WBJEE 2026 Physics and Chemistry Paper

Concept:The magnetic moment of a low spin $d^5$ metal complex depends on the number of unpaired electrons.Explanation:In $[\mathrm{Ru}(\mathrm{EDTA})]^{-}$, EDTA acts as a tetradentate/polydentate ligand with a $-4$ charge.Let oxidation state of Ru be $x$.$x + (-4) = -1$$x = +3$So Ru is in the $+3$ oxidation state: $\mathrm{Ru}^{3+}$.Ruthenium atomic number $= 44$.$\mathrm{Ru}^{3+}$ has $d^5$ electronic configuration.EDTA is a strong field ligand, so the complex is low spin.Low spin $d^5$: $t_{2g}^5 e_g^0$.Number of unpaired electrons, $n = 1$.Spin-only magnetic moment:$\mu = \sqrt{n(n+2)}$$\mu = \sqrt{1(1+2)}$$\mu = \sqrt{3}$$\mu = 1.73$ BM.Answer:Therefore, the calculated magnetic moment is $1.73$ BM, which matches option B.