ELECTROCHEMICAL OXIDATION AND REDUCTION OF CATIONIC CARBONYL HYDRIDE COMPLEXES OF GROUP-VI TRANSITION-METALS

Voltammetric experiments are reported which establish kinetic and thermodynamic properties associated with the redox chemistry of carbonyl hydride complexes [M(CO)(2)(P-P)(2)H](SO3CF3) M = Cr, (Mo, W; P-P = Ph(2)PCH(2)PPh(2) (dpm), Ph(2)PCH(2)CH(2)PPh(2) (dpe)). A systemic relationship is shown between redox data obtained for the hydride complexes and those for the M(CO)(2)(P-P)(2) complexes. Isomerization reactions of the seven-coordinate hydride complexes give rise to interesting mechanistic features which accompany the oxidation as well as the reduction, and both are found to proceed via metal-hydrogen bond cleavage. The oxidation process corresponds to an ECEC reaction pathway {(E) [Mo(CO)(2)(dpe)(2)H](+)reversible arrow[Mo(CO)(2)(dpe)(2)H](2+) + e(-); (C) [Mo(CO)(2)(dpe)2H](2+)--> trans-[Mo(CO)(2)(dpe)(2)](+) + H+; (E) trans-[Mo(CO)(2)(dpe)(2)](+)reversible arrow trans-[Mo(CO)(2)(dpe)(2)](2+) + e(-); (C) trans-[Mo(CO)(2)(dpe)2](2+)--> products} or a related mechanism while the reduction process involves the formation of hydrogen: [Mo(CO)(2)(dpe)(2)H](+) + e(-)reversible arrow cis-Mo(CO)(2)(dpe)(2) + 1/2H(2). In the case of the oxidation of the tungsten and the the [Mo(CO)(2)(dpe)(2)H](+) complexes, a chemically reversible one-electron oxidation process is detected at low temperature in butyronitrile. The investigation of the deuterated complex [Mo(CO)(2)(dpe)(2)D](+) allows the detection of a normal isotope effect on the metal-hydrogen bond cleavage reaction. The oxidized species [M(CO)(2)(P-P)(2)H](2+) can be viewed as extremely strong acids.