Linkage isomerism occurs in complexes with ambidentate ligands. For instance, thiocyanate ligand (NCS⁻) can bind through nitrogen (M–NCS) or sulphur (M–SCN), illustrated by complexes like [Co(NH₃)₅(NO₂)]Cl₂.
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Coordination isomerism involves the interchange of ligands between cationic and anionic entities. In [Co(NH₃)₆][Cr(CN)₆], NH₃ binds to Co³⁺ and CN⁻ to Cr³⁺. In its coordination isomer [Cr(NH₃)₆][Co(CN)₆], the ligand binding arrangement switches.
Ionisation isomerism occurs when the counter ion in a complex salt becomes a ligand. In [Co(NH₃)₅(SO₄)]Br, SO₄ displaces a ligand, while in [Co(NH₃)₅Br]SO₄, Br⁻ becomes the ligand. This is similar to ‘hydrate isomerism’ when water is involved as a solvent.
A coordination polyhedron is the spatial arrangement of ligand atoms directly attached to the central atom/ion. Common polyhedra include octahedral (e.g., [Co(NH₃)₆]³⁺), tetrahedral (e.g., [Ni(CO)₄]), and square planar (e.g., [PtCl₄]²⁻).
The coordination number is determined by the number of sigma bonds formed by ligand donor atoms directly with the central metal atom/ion. Pi bonds, if present, are not considered when determining the coordination number.