Experimental and Theoretical Studies of Bonding and Oxidative Addition of Germanes and Silanes, EH4-nPhn (E = Si, Ge; n = 0−3), to Mo(CO)(diphosphine)2. The First Structurally Characterized Germane σ Complex
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Abstract
Reaction of GeH4 and GeH3Ph with the agostic complex Mo(CO)(dppe)2 (dppe = Ph2PC2H4PPh2) provides germane σ complexes Mo(CO)(η2-GeH4-nPhn)(dppe)2 (n = 0, 1). The coordination in these complexes has been assigned as (η2-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 η2-complex Mo(CO)(η2-GeH2Ph2)(depe)2 is obtained. This complex was characterized by X-ray crystallography and NMR and IR spectroscopy. The Mo(CO)(η2-GeH3Ph)(dppe)2 and Mo(CO)(η2-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)(η2-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)(EH4-nvinn)(dhpe)2 model complexes (E = Si, Ge; n = 0−3; dhpe = H2PCH2CH2PH2; vin = CHCH2) and also the analogous H2 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 H2, 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.
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