Rare‐Earth‐ and Uranium‐Mesoionic Carbenes: A New Class of f‐Block Carbene Complex Derived from an N‐Heterocyclic Olefin

Neutral mesoionic carbenes (MICs) have emerged as an important class of carbene, however they are found in the free form or ligated to only a few d‐block ions. Unprecedented f‐block MIC complexes [M(N′′)(3){CN(Me)C(Me)N(Me)CH}] (M=U, Y, La, Nd; N′′=N(SiMe(3))(2)) are reported. These complexes were prepared by a formal 1,4‐proton migration reaction when the metal triamides [M(N′′)(3)] were treated with the N‐heterocyclic olefin H(2)C=C(NMeCH)(2), which constitutes a new, general way to prepare MIC complexes. Quantum chemical calculations on the 5f(3) uranium(III) complex suggest the presence of a U=C donor‐acceptor bond, composed of a MIC→U σ‐component and a U(5f)→MIC(2p) π‐back‐bond, but for the d(0)f(0) Y and La and 4f(3) Nd congeners only MIC→M σ‐bonding is found. Considering the generally negligible π‐acidity of MICs, this is surprising and highlights that greater consideration should possibly be given to recognizing MICs as potential π‐acid ligands when coordinated to strongly reducing metals.