Why do transition elements form a large number of complex compounds?

(N/A) Complex compounds are those in which the metal ions bind a number of anions or neutral molecules,giving complex species with characteristic properties. Examples include: $[Fe(CN)_{6}]^{3-}$,$[Fe(CN)_{6}]^{4-}$,$[Cu(NH_{3})_{4}]^{2+}$,and $[PtCl_{4}]^{2-}$.
The transition elements have a great tendency to form complex compounds because of:
$(i)$ Small size of metal ions.
$(ii)$ High nuclear charge.
$(iii)$ Availability of vacant $d$-orbitals of suitable energy to accommodate lone pairs of electrons donated by the ligands.