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

Abstract

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.