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.
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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.
The coordination number (CN) of a metal ion in a complex is the count of ligand donor atoms directly bonded to the metal. It determines the spatial arrangement around the central metal ion.
Ambidentate ligands, like NO₂⁻ and SCN⁻ ions, have two different donor atoms. NO₂⁻ can coordinate through nitrogen or oxygen, while SCN⁻ can coordinate through sulfur or nitrogen, offering flexibility in metal binding.