DFT study of tris(bis(trimethylsilyl)methyl)lanthanum and -samarium

DFT calculations have been carried out on the compounds Ln[CH(SiR2R')(SiR3)](3) for Ln La, Sm and W 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 beta-Si-C agostic interactions, whereas little structural changes are found for gamma-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 beta-Si-C agostic interactions are replaced by beta-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.