Lanthanide Complexes: Electronic Structure and H-H, C-H, and Si-H Bond Activation from a DFT Perspective

摘要

This paper discusses some relationships between the electronic structure and the reactivity of lanthanide complexes. The electronic structures of some representative lanthanide complexes are described. The 4f electrons do not participate in the Ln-X bond and a comparison between lanthanide and d~0 transition metal complexes from Groups 3 and 4 highlights the dominant ionic character in the bonding to lanthanide centers. However some covalent character cannot be excluded and this covalent character rationalizes the geometry of the complexes such as the non-planar structure of LaX_3 (X = H, Me, F). The consequences of the nature of the bonding on the β Si-C agostic interaction in La{CH(SiMe_3)_2}_3 is presented. The unusual O-bonding mode of CO to Cp~*_2Yb is briefly summarized. The reactivity of X_2Ln-Z (X = Cp, H, Cl, effective group potential; Z = H, Me) with simple molecules like H_2, CH_4, and SiH_4 is compared. It is shown that the strong ionic character of the lanthanide bonding is key to rationalizing the selectivity of the σ-bond metathesis with alkane and to the lack of it in the case of silane. In particular, the position β to the metal center in the diamond shape 4c-4e-transition state is not allowed for a methyl group (transition state of very high energy) whereas it is permitted to a silyl group. This is shown to be related to the relative ability of carbon and silicon to be hypervalent.