Formation of Low Spin Complexes by Elements of 3d, 4d, and 5d Series

Low spin complexes are those in which the electrons are paired up in the lower energy orbitals before they are filled in the higher energy orbitals. These complexes are usually formed by the elements of the 3d, 4d, and 5d series. Let's see how:

3d Series Elements

The 3d series elements have a partially filled d-orbital and can form low spin complexes due to the following reasons:

- The electrons in the d-orbitals experience strong repulsion from each other due to their close proximity.
- The pairing energy, i.e., the energy required to pair two electrons in the same orbital, is quite high in 3d series elements.
- The crystal field stabilization energy (CFSE), which is the energy difference between the high and low energy orbitals in a complex, is relatively low for these elements.

Due to these factors, the 3d series elements usually form low spin complexes.

4d and 5d Series Elements

The 4d and 5d series elements also have partially filled d-orbitals, but their d-orbitals are more diffuse compared to the 3d series elements. This means that the electrons in the d-orbitals are further away from each other and experience less repulsion. This, in turn, leads to a lower pairing energy for these elements.

However, the CFSE for 4d and 5d series elements is higher compared to the 3d series elements. This means that the energy difference between the high and low energy orbitals in a complex is larger for these elements. As a result, the 4d and 5d series elements can form both low spin and high spin complexes, depending on the ligand and the nature of the complex.

Conclusion

In conclusion, low spin complexes are generally formed by the elements of the 3d series, but the 4d and 5d series elements can also form low spin complexes depending on the ligand and the nature of the complex.