Electron spin resonance studies of one-electron reduction products of nickel(II) and palladium(II) complexes containing tetrathiomolybdate, tetrathiotungstate, and dialkyldithiocarbamate ligands, [M(M′S4)n(R2NCS2)2 –n)]n–(n= 0–2, M = Ni or Pd, M′= Mo or W)

Abstract

The redox properties of nickel(II) and palladium(II) complexes of the type [M(M′S₄)_n(R₂NCS₂)_{2 –n}]^n–(n= 0–2, M = Ni or Pd, M′= Mo or W) have been studied using d.c. and a.c. cyclic voltammetry at a platinum electrode in dichloromethane solution. The series of complexes show initial reversible or quasi-reversible one-electron reduction to give species containing a single unpaired electron. E.s.r. spectra have been used to identify the species produced after one-electron reduction. In the case of palladium, the reduction potential increases smoothly with increasing n whilst the unpaired electron is increasingly delocalised from the central metal ion with decreasing g anisotropy. In contrast, the reduction potential of the nickel complexes increases sharply from n= 0 to 1 with a corresponding increase in the g anisotropy. The reduction potential increases again for n= 2, however the e.s.r. spectrum shows an unusual ‘reversal’ of g anisotropy (g_‖ < g_⊥) compared to that expected for a d⁹ planar complex with the unpaired electron in a nickel d_xy orbital (g_‖ > g_⊥). It is suggested that in the case of n= 1 or 2 the unpaired electron is now occupying a molecular orbital composed of the low-lying molybdenum (or tungsten)d orbitals. This is supported by scattered wave Xα calculations of the electronic structure of the model complexes [Ni(H₂NCS₂)₂], [Ni(MoS₄)(H₂NCS₂)]^– and [Ni(MoS₄)₂]^2– and their oneelectron reduction products.