Q.$[\text{Co}(\text{NH}_3)_6][\text{Cr}(\text{CN})_6]$ and $[\text{Cr}(\text{NH}_3)_6][\text{Co}(\text{CN})_6]$ have identical overall composition. Name the type of isomerism shown, and explain how the two compounds differ structurally.

[!TLDR] Coordination isomerism — ligands redistribute between the two different metal centres in a cation+anion complex pair. [!ANSWER] $[\text{Co}(\text{NH}_3)_6][\text{Cr}(\text{CN})_6]$ and $[\text{Cr}(\text{NH} …

Coordination isomerism can only occur in a compound that contains both a complex cation and a complex anion (rather than a complex ion paired with a simple counter ion). Here, both $[\text{Co}(\text{NH}_3)_6][\text{Cr}(\text{CN})_6]$ and $[\text{Cr}(\text{NH}_3)_6][\text{Co}(\text{CN})_6]$ contain exactly the same overall atoms — one cobalt, one chromium, six ammonia, six cyanide — but the two ligand types are distributed differently between the two metal centres. In the first compound, cobalt is the centre of the complex cation, holding all six $\text{NH}_3$ ligands ($[\text{Co}(\text{NH}_3)_6]^{3+}$), while chromium is the centre of the complex anion, holding all six $\text{CN}^-$ ligands ($[\text{Cr}(\text{CN})_6]^{3-}$). In the second compound, this assignment is exactly reversed: chromium now holds all six $\text{NH}_3$ ($[\text{Cr}(\text{NH}_3)_6]^{3+}$) and cobalt now holds all six $\text{CN}^-$ ($[\text{Co}(\text{CN})_6]^{3-}$). The two compounds are structurally distinct (different metal-lig …

Recognize that coordination isomerism requires both a complex cation and a complex anion in the same compound; compare whic …

Trying to apply this type of isomerism to a compound with only ONE complex ion and a simple counter ion — coordination isomerism specifically requires TWO d …