For octahedral and tetrahedral complexes of the first-row transition metals, the correct statement(s) is/are (atomic numbers: Mn 25, Fe 26, Co 27, Ni…

Determine the metal oxidation state, the $d^n$ configuration and the field strength. In $\mathrm{[FeF_6]^{3-}}$, $\mathrm{Fe^{3+}}$ is $d^5$ with the weak-field $\mathrm{F^-}$, giving $t_{2g}^3e_g^2$ with five unpaired electrons, $\mu=\sqrt{5\times7}=5.92$ BM; in $\mathrm{[Fe(CN)_6]^{3-}}$ the strong-field $\mathrm{CN^-}$ gives $t_{2g}^5$ with one unpaired electron, $\mu=1.73$ BM: (A) is correct. $\mathrm{Co^{3+}}$ is $d^6$ and en is strong field, so $t_{2g}^6e_g^0$ and CFSE $=6(-0.4\Delta_o)=-2.4\Delta_o$: (B) is correct. $\mathrm{Ni^{2+}}$ ($d^8$) with $\mathrm{CN^-}$ is square planar and diamagnetic, whereas with the weak ligand $\mathrm{Cl^-}$ it is tetrahedral with two unpaired electrons: (C) is wrong. $\mathrm{Mn^{2+}}$ is $d^5$ but $\mathrm{CN^-}$ forces low spin, $t_{2g}^5$, leaving one unpaired electron, not five: (D) is wrong. Answer: A, B.