Bonding analysis of ylidone complexes EL₂ (E = C-Pb) with phosphine and carbene ligands L

Quantum chemical calculations using DFT and ab initio methods have been carried out of the compounds EL₂ with atoms E = C-Pb and the ligands L = PPh₃, N-heterocyclic carbene (NHC), bicyclic NHC, and cyclic alkyl-amino carbene (cAAC). The equilibrium structures are reported and the bonding situation was analyzed with a variety of charge-and energy decomposition methods. Some of the molecules are experimentally known, but most molecules have not yet been prepared. The bonding analysis suggests that all but one molecule should be considered as ylidones that possess dative bonds L→E←L. The sole exception is C(cAAC)₂, which has a linear structure and classical double bonds (cAAC)=C=(cAAC). The ylidones EL₂ have two lone-pair orbitals at atom E with σ and π symmetry. The π lone pair is somewhat delocalized due to L←E→L π-backdonation. The contribution of L←E→L π-backdonation relative to L→E←L σ-donation increases for the heavier elements. The compounds have rather large first and second proton affinities, which is a characteristic feature of ylidones. They are thus double Lewis bases that could be utilized in chemical reactions.