Bonding trends in molecular compounds of lanthanides: the double-bonded carbene cations LnCH(2) (+) (Ln=Sc, Y, La-Lu)

Quasi-relativistic Douglas-Kroll CASPT2 calculations are reported for the title molecules, mainly to provide primary data for a fit of double-bond covalent radii. Indeed, a well-developed sigma(2)pi(2) double bond is identified in all cases. For Eu and Yb, however, it is an excited state. The main valence orbitals of all Ln ions are 6s and 5d. In the sigma bonds, more 5d than 6s character is found at the Ln. The Ln=C bond lengths show a systematic lanthanide contraction of 13 pm from La to Lu. An agostic symmetry breaking is demonstrated for Ce but its effect on the Ln-C length is small.