In recent years a number of transition metal hydrido-silyl complexes exhibiting unusually short H⋅⋅⋅Si distances ( 2.0 Å) have been characterized. These "nonclassical" η²-silane complexes are believed to serve as models for the intermediates of catalytic hydrosilation processes. H⋅⋅⋅Si interactions have been observed for complexes throughout the transition metal series, but only a few theoretical studies have been attempted to study the nature of the H⋅⋅⋅Si coordination to transition metal centers. Three types of complexes have been chosen for this study: the Group 6 pseudooctahedral complex cis-Mo(CO)(depe)₂(H⋅⋅⋅SiH₃), the unusual Group 4 acetylene complex TiCp₂(trans-η²-^tBuC≡CSiHMe2), and the Group 5 metallocene complexs MCp₂(SiR₃)H(SiR₃). Ab initio theoretical calculations have been...[ Read more ]

In recent years a number of transition metal hydrido-silyl complexes exhibiting unusually short H⋅⋅⋅Si distances (< 2.0 Å) have been characterized. These "nonclassical" η²-silane complexes are believed to serve as models for the intermediates of catalytic hydrosilation processes. H⋅⋅⋅Si interactions have been observed for complexes throughout the transition metal series, but only a few theoretical studies have been attempted to study the nature of the H⋅⋅⋅Si coordination to transition metal centers. Three types of complexes have been chosen for this study: the Group 6 pseudooctahedral complex cis-Mo(CO)(depe)₂(H⋅⋅⋅SiH₃), the unusual Group 4 acetylene complex TiCp₂(trans-η²-^tBuC≡CSiHMe2), and the Group 5 metallocene complexs MCp₂(SiR₃)H(SiR₃). Ab initio theoretical calculations have been utilized to elucidate the nature of the H-Si σ-bond coordination in a variety of complexes. Results obtained from these theoretical studies can be summarized as: (1) The H⋅⋅⋅Si σ-bond is a moderately strong σ*-acceptor which can compete strongly with those widely known π-acceptors like CO and acetylene π-bond; (2) The H⋅⋅⋅Si interaction is usually quite strong for those "nonclassical" silane complexes, such that structures with close H⋅⋅⋅Si contacts are strongly preferred to alternative ones with separate H and SiR₃, ligands whenever the coordination environments permit; (3) Tautomerism between "classical" and "nonclassical" isomers of a hydrido-silyl complex is not very likely to occur, unlike that of η²-dihydrogen complexes; (4) While predominantly covalent H-Si bonding interactions are observed for those hydrido-silyl complexes with H⋅⋅⋅Si distances < 2.0 Å, significant interaction (as indicated by the polarization of electron density from the central hydride to the silicon center) still exists for H⋅⋅⋅Si distances up to (at least) 2.3 Å; (5) The "classical" and "nonclassical" classifications can be related quite well to the formal electronic configuration of the metal center (a variation in 2 of the oxidation state); (6) The feasibility of silicon to achieve 5-coordinate hypervalent coordination by bonding with the electronegative hydride is crucial in maintaining a strong H⋅⋅⋅Si interaction. Furthermore, the theoretical results suggest a new type of "nonclassical" H⋅⋅⋅Si complexes stabilized by σ-withdrawing effects, in addition to the more familiar "carbonyl-type" η²-silane complexes stabilized by Dewar-Chatt π-backbonding interactions.