The coordination number of a central metal atom in a complex is determined by

The co-ordination number of a central metal atom in a complex is determined by the number of ligands around a metal ion bonded by sigma bonds. It is also define as the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbors in a complex. For example, in Ni(CO)4​, four carbonyl ligands are bonded to Ni atom. Hence, the coordination  number of Ni in Ni(CO)4​ is four.

The number of ligands around a central metal atom bounded by sigma bonds is known as coordination number.