JEE Main · 2026 · 02 Apr 2026 · Shift 2 · Coordination Compounds

Coordination isomerism arises in a salt built from a complex cation and a complex anion when redistributing ligands between the two coordination spheres yields a genuinely different compound; this requires the two centres to be distinguishable, in practice two different metals (or one metal in two oxidation states). In $[\mathrm{Ag}(\mathrm{NH}{}_3){}_2][\mathrm{Ag}(\mathrm{CN}){}_2]$ both spheres hold silver in the same state, so swapping $\mathrm{NH}{}_3$ and $\mathrm{CN}{}^{-}$ merely regenerates the same salt and no isomer is possible; the same reasoning excludes $[\mathrm{Co}(\mathrm{NH}{}_3){}_6][\mathrm{Co}(\mathrm{CN}){}_6]$, where both spheres are cobalt(III). The other three salts each pair two different metals, so their ligands can be reshuffled into a distinct coordination isomer. Hence coordination isomerism is shown only by $[\mathrm{Co}(\mathrm{NH}{}_3){}_6][\mathrm{Cr}(\mathrm{CN}){}_6]$, $[\mathrm{Fe}(\mathrm{NH}{}_3){}_6][\mathrm{Co}(\mathrm{CN}){}_6]$ and $[\mathrm{Co}(\mathrm{NH}{}_3){}_6][\mathrm{Fe}(\mathrm{CN}){}_6]$.