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

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(SiMe₃)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 H₂C=C(NMeCH)₂ which constitutes a new, general way to prepare MIC complexes. Quantum chemical calculations on the 5f³ 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 d⁰f⁰Y and La and 4f³ 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.