Why do d-block elements form complexes?

1. Transition elements form complexes readily because of the small size of the ions and higher nuclear charge, they possess high charge density.
2. Due to the presence of vacant d-orbitals in which a ligand can donate its electron, forms a complex.
3. Transition metal ions have a large polarizing power as a result they can attract lone pairs from other atoms to form complexes.
4. For example, elements like ${\mathrm{Fe}}^{+2}\mathrm{and}{\mathrm{Fe}}^{+3},{\mathrm{Cu}}^{+}\mathrm{and}{\mathrm{Cu}}^{+2}$ shows variable oxidation state and can form a large number of complexes.