If Δ_o < pairing energy, then what type of complex would be formed?

High spin complex

The correct answer is option 1. High spin complex.

When the crystal field splitting parameter (\( \Delta_o \)) is less than the pairing energy of the electron, it indicates that the energy required to pair electrons in the lower energy orbitals is less than the energy gained by transferring electrons to the higher energy orbitals due to the crystal field splitting.

In such scenarios, the electrons prefer to occupy the higher energy orbitals rather than pairing up in the lower energy orbitals. This preference arises because the energy penalty for pairing up in the lower energy orbitals is greater than the energy gained by occupying the higher energy orbitals.

As a result, the electrons are distributed among the d orbitals in a way that maximizes the number of unpaired electrons, leading to the formation of high spin complexes. In high spin complexes, the number of unpaired electrons is maximized, even if it means pairing up electrons in higher energy orbitals. This phenomenon typically occurs when the ligand field is weak or the metal ion has a high oxidation state.

Therefore, when \( \Delta_o \) is less than the pairing energy, high spin complexes are formed, as electrons prefer to occupy the higher energy orbitals to maximize the number of unpaired electrons.