Pertaining to

It is important to note here that coordination number of the central atom/ion is determined only by the number of sigma bonds formed by the ligand with the central atom/ion. Pi bonds, if formed between the ligand and the central atom/ion, are not counted for this purpose. (e) Coordination sphere The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the coordination sphere . The ionisable groups are written outside the bracket and are called counter ions.

For example, in the complex K [Fe(CN) ], the coordination sphere is [Fe(CN) ] – and the counter ion is K + . (f) Coordination polyhedron The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and