This is because pairing in these (5d & 4d series) elements does not depend upon strength of ligand. it takes almost always, forming low spin complex, unlike 3-d series elements , pairing in 3d series elements depends upon strength of ligand

Low Spin Complexes Formed by d-series Elements

Low spin complexes are those complexes in which the pairing of electrons in the d-orbitals occurs after filling all the orbitals singly. It usually happens when the ligand field is weak, and the crystal field splitting energy is low. The elements of the d-series, which include transition metals, can form low spin complexes.

Formation of Low Spin Complexes

The formation of low spin complexes depends on several factors, including:

- The strength of the ligand field
- The number of unpaired electrons in the metal ion
- The crystal field splitting energy

When the ligand field is weak, the energy required to pair electrons in the d-orbitals is lower than the energy required to fill them singly. As a result, the electrons pair up, forming low spin complexes.

Elements of 4d and 5d Series

The 4d and 5d series of elements are transition metals that can form low spin complexes. These elements have partially filled d-orbitals, which can participate in bonding with ligands. Since the ligand field strength and crystal field splitting energy are important factors in the formation of low spin complexes, the elements of the 4d and 5d series can form such complexes depending on the ligands present.

Conclusion

In conclusion, low spin complexes are formed by the elements of the d-series, particularly the 4d and 5d series. However, the formation of low spin complexes depends on several factors, including the strength of the ligand field, the number of unpaired electrons, and the crystal field splitting energy.