Abstract
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.