Probing the Dichotomy of Square Planar d¹⁰ Complexes: Geometric and Electronic Structure of Nickel π-Complexes

Ni π-complexes are widely postulated as intermediates in organometallic chemistry. However, the nature of the bonding in such complexes has not been extensively studied. Herein, we probe the geometric and electronic structure of a series of nickel π-complexes using a combination of 31P NMR, Ni K-edge XAS, Ni Kβ XES, and supporting density-functional computations. These complexes are best described as square planar d10 complexes with π-backbonding acting as the dominant factor in the M-L bond to the π ligand. The degree of backbonding correlates with both 2JPP and the energy of the clearly observable Ni 1s→4pz pre-edge transition in the Ni K-edge XAS data. The degree of backbonding is determined by the energy of the π*ip ligand acceptor orbital: unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. The strength of the backbonding from the neutral Ni(dtbpe) molecular fragment is dramatically increased via σ donation from the diphosphine ligands. In fact, in unactivated pi complexes, backbonding is dominated by charge donation from the phosphines, which allows for strong backdonation even though the metal centre retains a formal d10 electronic configuration. We describe this interaction as a formal 3-centre-4-electron (3c-4e) interaction where the nickel centre mediates charge transfer from the phosphine σ-donors to the π*ip ligand acceptor orbital. The implications of this unusual bonding motif are described with respect to both geometric structure and reactivity.