The coordination number of the complex formed between Ni2+ and ethylenediamine en is 6. Ethylenediamine is a bidentate ligand, meaning it can form two coordinate bonds with the central metal ion. In the case of Ni2+, three ethylenediamine molecules can bind to the metal ion, resulting in a total of six coordinate bonds.The structure of the complex is [Ni en 3]2+. Since the coordination number is 6, the complex is likely to be octahedral in geometry.