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.