(O)R}((PPr3)-Pr-i)(2) (R = Ph (2), CH3 (3)), as a result of the ortho-CH activation of the aromatic group of the ketones. Complex 1 is also capable of activating ortho-CF bonds of fluorinated aromatic ketones. Thus, the reactions of this complex with pentafluoroacetophenone, decafluorobenzophenone, and 2,6-difluoroacetophenone give OsH3{C6F4C(O)R}(PiPr(3))(2) (R = CH3 (4), C6F5 (5)) and OsH3{C6H3FC(O)CH3}((PPr3)-Pr-i)(2) (6). The structure of 4 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. Complexes 4 and 6 can be also obtained by reaction of 1 with 2,3,4,5-tetrafluoroacetophenone and 2-fluoroacetophenone, respectively. This selective C-H activation of the ortho-CH bond of the above-mentioned ketones is in contrast with the selective C-F activation observed for the reaction of 1 with 2,3,4,5,6-pentafluorobenzophenone, which affords OsH3{C6F4C(O)C6H5}((PPr3)-Pr-i)(2) (7). The structure of 7 has also been determined by X-ray diffraction. The geometry around the osmium is the same as that of 4. DFT calculations suggest that in fluorinated aromatic ketones the ortho-CF activation is thermodynamically favored over the ortho-CH activation and that the kinetically preferred ortho-CH activation of 2,3,4,5-tetrafluoroacetophenone and 2-fluoroacetophenone is in part due to the preferred anti arrangement of the F-C-C-C=O unit of the starting ketones. In solution, the hydride ligands of the OsH3 unit of 2-7 undergo two different thermally activated exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The exchange involving the hydride ligand disposed cis to the carbonyl group is faster than the other one in all the cases. For 2, 3, and 6, quantum exchange coupling is also observed between the hydride ligands involved in the faster thermally activated exchange process.

The dihydride-dichloro complex OsH2Cl2((PPr3)-Pr-i)(2) (1) reacts with cyclohexanone oxime in toluene under reflux to give after 12 h OsHCl2{N=C(CH2)(4)CH2 }((PPT3)-P-i)(2) (2), which can be also obtained by reaction of the oximate compound, OsH2Cl{kappa-N,kappa-O[ON=C(CH2)(4)CH2]}-((PP3)-P-i)(2) (3) with HCl. Complex OsHCl2{N=C(CH3)(2)}((PPr3)-Pr-i)(2) (4) has been similarly prepared by treatment of compound OsH2Cl{kappa-N,kappa-O[ON=C(CH3)(2)]}((PPr3)-Pr-i)(2) (5) with HCl. When the reaction of I and cyclohexanone oxime, in toluene under reflux, was quenched after 1 h, a mixture of 1, 2, 3, and the trichloroazavinylidene OsCl3{N=C(CH2)(4)CH2}((PPr3)-Pr-i)(2) (6) was obtained. The structures in the solid state of 2 and 6 have been determined by X-ray diffraction studies. In both cases, the geometry around the metal center can be described as a distorted octahedron with the phosphorus atoms of the phosphines occupying trans positions and the C=N group and the carbon atoms bonded to this group lying in a plane that is parallel to the Cl-Os-Cl plane. CCSD(T)//B3LYP calculations on the model complexes OsXCl2(N=CH2)(PH3)(2) [X = H (2t), X = Cl (6t)] state that the above-mentioned conformation is 15.3 (2t) or 12.1 (6t) kcal mol(-1) more stable than that with the azavinylidene ligand parallel to the P-Os-P plane. In solution the azavinylidene ligands of 2 and 4 rotate around the Os-N-C axis. The activation parameters of the process are Delta H double dagger = 14.4 +/- 0.8 kcal mol(-1) and Delta S double dagger = -1.1 +/- 1.3 cal mol(-1) K-1 for 2 and Delta H double dagger = 13.1 +/- 0.8 kcal mol(-1) and Delta S double dagger = 0.0 +/- 2.8 cal mol(-1) K-1 for 4.

The dihydride-dichloro complex OsH2Cl2((PPr3)-Pr-i)(2) (1) reacts with cyclohexanone oxime and acetone oxime in the presence of Et3N to give the dihydride derivatives OsH2Cl{kappa N,kappa O-[ON=C(CH2)(4)CH2]}(PiPr(3))(2) (2) and OsH2Cl{kappa N,kappa O-[ON=C(CH3)(2)]}((PPr3)-Pr-i)(2) (3), respectively. The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid, with the triisopropylphosphine ligands occupying two relative trans positions. The remaining perpendicular plane is formed by the hydride ligands, the chlorine, and the oximate group, which acts with a bite angle of 36.6(1)degrees. In solution the hydride ligands of 2 and 3 undergo an intramolecular thermally activated site exchange process, The activation parameters off;his process are Delta H-double dagger = 11.9(+/-0.7) kcal mol(-1) and Delta(double dagger) = -0.5(+/-1.4) cal mol(-1) K-1 for 2 and Delta H-double dagger = 11.7(+/-0.8) kcal mol(-1) and Delta S-double dagger = -0.8(+/-2.0) cal mol(-1) K-1 for 3. To understand why complexes 2 and 3 are dihyride derivatives, while the previously reported complex OsCl-{NH=C(Ph)C6H4}(eta(2)-H-2)((PPr3)-Pr-i)(2) is an elongated dihydrogen-compound, a quantitative theoretical analysis of the interaction between the H-2 moiety and the OsClL2(PH3)(2)(L-2 = ON=CH2, NH=CHCH=CH) complex fragments, along the oxidative addition pathway, is also reported.

The interaction between H-2 and M(CO)(n)(PH3)(5-n) (M = Cr, Mo, W; n = 0, 3, 5) metal fragments has been studied by means of CCSD(T)//B3LYP calculations. Three steps in the dihydrogen addition path that starting from the ML5 and H-2 Separated fragments leads to a stable dihydride have been considered: (i) dihydrogen coordination; (ii) cleavage of the H-H bond in a dihydrogen-like structure, leading to a PB1 cis-dihydride; (iii) reorganization of the pentagonal bipyramidal cis-dihydride formed to a more stable PB2 dihydride structure. From the thermodynamic results and the energy profiles for the oxidative addition the nine complexes under study can be classified in three groups: (i) only dihydrogen observable; M(CO)(5)H-2 (M = Cr, Mo, W); and M(CO)(3)(PH3)(2)H-2 (M = Cr, Mo); (ii) equilibrium between dihydrogen and PB2 dihydride: W(CO)(3)(PH3)(2)H-2 and M(PH3)(5)H-2 (M = Cr, MO); (iii) only dihydride observable; W(PH3)(5)H-2. The different behavior for dihydrogen addition is related to the energetics of the M-H-2 and M-H bonds and to the singlet-triplet; separation in the ML5 fragment.

The dihydrogen (DiH(2)) and the dihydride Terms of the W(CO)(3)(PH3)(2)(H-2) complex are studied by means of DFT (B3LYF) and ab initio (MP2, MP4(SDTQ), and CCSD(T)) calculations. The oxidative-addition process involving simple H-H breaking (DiH(2) --> PB1) is found to be a very low energy process (Delta H double dagger = 2.50 kcal/mol at the CCSD(T) level). An essentially zero energy barrier for the reverse reaction explains why this structure with both the hydride and phosphorus ligands equivalent is not that observed in low-temperature NMR, experiments. A new structure is proposed for the dihydride form, PB2, which accounts for both the spectroscopic and thermodynamic experimental data. It can be described as a pentagonal-bipyramidal structure, with two axial carbonyl ligands and two equatorial hydrides separated by a phosphine ligand. The enthalpy difference between DiH(2) and PB2 is computed to be equal to 1.29 kcal/mol (in favor of DiH(2)) at the CCSD(T) level, in good agreement with the experimental data on related complexes (1.2-1.5 kcal/mol).