1. Transition elements (d-block elements) have a tendency to form coordination compounds (complexes) with a species that has an ability to donate an electron pair to form a coordinate
2. Transition metal ions are small and highly charged and they have vacant low energy orbitals to accept an electron pair donated by other groups. Due to these properties, transition metals form large number of complexes.
3. Examples – [Fe(CN)6]4-, [CO(NH3)6]3+
