Explain why transition metals form complex compounds.

Explanation

Transition metals are d-block elements with valence shell configuration `ns^2(n-1) d^(1-10)`. These metals and their ions easily coordinate to a number of negative ions or neutral molecules having lone pairs of electrons to form complex. This is due to :
(1) Small size and high nuclear charge of these metals.
(2) Availability of vacant d-orbitals of suitable energy to accommodate lone pairs of electrons donated by the ligands.