Question

Arrange the following coordination compounds in the decreasing order of magnetic moment. (A) $[NiCl_4]^{2-}$ (B) $[Ni(CN)_4]^{2-}$ (C) $[CoF_6]^{3-}$ (D) $[Fe(CN)_6]^{3-}$ Choose the correct answer from the options given below.

• A. [CoF_6]^{3-} > [NiCl_4]^{2-} > [Fe(CN)_6]^{3-} > [Ni(CN)_4]^{2-}
• B. [NiCl_4]^{2-} > [CoF_6]^{3-} > [Fe(CN)_6]^{3-} > [Ni(CN)_4]^{2-}
• C. [CoF_6]^{3-} > [NiCl_4]^{2-} > [Ni(CN)_4]^{2-} > [Fe(CN)_6]^{3-}
• D. [NiCl_4]^{2-} > [CoF_6]^{3-} > [Ni(CN)_4]^{2-} > [Fe(CN)_6]^{3-}

Answer 1

Answer: (A)

[CoF_6]^{3-} > [NiCl_4]^{2-} > [Fe(CN)_6]^{3-} > [Ni(CN)_4]^{2-}

Answer 2

To determine the order of magnetic moment, we need to find the number of unpaired electrons ($n$) for each complex and then use the formula $\mu = \sqrt{n(n+2)}$.

1. $[NiCl_4]^{2-}$: Ni$^{2+}$ is $d^8$. This is a tetrahedral complex with weak field ligands (Cl$^-$). The electron configuration is $e^4 t_2^2$. Number of unpaired electrons ($n$) = 2. Magnetic moment $\mu_A = \sqrt{2(2+2)} = \sqrt{2 \times 3} = \sqrt{8}$ B.M.

2. $[Ni(CN)_4]^{2-}$: Ni$^{2+}$ is $d^8$. This is a square planar complex with strong field ligands (CN$^-$). All electrons are paired. Number of unpaired electrons ($n$) = 0. Magnetic moment $\mu_B = \sqrt{0(0+2)} = 0$ B.M.

3. $[CoF_6]^{3-}$: Co$^{3+}$ is $d^6$. This is an octahedral complex with weak field ligands (F$^-$). It is a high spin complex. The electron configuration is $t_{2g}^4 e_g^2$. Number of unpaired electrons ($n$) = 4. Magnetic moment $\mu_C = \sqrt{4(4+2)} = \sqrt{4 \times 6} = \sqrt{24}$ B.M.

4. $[Fe(CN)_6]^{3-}$: Fe$^{3+}$ is $d^5$. This is an octahedral complex with strong field ligands (CN$^-$). It is a low spin complex. The electron configuration is $t_{2g}^5 e_g^0$. Number of unpaired electrons ($n$) = 1. Magnetic moment $\mu_D = \sqrt{1(1+2)} = \sqrt{1 \times 3} = \sqrt{3}$ B.M.

Now, arrange the magnetic moments in decreasing order: $\sqrt{24} > \sqrt{8} > \sqrt{3} > 0$ This corresponds to C > A > D > B.

Therefore, the decreasing order of magnetic moment is $[CoF_6]^{3-} > [NiCl_4]^{2-} > [Fe(CN)_6]^{3-} > [Ni(CN)_4]^{2-}$.