Cobalt solvation entropy

Apart from its own susceptibility to oxidation or reduction, a solvent can affect redox equilibria by modifying the relative stabilities of oxidation states of solutes. Thus Cu+ is unstable in aqueous solution to disproportionation (Section 5.4) but it is quite stable in acetonitrile. This arises from the relative magnitudes of the solvation energies and entropies of Cu+ and Cu2+ in the different solvents. In ammonia, cobalt(III) is much more stable relative to cobalt(II) than in water. The... 

More significant catalytically may be the role of an active-site tyrosine [28, 29], which appears to sterically drive the adenosyl group off the Co, as a result of a conformational change upon substrate binding. EPR spectroscopy provides information about the distance of the dominant product radical from cob(ii)alamin [30]. Finally, it is important to consider the role of entropy and the solvating power of the protein in promoting cobalt-carbon bond fission [31]. 

Whereas considerations of transition enthalpies and of activation volumes have led to evidence useful in confirming the mechanism of aquation in cobalt(m)-ammin6-ligand complexes, an attempt to derive similarly useful information from a comparison of kinetic and solubility parameters proved unsuccessful. No correlation was found between enthalpies of activation for aquation of complexes [Co(NH3)5X] +, with X = F, Cl, Br, I, NCS, N3, or NO2, and the respective enthalpies of solution of their perchlorate salts. In fact it seems impossible to correlate these activation enthalpies with any other apparently reasonable parameter, though it does seem that there is a correlation between activation entropies and the entropies of solvation of the leaving anions. ... 

---