JEE Main · 2019 · 12 Apr 2019 · Shift 1 · Coordination Compounds

With the strong-field phen ligand, $\mathrm{Fe}{}^{2+}$ (a $d^6$ ion) places all six electrons in the lower $t_{2g}$ set and enjoys a large crystal-field stabilization. On oxidation to $\mathrm{Fe}{}^{3+}$ the configuration becomes $d^5$; counting the stabilization while ignoring pairing energy, the five $d$ electrons are spread so that the stabilization sums to zero. The complex that therefore surrenders its crystal-field stabilization energy on oxidation to the $+3$ state is $[\mathrm{Fe}(\mathrm{phen}){}_3]{}^{2+}$.