JEE Main 2025 Jan 24 Shift 2, Chemistry Q13: Colour, Magnetism And Stability Of Co-ordination Compounds

The configuration $t_{2g}^3{e}_g^1$ corresponds to a $d^4$ metal ion in an octahedral field with all four electrons unpaired, i.e. four parallel spins distributed across $t_{2g}$ and $e_g$ rather than being paired up in $t_{2g}$.

This happens only when the pairing energy $P$ is greater than the crystal-field splitting ${\Delta }_o$, which is the case for weak-field ligands. The result is a high-spin complex.

For comparison, a strong-field ligand on the same $d^4$ ion would force the configuration $t_{2g}^4{e}_g^0$ (low spin).

Correct option: (1) — weak field ligand, high spin complex