Electron spin resonance studies on bis(dimethylglyoximato)cobalt(II) and its complexes with pyridine

Abstract

The e.s.r. spectra of bis(dimethylglyoximato)cobalt(II) and its pyridine complexes have been studied in methanol. The spectra obtained in both liquid and frozen methanol show the existence of three paramagnetic centres, viz, [Co(Hdmg)₂], [Co(Hdmg)₂(py)], and [Co(Hdmg)₂(py)₂](Hdmg is the monoanion of dimethylglyoxime), and that of a diamagnetic adduct [{Co(Hdmg)₂(py)}₂]. The magnetic parameters have been determined for each paramagnetic species; a significant temperature dependence has been observed in the case of [Co(Hdmg)₂]. The parameters indicate a low-spin ²A₁ ground state. The Fermi-contact term for [Co(Hdmg)₂] varies between –20.0 × 10^–4 and –10.5 × 10⁴ cm^–1 on decreasing the temperature from 62 °C to the freezing point of methanol; the values for [Co(Hdmg)₂(py)] and [Co(Hdmg)₂(py)₂] are –3.9 × 10^–4 and 9.8 – 10^–4 cm^–1, respectively, at liquid-nitrogen temperature. The observed trend is consistent with the variation in the axial ligand field, which affects the Fermi term through 3d–4s mixing. The linewidth variation of the hyperfine multiplet in the e.s.r. spectrum of [Co(Hdmg)₂] has been studied in methanol as a function of temperature. The widths of the strongly overlapping lines have been determined by an automatic-fitting procedure. Good simulation and rapid convergence have been obtained by assuming Lorentzian lineshapes. The predominant factors contributing to the linewidths are spin-rotational relaxation at high temperatures and anisotropic relaxation at low temperatures.