Analysis of a Diimine-Organonickel Redox Series

Complex cations [Ni(C5R5)(L)](+), L = 1,4-diphenyl-2,3-dimethyl-1,4-diaza-1,3-butadiene, were compared as structurally characterized BF4-(R = H) and SbF(6)(-)salts (R = Me). These Ni-II(L-0) compounds are reversibly reduced (R = H,Me) or oxidized (R = Me) and their one-electron oxidized and reduced forms studied by EPR and UV/Vis spectroelectrochemistry, supported by DFT calculations. Surprisingly similar EPR features were observed for [Ni(C5Me5)(L)](0)and [Ni(C5Me5)(L)](2+). Whereas the oxidation is largely metal centered to yield a Ni(III)species (R = Me), the reduction reveals energetically close-lying alternatives (redox isomers) Ni-I(L-0) and Ni-II(L center dot-).