SYNTHESIS OF DIPOSITIVE MOLYBDENUM(II) AND TUNGSTEN(II) CARBONYL CATIONS BY ELECTROCHEMICALLY GENERATED INTERNAL ADDITION-REACTIONS

Electrochemical oxidation of M(CO)3(η1-L-L)(η2-L-L) (M = Mo, W; L-L = various ditertiary phosphines and a mixed phosphine/arsine) in acetone (0.1 M Et4NC1O4) or in dichloromethane (0.1 M Bu4NC1O4) solution under carbon monoxide at a platinum electrode yields isolable [M(CO)3(η2-L,-L)2]2+ cations by the electrochemically generated internal addition reaction M(CO)3(η1-L-L)(η2-L-L) → [M(CO)3(η2-L-L)2]2+ + 2e-. Electrochemical synthesis therefore provides at present the only route to novel dipositive carbonyl cations in the special case where a pendent phosphorus (or arsenic) atom is available to provide the additional donor ligand required to achieve an 18-electron configuration. Chemical oxidations do not give the same products because of nucleophilic attack by solvent, the anion of the oxidant, or adventitious impurities on the dipositive cation. The [M(CO)3(η2-L-L)2]2+ cations decompose very slowly under carbon monoxide and much more rapidly under nitrogen to give first the 16-electron trans-[M(CO)2(η2-L-L)2]2+ species (or weakly solvated species), which then undergo nucleophilic attack by the electrochemical supporting electrolyte NBu4ClO4 to generate [M(CO)2(η2-L-L)2(ClO4)]+: [M(CO)3(η2-L-L)2]2+ → trans-[M(CO)2(η2-L-L)2]2+ → [M(CO)2(η2-L-L)2(ClO4)]+. The product of the oxidation of Mo(CO)3(η1-dpm)(η2-dpm)2 (dpm = Ph2PCH2PPh2) by AgClO4 has been isolated and shown to be Mo(η2-dpm)2(ClO4)2, but its structure is not known. © 1990, American Chemical Society. All rights reserved.