Comparison between the bis(2-thiopyridine N-oxide) derivatives of CuII and NiI: an electron spin resonance study

Abstract

The e.s.r. spectra for dilute solutions of Cu^II and Ni^I in bis(2-thiopyridine N-oxide)nickel(II) have been studied at 77 K. The latter, formed by exposure to ionizing radiation, was also prepared by electron addition to the Ni^II complex in CD₃OD and 2-methyltetrahydrofuran. The spectra are typical of 3d⁹ complexes with a d_x²–y² semi-occupred molecular orbital. Analysis of the ⁶¹Ni and ⁶³Cu hyperfine coupling constants gave spin densities of 59 and 87% on copper and nickel respectively. The reduced delocalisation for the Ni^I complex is discussed in terms of the smaller positive charge and consequent weaker bonding. This is also thought to be responsible for the low stability of the Ni^I complex, which changed irreversibly on annealing to ca. 140 K. Analysis of the g-tensor components shows that the d_xz and d_yz levels are split, as expected because of the low symmetry, and are higher in energy than the in-plane d_xy orbital. Unfortunately, attempts to detect the Ni^III complex after radiolysis were not successful.