ELECTRON SPIN RESONANCE OF LOW-SPIN ISOCYANIDE COMPLEXES OF COBALT(2) .I. HALIDES

The electron spin resonance (esr) spectra of the low-spin CoII (phenyl isocyanide)4X2, where X = Cl-, Br-, or I-, show that the two halogen ions are equivalently coordinated in liquid as well as in frozen solution and the molecule shows rhombic distortion for X- = Cl- and Br-. From the observed large halogen hyperfine splittings the fractional spin transfer to the halogen ion is calculated as 8.7, 15.3, and 14.3% per halogen ion, respectively, for the chloride, bromide, and iodide. The fact that the 59Co hyperfine structure was resolved only in the parallel band is attributed to the positive isotropic contact term which means that the unpaired electron orbital of the complexes consists of predominant 3d3z2-r2 plus a small amount of s orbitals. The existence of an equilibrium between the para- and the diamagnetic forms was demonstrated for the bromide and iodide by esr and by visible absorption spectra.