The cations of d-block elements have strong tendency to form complexes with certain molecules (e.g. CO, NO, NH3….etc) or ions (e.g. F-, Cl-, CN- ….etc) called ligands. Their tendency to form complexes is due to two reasons.
i) Small size and high positive charge density.
ii) Presence of vacant (n-1)d orbitals which are of appropriate energy to accept lone pair and unshared pair of electrons from the ligands for bonding with them.
Examples of some complex compounds are,
[Cu(NH3)4]2+, [Ag(NH3)2]+, [Fe(CN)6]4-,….etc.