Nickel-(II) and -(III) complexes with NiS4 environments: electron paramagnetic resonance and electrochemical studies

Abstract

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 [NiL¹₂]^2– and thione [Ni(H₂L¹)₂]²⁺ complexes with E_½ 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≈ 2.30, g_yy≈ 2.05, g_zz≈ 2.03) compatible with the uncommon configuration d_xy¹(g‖ > g⊥) 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.