Which pair of complexes has the same spin-only magnetic moment but the central metal ion in the two members has different hybridisation (use the…

Work out the $d$-configuration, number of unpaired electrons $n$ and hybridisation. A: $\mathrm{Fe^{2+}}$ ($d^6$) and $\mathrm{Co^{3+}}$ ($d^6$), both high-spin, $n=4$, both $sp^3d^2$, so the same moment and the same hybridisation. B: $\mathrm{Fe^{3+}}$ low-spin $d^5$ has $n=1$, whereas $\mathrm{Cr^{3+}}$ $d^3$ has $n=3$, so the moments differ. C: $\mathrm{Ni^{2+}}$ ($d^8$) with $\mathrm{CN^-}$ and $\mathrm{Pt^{2+}}$ ($d^8$) are both square planar, $n=0$, $dsp^2$, again the same hybridisation. D: $\mathrm{Mn^{3+}}$ is $d^4$, which with strong-field $\mathrm{CN^-}$ pairs up to $t_{2g}^4$, so $n=2$, $d^2sp^3$ (octahedral). $\mathrm{Ni^{2+}}$ in the tetrahedral chloride has $n=2$, $sp^3$. Both have $\mu=\sqrt{2\times4}=2.83\ \mathrm{BM}$, but the hybridisation differs. Answer: D.