Complex Formation

The strong tendency of transition metal ions to form complexes arises from several features of their ions. They are relatively small with high positive charge, giving a high charge-to-size ratio and strong polarising power that attracts ligand electron pairs. Crucially, they also possess vacant d-orbitals of suitable energy that can accept lone pairs of electrons donated by ligands to form coordinate bonds. Together these allow transition metal ions to bind several ligands and form stable coordination complexes such as the hexamminecobalt or the hexacyanoferrate ions. The option of large size and low charge would weaken complex formation. The option of colourless nature is irrelevant to complexing ability. The option of low melting points is both false and unrelated. The basis of complex formation is a standard NCERT link between the d-block and coordination chemistry. It is worth emphasising that this is not a special case but a representative example of how complex formation operates throughout d- and f-block elements. This concept also bridges to Coordination Compounds and Chemical Bonding and Molecular Structure, so mastering it strengthens performance on linked questions from those topics as well. Plausibility check: the abundance of stable transition-metal complexes with diverse ligands directly reflects their small high-charge ions and accessible d-orbitals, confirming the answer.