: 260
Structural isomerism in coordination compounds involves different connectivity or composition, not just spatial arrangement. Four main types exist:
Ionisation isomerism arises when ions exchange between the coordination sphere and outer sphere. [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4 have the same formula but produce different ions in solution, distinguishable by AgNO3 or BaCl2 tests. Conductivity measurements also differ.
Linkage isomerism occurs with ambidentate ligands that can coordinate through different donor atoms. NO2- bonds through N (nitro, yellow, more stable) or O (nitrito, red, kinetically formed). SCN- bonds through S (thiocyanato, with soft metals) or N (isothiocyanato, with hard metals).
Coordination isomerism requires salts with both cationic and anionic complex ions. Ligands exchange between the two metal centres: [Co(NH3)6][Cr(CN)6] vs [Cr(NH3)6][Co(CN)6].
Hydrate (solvate) isomerism involves different distributions of water between coordination sphere and crystal lattice. CrCl3.6H2O exists as three isomers with distinct colours and conductivities: [Cr(H2O)6]Cl3 (violet, 3 free Cl-), [Cr(H2O)5Cl]Cl2.H2O (blue-green, 2 free Cl-), and [Cr(H2O)4Cl2]Cl.2H2O (dark green, 1 free Cl-).
Polymerisation isomerism is not true isomerism — compounds have the same empirical formula but different molecular formulas, e.g., [Pt(NH3)2Cl2] and [Pt(NH3)4][PtCl4].