Peculiar Hydride−Silyl Interactions in Group 5 Bent Metallocene Complexes, Studied by ab Initio Calculations

The nature of possible H···SiR 3 interligand interactions in the complexes MCp 2 XH(SiR 3 ) (M = Nb, Ta; X = SiR 3, Cl, H, CH 3 ) has been studied by ab initio calculations at the MP2 level. The optimized structure of NbCp 2 (SiClH 2 )H(SiClH 2 ) shows symmetric displacements of the silyl ligands about the hydride, with a H···Si distance of 2.180 Å, consistent with the experimental findings. Analysis of the Laplacian of the valence electron density (−∇ 2 ρ) reveals significant polarization of the electron density around H toward both silicon centers (without an observable H···Si bond path), indicating the presence of weak H···Si interaction. The extent of the H···Si interaction can be increased by replacing one of the SiR 3 ligands by a halide or CH 3 and using halide-substituted silyl ligands. An extreme case is MCp 2 ClH(SiCl n H 3 - n ), n ≥ 1, with H···Si interaction distances less than 2.0 Å, and a curved H−Si bond path is observed in each of the respective Laplacian plots. Moreover, an in-plane Cl substituent on silicon causes a more drastic increase of the H···Si interaction than an out-of plane chloride due to favorable orbital interactions. For the tantalum analogue TaCp 2 (SiH 3 )H(SiH 3 ), the theoretical results indicate a symmetric structure similar to NbCp 2 (SiClH 2 )H(SiClH 2 ) rather than unsymmetric (with one H···Si distance of ∼1.6 Å), as suggested by experimental X-ray results. Other theoretical evidences also support the symmetric C 2 v structure of the complex. The “nonclassical” H···Si coordination in the Cl- and CH 3 -substituted complexes suggest a new type of η 2 -silane complexes stabilized by σ-withdrawing effects, without the presence of any strong π-acceptor like CO and PR 3 .