Six-coordinate Co2+ with H2O and NH3 ligands:: Which spin state is more stable?

Octahedral, six-coordinate Co2+ can exist in two spin states. For biological ligands, H2O and NH3, the most stable spin state is high spin (S = (3)/(2)). The difference in energy between high and low spin is dependent upon the ligand mix and coordination stereochemistry. High spin optimized geometries for these model compounds give structures close to octahedral symmetry. Low spin permits significant Jahn-Teller distortion. H2O ligands preferentially assume axial positions. Continuum solvent has a greater effect on low spin Co2+, and it reduces the energy difference between the two spin states. For some ligand combinations optimized in the presence of solvent, there is no significant difference in energy between spin states.