Insight into the nature of M-C bonding in the lanthanide/actinide-biscarbene complexes: a theoretical perspective

We have investigated M-C bonds in lanthanide and actinide complexes ML2 (M = Ce, Th, U, Np and Pu; L = C(PPh2NMes)2) using scalar-relativistic theory. The M-C bonds possess typical σ and π bonding character, except for the nearly π-only Th-C bonds. The metal valence electrons significantly reside in the valence d and f orbitals for CeL2, UL2, NpL2 and PuL2, while for ThL2 most electron population is in 6d orbitals. The contribution of 6d orbitals to the An-C bonds decreases and that of 5f orbitals increases across the actinide series. QTAIM (quantum theory of atoms in molecules) and NBO (natural bond orbital) analyses confirm that the M-C bonds possess significant covalent character. This work provides insights into the contributions of d and f valence orbitals to M-C bonding. And inclusion of Np and Pu in this evaluation extends understanding to later actinides.