Question

Answer the question on the basis of the passage given below:
Transition metals form a large number of complexes or coordination compounds in which the metal atoms are bound to a number of anions or neutral molecules. The valence bond theory explains the formation, magnetic behaviour, and geometrical shapes, while the crystal field theory explains the effect of different crystal fields on the degeneracy of \(d\)-orbitals energies of the central metal atom/ion. This provides for the quantitative estimation of orbital separation energies, magnetic moments, and spectral and stability parameters.
Amongst the following ions, which should have the highest magnetic moment value?

• A. \( [NiCl_4]^{2-} \)
• B. \( [Mn(CN)_6]^{4-} \)
• C. \( [Cr(NH_3)_6]^{3+} \)
• D. \( [CoF_6]^{3-} \)

Hint:

The magnetic moment of a complex is directly related to the number of unpaired electrons. Complexes with strong field ligands (such as CN⁻) usually have low magnetic moments, while those with weak field ligands (such as NH₃ or F⁻) tend to have higher magnetic moments due to the presence of unpaired electrons.

Answer 1

The Correct Option is B

The magnetic moment of a metal complex depends on the number of unpaired electrons in the d-orbitals of the central metal ion. The formula for magnetic moment \( \mu_{\text{eff}} \) is given by: \[ \mu_{\text{eff}} = \sqrt{n(n+2)} \] Where \( n \) is the number of unpaired electrons. The more unpaired electrons there are, the higher the magnetic moment. Let’s analyze the complexes: 
- (1) \( [NiCl_4]^{2-} \): For Ni²⁺, which has an electronic configuration of \( 3d^8 \), in the presence of chloride ions (Cl⁻), a weak field ligand, it results in two unpaired electrons. The magnetic moment will be moderate. 
- (2) \( [Mn(CN)_6]^{4-} \): \( \text{Mn}^{2+} \) has an electronic configuration of \( 3d^5 \), and \( \text{CN}^- \) is a strong field ligand, so it causes pairing of the electrons. In this complex, no unpaired electrons are present, making the magnetic moment zero (since \( [Mn(CN)_6]^{4-} \) is diamagnetic). 
- (3) \( [Cr(NH_3)_6]^{3+} \): Cr³⁺ has an electronic configuration of \( 3d^3 \), and ammonia (NH₃) is a weak field ligand, so there will be three unpaired electrons, making this complex paramagnetic with a relatively high magnetic moment. 
- (4) \( [CoF_6]^{3-} \): Co³⁺ has an electronic configuration of \( 3d^6 \), and fluoride ions (F⁻) are weak field ligands. This results in four unpaired electrons, so the complex will have a high magnetic moment. 
Thus, the highest magnetic moment will be found in \( [CoF_6]^{3-} \) and \( [Cr(NH_3)_6]^{3+} \), with Cr³⁺ having a relatively higher number of unpaired electrons compared to the other complexes. The correct answer is Option (2).