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The Atomic numbers of Mn, Fe and Co are 25, 26 and 27 respectively. The order of Magnetic moments among the following is

$(a)\; [MnCl_4]^{2-} \rightarrow [Fe(CN)_6]^{4-} \rightarrow [CoCl_4]^{2-}$ $(b)\; [Fe(CN)_6]^{4-}\rightarrow [MnCl_4]^{2-}\rightarrow [CoCl_4]^{2-}$ $(c)\; [MnCl_4]^{2-} \rightarrow[CoCl_4]^{2-} \rightarrow [Fe(CN)_6]^{4-}$ $(d)\; [Fe(CN)_6]^{4-}\rightarrow [CoCl_4]^{2-} \rightarrow [MnCl_4]^{2-}$

Answer: $[MnCl_4]^{2-} \rightarrow[CoCl_4]^{2-} \rightarrow [Fe(CN)_6]^{4-}$
$Mn^{2+}$, $Co^{2+}$ and $Fe^{2+}$ have 5, 3 and 0 unpaired electrons respectively, therefore the correct order is $[MnCl_4]^{2-} \rightarrow[CoCl_4]^{2-} \rightarrow [Fe(CN)_6]^{4-}$
edited Aug 5, 2014