JEE Main 2025 Apr 4 Shift 2, Chemistry Q16: Colour, Magnetism And Stability Of Co-ordination Compounds

Among the four Fe(III) complexes, the most stable is $[Fe(C_2{O}_4{)}_3{]}^{3-}$. Oxalate is a bidentate ligand and forms five-membered chelate rings with Fe(III); the chelate effect gives this complex extra thermodynamic stability over the monodentate-ligand complexes.

$F{e}^{3+}$ is a $d^5$ ion. Oxalate lies below $H_{2}O$ in the spectrochemical series, so it is a weak-to-moderate field ligand and the complex is high spin: $t_{2g}^3{e}_g^2$ (magnetic moment $\approx 5.9$ BM, five unpaired electrons — experimentally established).

Number of electrons in $t_{2g}$ orbitals: $X=3$.

So the oxide is $V_2{O}_3$. The oxides of vanadium become more acidic as the oxidation state rises: $V_2{O}_3$ (V in +3) is basic, $V{O}_2$ is amphoteric, and $V_2{O}_5$ is amphoteric.

$\therefore V_2{O}_X=V_2{O}_3$ is basic.