NICKEL-(II) AND NICKEL-(III) COMPLEXES WITH NIS4 ENVIRONMENTS - ELECTRON-PARAMAGNETIC RESONANCE AND ELECTROCHEMICAL STUDIES

A group of nickel(II) complexes having a square-planar NiS4 co-ordination sphere have been synthesised using the bidentate quinoxaline-2,3-dithiol ligand (H2L1). In dimethylformamide solution the nickel(III)-nickel(II) couple is electrochemically observable for both the thiolate (NiL1(2)]2- and thione [Ni(H2L1)2]2+ Complexes with E1/2 values of +0.12 and +0.32 V respectively vs. saturated calomel electrode. The nickel(III) species can be electrogenerated in solution. Upon freezing (77 K) rhombic EPR spectra (g(xx) almost-equal-to 2.30, g(yy) almost-equal-to 2.05, g(zz) almost-equal-to 2.03) compatible with the uncommon configuration d(xy)1 (g(parallel-to) > g(perpendicular-to)) are observed. The fact that the thione (neutral sulfur) complexes are more difficult to oxidise is consistent with the effectiveness of the polarisable sulfur atoms in the thiolate complex, resulting in the stabilisation of Ni(III) and the observation of relatively low values for the Ni(II)-Ni(III) potential.