γ Agostic C–H or β agostic Si–C bonds in La{CH(SiMe3)2}3? A DFT study of the role of the ligand

DFT calculations show that La{CH(SiMe3)2}3, whose X-ray structure has previously been determined, should be considered as having a β agostic Si–C bond and not a γ agostic C–H bond. The role of the isolated ligand CH(SiMe3)2− in agostic interactions involving an extremely electropositive metal centre is discussed. Delocalization of the electrons of the lone pair located on the C centre in the neigbouring trimethylsilyl group (negative hyperconjugation) is suggested as being important. The metal centre has an electrostatic effect which elongates preferentially the β Si–C bond of the closest methyl group and contracts the La–C–Si angle.