Hydration Isomerism in Cr(III) and Co(III) [closed]








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Cr(III) and according to a paper I found also Co(III) are famous for the property called hydration isomerism. I already asked about how changes in the environment of a complex, for example temperature causes a ligand substitution but Cr(III) seems to be treated differently here. I guess it's somehow basically the same reaction but Cr(III) often adapts to changes quite slowly.


I once made a Cr(III) salt, which was green in solution and it took a week till the color suddenly changed to purple. On the other hand if I dissolve some anhydrous CrCl3 and wait for a day the purple color will change to green. This exchange of ligands and counter-ions or solvent seems to be a bit slower for Cr(III) and of course some reactions don't even happen for other metals. The chlorocuprate(II) would only form at elevated temperatures when in equilibrium with the aqua-complex.


But perhaps this temperature is just lowered for Cr(III) and room temperature is enough to cause the change here. Because according to a paper I found is this property of hydration isomerism something very unique in Cr(III) and Co(III) or d3 and d6 ions. 


But then I again I don't see that much of a difference to other changes in ligands. Most anhydrous compounds will change from a situation where the counter-ions are coordinated to the metal to an aqua-complex. They do it faster than Cr(III) and they do the opposite reaction but still I don't see that much difference here. 


The problem is that I cannot find any good explanation about what makes Cr(III) so special. Does this have to do with the d6 configuration? I know Cr(III) tends to act as an acid as well forming polynuclear compounds, so perhaps the Cr-OH2 bond is weaker than the Cr-Cl bond or something like that?


To summarize it and formulate is as a question, because it doesn't really matter if we give this property a name or not, what makes Cr(III) so special, that it will exchange its aqua ligands for chloride ligands when dissolved in water? Or the other way around? Because there are also reports where, if you dissolve the aqueous CrCl3 in water the green solution will turn purple and in my case the purple solution turned green. And with other Cr(III) complexes I watched transitions in both directions as well. 


