GAS-PHASE REACTIVITY OF COORDINATIVELY UNSATURATED TRANSITION-METAL COMPLEX-IONS TOWARD MOLECULAR-OXYGEN

Electrospray mass spectrometry is used to investigate the reactivity o f the coordinatively unsaturated ions [M(bipy)(2)](2+) (M = Cr, Ru, Os ) and [M(bipy)](+) (M = Co, Ni) toward O-2 to give the dioxygen adduct s [M(bipy)(2)O-2](2+) (M = Cr, Ru, Os) and [M(bipy)O-2](+) (M = Co, Ni ). CID experiments with the [M(bipy)(2)O-2](2+) and [M(bipy)O-2](+) ad ducts show that the loss of a mass equivalent to two oxygen atoms is t he predominant process for [M(bipy)(2)O-2](2+) (M = Cr, Ru) and [M(bip y)O-2](+) (M = Ni, Co). In contrast, weak signals assigned to [Os(bipy )O-2](+), [Os(bipy)O-2](2+), and bipy(+.) indicate loss of a bipyridin e ligand from [Os(bipy)(2)O-2](2+) rather than O-2 loss. This result m ay have structural implications and a dioxo Os(VI) complex cation, [Os (bipy)(2)(O)(2)](2+), is proposed for the Os-dioxygen adduct. This ass ignment is consistent with the known solution and solid-state chemistr y of osmium. Formation of [Os-VI(bipy)(2)(O)(2)](2+) involves a formal oxidation of the Os(II) to an Os(VI) and concomitant O-2 four-electro n reduction of O-2. Even though loss of O-2 rather than bipy was the p redominant process observed in CID experiments for the Ru adduct, a st ructural formulation of an analogous dioxo species is not excluded for [Ru(bipy)(2)O-2](2+).