Question

Would you expect the ligand substitution reactions of transition metal ions to be faster or slower than those of lanthanide ions and the pre-transition metal ions? Explain why.

Answer 1

Ligand substitution reaction involves the exchange of one ligand for another, with no change in oxidation state at the metal center.

Ligand substitution reactions of transition metal ions are faster than those of lanthanide ions and the pre-transition metal ions due to the following reasons:

• Generally, complexes formed with lanthanides are unstable. f-orbitals of lanthanide ions are diffused (i.e., larger), so these orbitals extend towards ligands and overlap with ligands orbitals more effectively and thus a strong Metal-Ligand bond is formed and also due to sterric hinderance among the ligands round the metal cations, there is less tendency for a given ligand to move from cordination sphere. And hence ligand substitution becomes difficult.
• The pre-transition metal (alkali metals and alkaline earth metals) ions are highly electropositive and for complex formation metal should have tendency to disperse electron density coming from ligand, as a result tendency of complex formation of pre transition metal ions is not as good as transition metal ions. Hence, ligand substituion reaction would be slower than transition metal ions.