DFT Study of Tris(bis(trimethylsilyl)methyl)lanthanum and -samarium

Abstract

DFT calculations have been carried out on the compounds Ln[CH(SiR2R‘)(SiR3)]3 for Ln = La, Sm and (i) R = R‘ = Me, (ii) R = H, R‘ = Me, and (iii) R = R‘ = H. The results are compared with the X-ray structures that are available from the literature for both metals and R = R‘ = Me. The calculations correctly reproduce the experimental structural features in these complexes exhibiting the peculiar pyramidal coordination geometry. The results show significant increases in the Si−C bond lengths associated with β-Si−C agostic interactions, whereas little structural changes are found for γ-C−H agostic interactions. The latter are in fact repulsive. The simplified model system with R = H and R‘ = Me that retains one agostic methyl interaction in each alkyl ligand also correctly reproduces the essential geometrical features. The simplest model with only SiH3 groups, while also adopting a pyramidal coordination geometry, no longer accurately describes the real molecule, since the β-Si−C agostic interactions are replaced by β-Si−H interactions. A Mulliken analysis of the electronic structure shows a relatively covalent Ln−C interaction with significant 5d orbital participation in the bonding. A number of calculations with different basis sets on the model system Sm[CH(SiH2Me)(SiH3)]3 shows that the addition of polarization functions (d functions on Si and C, p functions on H, or f functions on Sm) has little or no beneficial effect on the quality of the results.