Which of the following are paramagnetic? A. $[\mathrm{NiCl}{}_4]{}^{2-}$ B. $\mathrm{Ni}(\mathrm{CO}){}_4$ C. $[\mathrm{Ni}(\mathrm{CN}){}_4]{}^{2-}$ D. $[\mathrm{Ni}(\mathrm{H}{}_2\mathrm{O}){}_6]{}^{2+}$ E. $\mathrm{Ni}(\mathrm{PPh}{}_3){}_4$

Solution & Explanation

\textbf{Answer:} (A) A and D only $([\mathrm{NiCl}{}_4]{}^{2-}\mathrm{and}[\mathrm{Ni}(\mathrm{H}{}_2\mathrm{O}){}_6]{}^{2+})$ are paramagnetic. \textbf{Solution:} Evaluate each Ni species by oxidation state, $d$-count, ligand field strength and hybridisation: A. $[\mathrm{NiCl}{}_4]{}^{2-}$: Ni(II), $3d^8$; $\mathrm{Cl}{}^{-}$ is weak-field, no pairing $\Rightarrow$ $s{p}^3$, tetrahedral, 2 unpaired electrons $\Rightarrow$ \textbf{paramagnetic}. B. $\mathrm{Ni}(\mathrm{CO}){}_4$: Ni(0), $3d^{10}$; $s{p}^3$ tetrahedral, all paired $\Rightarrow$ diamagnetic. C. $[\mathrm{Ni}(\mathrm{CN}){}_4]{}^{2-}$: Ni(II), $3d^8$; $\mathrm{CN}{}^{-}$ strong-field pairs the electrons $\Rightarrow$ $ds{p}^2$, square planar, all paired $\Rightarrow$ diamagnetic. D. $[\mathrm{Ni}(\mathrm{H}{}_2\mathrm{O}){}_6]{}^{2+}$: Ni(II), $3d^8$ octahedral; $\mathrm{H}{}_2\mathrm{O}$ weak/moderate field, 2 unpaired electrons $\Rightarrow$ \textbf{paramagnetic}. E. $\mathrm{Ni}(\mathrm{PPh}{}_3){}_4$: Ni(0), $3d^{10}$; all paired $\Rightarrow$ diamagnetic. Only A and D are paramagnetic, so option (A).