ELECTROCHEMISTRY OF THE COPPER-NICKEL SERIES OF HETEROPOLYMETALLIC COMPLEXES (4)-O)(NC5H4[C(O)NET2]-3)(4)CU4-X(NI(H2O))(X)CL-6] (X=0-4)

The electrochemistry of the tetranuclear copper-nickel heteropolymetal lic complexes [(mu(4)-O)L4Cu4-x{Ni-(H2O)}xCl(6)][x = 0-4, L = N,N-diet hylnicotinamide (denc)] were studied at a platinum electrode in dimeth yl sulfoxide with 0.20 M tetrabutylammonium hexafluorophosphate as sup porting electrolyte. At potentials more cathodic than -1.0 V the compl exes are electrodeposited as Cu-Ni alloy and metal oxide films and dis play a complicated set of cyclic voltammographs. The voltammographs of all the Cu-containing complexes show a quasi-reversible redox couple in the potential range 0.250 to -0.450 V vs. Ag-AgPF6 (0.01 M)-CH3CN. As the number of Cu atoms decreases in the complex, the peak currents i(pa) and i(pc) decrease proportionally and the peak potential shifts anodically. The cyclovoltammetric (CV) results indicate that electron transfer initially occurs only to the Cu-II centres and that the elect ron-transfer reaction appears to be quasi-reversible. Using steady-sta te voltammetry at an ultramicroelectrode in combination with chronoamp erometry at a microelectrode and exhaustive electrolysis at a Hg-pool electrode, the number of electrons (n) transferred for this initial re duction of the Cu-4, Cu3Ni, Cu2Ni2 and CuNi3 complexes were 3.1, 2.1, 1.8 and 0.57, respectively. The diffusion coefficient for all the comp lexes was 2.2(+/-0.1) x 10(-6) cm(2) s(-1). The electronic spectrum of the Cu-4 complex taken after exhaustive electrolysis shows that one q uarter of the Cu atoms remain in the Cu-II form and that the Cu-I comp lex remains stable. Since only a single CV peak results for all of the complexes, the electron transfer is most likely consecutive with very closely spaced E degrees potentials. A model based on statistically d etermined electron transfer to Cu-II in particular faces is also propo sed.