Why do transition elements form complex compounds?

by BY P GHOSH on November 10, 2021 in INORGANIC

Why do transition elements form complex compounds?

Most of the transition metal atoms or ions have a common tendency to form complex compoundscomplex ions when they are attached to the neutral molecules [such as CO, NO, NH₃, H₂O] or ions [Cl^–, CN^–, NO₂^–].

The above said neutral molecules or ions are called ligand. One or more atoms in ligand are capable to form co-ordinate bond with the atoms or ions of transition metal by sharing of electrons.

The inner (n–1)d orbital of transition metal atoms takes part in the formation of co-ordinate bond with the ligand atoms or ions.

For examples, [Fe(CN)₆]^4–, [Fe(CN)₆]^3–, [Co(NH₃)₆]³⁺, [PtCl₄]^2–, [Cu(NH₃)₄] ²⁺etc. Now the question arises, why do the transition elements form complex compound?

It has been experimentally found that most of the transition metal atoms show different types of oxidation state and they form complex compounds with different ligand in their different oxidation sate.

There are three principal causes of transition metal elements for the formation of complex compound.

Such as, small size of transition metal atoms, high electric charges of transition metal ions and presence of vacant d-orbitals in their outer most shell.

Small size and high positive charge of transition metal atoms favor for the formation of complex compounds.

Because of small size and high charge increase the positive charge density around the surface of transition metal ions.

As a result, the electron-pair donor ligands are easily attracted by the transition metal ions.

In this way, the transition metal ions accept two or more lone pair of electrons from ligand to their vacant d-orbitals resulting in the formation of L – M co-ordinate bond and finally the expected complex compounds.

For example, the transition metal Mn (I) form methylcyclopentadienyl manganese tricarbonyl complex with carbonyl and methylcyclopentadienyl ligands.