STRUCTURE AND ELECTROCHEMICAL PROPERTIES OF THE NEW TRIPLY BRIDGED MOLYBDENUM(V) COMPLEX [NBU(4)(N)](2)[MO2O2(MU-O)- (SCH2CO2)(2)(MU-SCH2CO2)]

The reaction of molybdate ions with an excess of thioglycolic acid has been studied in aqueous solution at room temperature by UV/VIS and NM R spectroscopy. As previously observed, spectroscopic data suggest ini tial formation of the complex [MoO2(SCH2CO2)(2)](2-) 1, which is subse quently reduced by the excess of acid. Two molybdenum(V) complexes are formed in equilibrium in this redox reaction. The main product has be en isolated as an orange-red solid with the formula [NBu(4)(n)](2)[Mo2 O3(SCH2CO2)(3)]. 2H(2)O 2. It crystallizes in the monoclinic space gro up P2(1)/a with a = 20.761(8), b = 17.441(7), c = 14.587(6) Angstrom, beta = 101.08(6)degrees, and Z = 4. The structure contains two cofacia l (MoO)-O-V,(SCH2CO2), distorted octahedra sharing one oxygen atom and one thioglycolate ligand on a pseudo-symmetry plane (O-t-Mo 1.677, Mo -Mo 2.643, Mo-S-l,S-b 2.489, Mo-O-l,O-b 2.337, Mo-O-b 1.929 Angstrom, Mo-S-l,S-b-Mo 64.1, Mo-O-l,O-b-Mo 68.9, Mo-O-b-Mo 86.5 degrees; t = te rminal, b = bridging). Cyclic voltammetry shows that complexes 1 and 2 undergo a two-electron irreversible reduction at -1.27 and -1.38 V vs . the saturated calomel electrode, respectively, in methanol. In both reduction processes the monooxomolybdenum(IV)) species [MoO(SCH2CO2)(2 )](2-) and [MoO(SCH2CO2)(solv)(2)] are generated. A reaction mechanism for the oxidation of thioglycolic acid by molybdate ions is proposed from the combined analysis of spectrophotometric, NMR, structural. and voltammetric data. Dimeric molybdenum(V) products are generated from reaction of the above monooxomolybdenum(IV) species with the starting dioxomolybdenum(VI) complex 1. A particularly remarkable supporting fe ature is the formation of Me(2)S when the reaction is carried out in t he presence of Me(2)SO.