Temperature dependence of the magnetic susceptibility of cobalt (II) stellacyanin.

Magnetic susceptibility measurements on cobalt(II) stellacyanin ($\text{Rhus vernicifera}$) have been performed between 2.2 and 50 K. The effective magnetic moment of Co(II) in the protein is 3.91 ± 0.12 (μ_B). Nonlinear behavior below 3 K evidences the presence of zero-field splitting attributable to a low-symmetry component of the ligand field. The results are consistent with a structural model based on a distorted tetrahedral Co(II) site involving one or more extremely covalent metal-ligand bonds.