Structural Isomerism in Coordination Compounds

Structural isomers of a coordination compound share exactly the same overall chemical formula (the same atoms in the same total quantities) but differ in which groups are actually bonded to the metal, inside the coordination sphere, versus which groups sit outside it as free counter ions — or, in one variant, in exactly which atom of an ambidentate ligand is doing the bonding. Several distinct types are recognized.

Ionization isomerism occurs when a compound can produce different ions in solution depending on whether a particular group sits inside the coordination sphere (bonded to the metal) or outside it as a free counter ion, with two such groups effectively swapping places between two isomers. $[\text{Co}(\text{NH}_3)_5\text{Br}]\text{SO}_4$ and $[\text{Co}(\text{NH}_3)_5\text{SO}_4]\text{Br}$ are ionization isomers: in the first, $\text{Br}^-$ is coordinated to cobalt and free $\text{SO}_4^{2-}$ is the counter ion, so its solution gives a white precipitate with $\text{BaCl}_2$ (a positive test for free sulfate) but no immediate precipitate with $\text{AgNO}_3$; in the second, $\text{SO}_4^{2-}$ is coordinated and free $\text{Br}^-$ is the counter ion, giving exactly the opposite pattern — a pale yellow $\text{AgBr}$ precipitate with $\text{AgNO}_3$ but no immediate reaction with $\text{BaCl}_2$.

Hydrate isomerism (sometimes called solvate isomerism more generally) is a specific case of ionization isomerism in which water is the group that moves between being coordinated and being free (as water of crystallization). The classic example is $\text{CrCl}_3.6\text{H}_2\text{O}$, long known to exist as three distinct, differently coloured compounds: violet $[\text{Cr}(\text{H}_2\text{O})_6]\text{Cl}_3$ (all three chlorides free, giving 3 moles of $\text{AgCl}$ on treatment with excess $\text{AgNO}_3$), grey-green $[\text{Cr}(\text{H}_2\text{O})_5\text{Cl}]\text{Cl}_2.\text{H}_2\text{O}$ (only two chlorides free, giving 2 moles of $\text{AgCl}$, with one water molecule outside the coordination sphere as water of crystallization), and dark green $[\text{Cr}(\text{H}_2\text{O})_4\text{Cl}_2]\text{Cl}.2\text{H}_2\text{O}$ (only one chloride free, giving 1 mole of $\text{AgCl}$, with two waters of crystallization).

Linkage isomerism occurs specifically with an ambidentate ligand (Section 5.2), which can bind the metal through either of two different donor atoms. $[\text{Co}(\text{NH}_3)_5(\text{NO}_2)]\text{Cl}_2$ (the nitro isomer, cobalt bonded through nitrogen, $\text{Co-NO}_2$) and $[\text{Co}(\text{NH}_3)_5(\text{ONO})]\text{Cl}_2$ (the nitrito isomer, cobalt bonded through oxygen, $\text{Co-ONO}$) are linkage isomers of one another; the nitrito form is the less stable of the two and slowly converts into the nitro form on standing or gentle heating. …