An octahedral complex of a d⁴ ion has Δo = 130 kJ/mol with a weak ligand and Δo = 310 kJ/mol with a…

Step 1. The weak ligand. $130\ \text{kJ/mol}$ is below $P = 280\ \text{kJ/mol}$. The fourth electron goes to the $e_g$ set. The arrangement is $t_{2g}^3 e_g^1$, high spin, with 4 unpaired electrons. Its CFSE is $(-1.2 + 0.6) \times 130 = -78\ \text{kJ/mol}$.

Step 2. The strong ligand. $310\ \text{kJ/mol}$ is above $P$. The fourth electron pairs up in $t_{2g}$. The arrangement is $t_{2g}^4$, low spin, with 2 unpaired electrons.

Step 3. Its CFSE. $-0.4 \times 4 \times 310\ \text{kJ/mol} = -496\ \text{kJ/mol}$.