Which of the following are paramagnetic? A. \ce{[NiCl4]^2-} B. \ce{Ni(CO)4} C. \ce{[Ni(CN)4]^2-} D. \ce{[Ni(H2O)6]^2+} E. \ce{Ni(PPh3)4}

Solution & Explanation

\textbf{Answer:} (A) A and D only \ce{( [NiCl4]^2- and [Ni(H2O)6]^2+ )} are paramagnetic. \textbf{Solution:} Evaluate each Ni species by oxidation state, $d$-count, ligand field strength and hybridisation: A. \ce{[NiCl4]^2-}: Ni(II), $3d^8$; \ce{Cl-} is weak-field, no pairing $\Rightarrow$ $sp^3$, tetrahedral, 2 unpaired electrons $\Rightarrow$ \textbf{paramagnetic}. B. \ce{Ni(CO)4}: Ni(0), $3d^{10}$; $sp^3$ tetrahedral, all paired $\Rightarrow$ diamagnetic. C. \ce{[Ni(CN)4]^2-}: Ni(II), $3d^8$; \ce{CN-} strong-field pairs the electrons $\Rightarrow$ $dsp^2$, square planar, all paired $\Rightarrow$ diamagnetic. D. \ce{[Ni(H2O)6]^2+}: Ni(II), $3d^8$ octahedral; \ce{H2O} weak/moderate field, 2 unpaired electrons $\Rightarrow$ \textbf{paramagnetic}. E. \ce{Ni(PPh3)4}: Ni(0), $3d^{10}$; all paired $\Rightarrow$ diamagnetic. Only A and D are paramagnetic, so option (A).