Infrared Photodissociation Spectroscopy of Mass-Selected Dinuclear Transition Metal Boride Carbonyl Cluster Cations

The transition-metal-boron bonding interactions and geometric structures of heterodinuclear transition metal carbonyl cluster cations BM(CO)⁺ (M = Co, Ni, and Cu) are studied by a combination of the infrared photodissociation spectroscopy and density functional theory calculations at the B3LYP/def2-TZVP level. The BCu(CO)₅⁺ and BCo(CO)₆⁺ cations are characterized as an (CO)₂B-M(CO)_3/4⁺ structure involving an σ-type (OC)₂B → M(CO)_3,4⁺ dative bonding with end-on carbonyls, while for BNi(CO)_5,6⁺ complexes with a bridged carbonyl, a 3c-2e bond involving the 5σ electrons of the bridged carbonyl and an electron-sharing bond between the B(CO)₂ fragment and the Ni(CO)_2,3⁺ subunits were revealed. Moreover, the fundamental driving force of the exclusive existence of a bridged carbonyl group in the boron-nickel complexes has been demonstrated to stem from the desire of the B and Ni centers for the favorable 8- and 18-electron structures.