JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Coordination Compounds

In an octahedral $d^4$ complex, crystal field splitting determines whether electrons pair up before occupying the higher $e_g$ set. A strong field ligand produces a large splitting ${\Delta }_0$ that exceeds the pairing energy, so all four electrons crowd into $t_{2g}$ before any occupies $e_g$, giving the low-spin configuration $t_{2g}^4{e}_g^0$.

A weak field ligand produces a small ${\Delta }_0$, smaller than the pairing energy, so the electrons follow Hund's rule and singly occupy all five d-orbitals before any pairing occurs, giving the high-spin configuration $t_{2g}^3{e}_g^1$.

The condition that favours $t_{2g}^3{e}_g^1$ is a weak field ligand producing a high-spin complex.