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.
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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 sphere is the central atom/ion and the ligands enclosed in square brackets, such as [Fe(CN)₆]⁴⁻ in K₄[Fe(CN)₆]. Counter ions, like K⁺, are written outside the brackets.
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.
In [PtCl₆]²⁻, Pt has a coordination number of 6, and in [Ni(NH₃)₄]²⁺, Ni has a coordination number of 4. Similarly, [Fe(C₂O₄)₃]₃³⁻ and [Co(en)₃]³⁺ both have a coordination number of 6.