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 σ2π2 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 σ bonds, more 5d than 6s character is found at the Ln. The LnC 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 LnC length is small.