= Solution
<iron> remains Fe(II), $d^6$, in both states; the change is a <spin crossover>, rather than a change in <oxidation state>. At the higher <temperature> the <high-spin complex> has $t_{2g}^4e_g^2$: four <unpaired electrons>, $S=2$, and
$$
\mu_{\mathrm{eff}}=\sqrt{n(n+2)}\,\mu_B=\sqrt{24}\,\mu_B=4.90\,\mu_B.
$$
This matches the observed moment. At low <temperature> a <low-spin complex> with $t_{2g}^6e_g^0$ has $S=0$ and shows <diamagnetism>. \b[Cooling switches high-<spin> $d^6$ to low-<spin> $d^6$.] Emptying the <metal>–ligand antibonding $e_g$ orbitals strengthens and shortens the Fe–N bonds, accounting for the contraction by roughly $0.2\ \text{Å}$.
The <octahedral splitting> and <electron pairing energy> must be sufficiently close for both states to be accessible. The low-<spin> state gains electronic energy, while the high-<spin> state has greater <spin> degeneracy and usually greater vibrational <entropy> because of its softer bonds. The resulting <Gibbs free energy> balance favors high <spin> at higher <temperature>. The two endpoint observations do not by themselves specify <hysteresis> or transition cooperativity; those require a <temperature> series. Coupled structural and magnetic measurements distinguish this explanation from a mere bond-length anomaly.
Back to article page
