Q.Why do transition elements form complexes ?

Transition metal ions are strong Lewis acids — small, highly charged, and equipped with empty d-orbitals of suitable energy — which lets them readily accept electron pairs donated by ligands (Lewis bases) to form coordinate (dative) bonds, i.e. complexes.

Reasons transition elements readily form complexes:

1. Small ionic size and high nuclear charge: transition metal ions are comparatively small and carry a high positive charge, giving a high charge density (charge/size ratio) that strongly attracts electron-rich ligands.
2. Availability of vacant d-orbitals of suitable energy: transition metal ions have empty (or partially filled) $(n-1)d$ orbitals of energy comparable to the $ns$, $np$ orbitals. These accept lone pairs of electrons donated by ligands to form coordinate covalent bonds.
3. Variable oxidation states: transition elements show multiple oxidation states, letting them form a range of complexes of different stabilities and geometries. …