The coordination number of a complex is the number of ligand attachment points to the central metal atom. In the nickel II complex [Ni en 3], the central metal atom is Ni, and the ligand is ethylenediamine en , which is a bidentate ligand. This means that each en ligand can attach to the central metal atom at two points.Since there are three en ligands in the complex, the coordination number of the complex is 3 ligands 2 attachment points per ligand = 6.The geometry of a complex with a coordination number of 6 is typically octahedral. In the case of [Ni en 3], the three en ligands are arranged around the central Ni atom in an octahedral geometry.