Unsaturated binuclear homoleptic nickel carbonyl anions Ni 2 (CO) n − (n = 4−6) featuring double three-center two-electron Ni−C−Ni bonds

The homoleptic homodinuclear nickel carbonyl anions Ni2(CO)n− (n = 4−6) are mass-selected in the gas phase and examined with anion photoelectron velocity-map imaging spectroscopy combined with the density functional calculations. The doubly carbonyl-bridged structures are found to be favorable for Ni2(CO)n− (n = 4−6). The nature of Ni−Ni bonding in these complexes is ascertained with the aid of a range of state-of-the-art quantum chemistry methods. Despite the absence of direct multiple Ni−Ni bonds, the two nickel atoms in Ni2(CO)n− (n = 4−6) complexes are joined by two bridging carbonyl ligands via the sharing three-center two-electron Ni−C−Ni bond to in turn achieve the (16,16), (16,18), and eventually the favored (18,18) configurations.