Question

Why do transition elements form complexes?

Answer 1

The cations of d-block elements have strong tendency to form complexes with certain molecules $(e.g.CO,NO,N{H}_3...etc)$ or ions $(e.g.F^{-},C{l}^{-},C{N}^{-})$ called ligands. Their tendency to form complexes is due to two reasons.

1. Small size and high positive charge density of ions of transition metals.
2. Presence of vacant $(n-1)d$ orbitals which are of appropriate energy to accept one pair and unshared pair of electrons from the ligands for bonding with them. e.g. of some complex compounds are:

${\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+},{\left[Ag{\left(N{H}_3\right)}_2\right]}^{{}^{+}},{\left[Fe{\left(CN\right)}_6\right]}^{4-}...$etc.