Hi MM, Co in its +2 oxidation state typically forms a tetrahedrally coordinated species in high pH environment; and pentagonally coordinated species with five ligands at low pH. A Co ion in its +3 oxidation state forms a octahedrally coordinated species with six ligands. Co +3 is unusual in biological molecules. However it is seen in crystal structures, sometimes with partial occupancy and is a likely product of radiation damage caused by the X-ray beam. In the past I have found that Co+2 is prone to oxidation in the beam if the crystallizing condition has a pH lower than 6.5. Here is a article you might enjoy.
Comparison of solution and crystal properties of Co(II)-substituted human carbonic anhydrase II. Avvaru BS, Arenas DJ, Tu C, Tanner DB, McKenna R, Silverman DN. Cheers Balu
