Paramagnetic anisotropy and electronic structure of NN′-ethylenebis-(acetylacetoneiminato)cobalt(II) and NN′-ethylenebis(thioacetylacetone-iminato)cobalt(II)

Abstract

Magnetic-susceptibility measurements have been made on single crystals of the title complexes, [Co(acacen)] and [Co(tacacen)], over the range 100–300 K. Both complexes exhibit anisotrppy typical of planar low-spin cobalt(II) species, with [Co(tacacen)] being much less anisotropic in the xy ligand plane than [Co(acacen)]. The anisotropy in both is dominated by the close approach of the d_yz orbital to the d_z² ground-state orbital. The temperature-dependent principal susceptibilities have been fitted to a recently developed model for low-spin Co^II which takes into account excited doublet and quartet states. In conjunction with e.s.r. parameters, it is found that quartet-state components lie only a few thousand wavenumbers in energy above the ground state. Correlations are made between the energies of out-of-plane metal π orbitals and the ability of the chelates to bind molecules such as pyridine and O₂.