Nickel dithiolenes revisited:: Structures and electron distribution from density functional theory for the three-member electron-transfer series [Ni(S2C2Me2)2]0,1-,2-

The complexes [Ni(S2C2Me2)(2)](z) (z = 0, 1-, 2-) have been isolated for the purpose of investigating their electronic structures in a reversible three-member electron-transfer series. Members are interrelated by reversible redox reactions with E-1/2(0/1-) = -0.15 V and, E-1/2(1-/2-) = -1.05 V versus SCE in acetonitrile. The three,complexes have nearly planar structures of idealized D-2h symmetry. As the series is traversed in the reducing direction, Ni-S and C-S bond lengths increase; the chelate ring C-C bond length decreases from the neutral complex to the monoanion and does not change significantly in the dianion. Structural trends are compared with previous results for [Ni(S2C2R2)(2)](1-,2-). Following the geometrical changes, values of v(Ni-S) and v(C-S) decrease, while the value of v(C-C) increases with increased reduction. Geometry optimizations at the density functional theory (DFT) level were performed for all members of -the series. Geometrical parameters obtained from the calculations are in. good agreement with the experimental findings. The 5b(2g) orbital was identified as the LUMO in [Ni(S2C2Me2)(2)], the SOMO in [Ni(S2C2Me2)(2)](1-), and the HOMO in [Ni(S2C2Me2)(2)](2-). Unlike in the situation in the [M(CO)(2)-(S2C2Me2)(2)](z) series (M, = Mo, W; z = 0, 1-, 2-), the apparent contribution from the metal d orbital in the electroactive orbital is not constant. In the present series, the d(xz) contribution increases from 13 to 20 to 39% upon passing from the, neutral to the monoanionic to the dianionic complex. Accurate calculation of EPR g-values Of [Ni(S2C2Me2)(2)](1-) by DFT serves as a test for the reliability of the electronic structure calculations.