Question

The number of unpaired electrons responsible for the paramagnetic nature of the following complex species are respectively : $ [Fe(CN)_6]^{3-}, [FeF_6]^{3-}, [CoF_6]^{3-}, [Mn(CN)_6]^{3-} $

• A. 1, 5, 4, 2
• B. 1, 5, 5, 2
• C. 1, 1, 4, 2
• D. 1, 4, 4, 2

Hint:

To determine the number of unpaired electrons in coordination complexes, first find the oxidation state of the central metal ion and its \( d \) electron configuration. Then, consider the nature of the ligand (strong field or weak field) to determine the electron pairing in the \( d \) orbitals based on the magnitude of crystal field splitting (\( \Delta_o \) compared to the pairing energy \( P \)). Strong field ligands favor pairing in the lower energy \( t_{2g} \) orbitals, leading to low spin complexes, while weak field ligands favor high spin complexes with electrons occupying both \( t_{2g} \) and \( e_g \) orbitals according to Hund's rule.

Answer 1

The Correct Option is A

To ascertain the number of unpaired electrons contributing to the paramagnetism of the provided complex species, we must examine the electronic configuration of the central metal ion in each complex. The presence of unpaired electrons within the d-orbitals of the central metal ion confers paramagnetic properties upon the complex.

1. Complex: [Fe(CN)_6]^{3-}
   • The oxidation state of Fe in this complex is +3.
   • Electronic configuration of Fe: [Ar] 3d^6 4s^0
   • Fe³⁺ ion: [Ar] 3d^5
   • CN^- acts as a strong field ligand, inducing electron pairing in the 3d orbitals.
   • With all five d electrons paired, there is 1 unpaired electron.
2. Complex: [FeF_6]^{3-}
   • Oxidation state of Fe is +3.
   • Fe³⁺ ion: [Ar] 3d^5
   • F^- is a weak field ligand and does not cause electron pairing.
   • Consequently, all five d electrons remain unpaired, resulting in 5 unpaired electrons.
3. Complex: [CoF_6]^{3-}
   • Oxidation state of Co is +3.
   • Electronic configuration of Co: [Ar] 3d^7 4s^0
   • Co³⁺ ion: [Ar] 3d^6
   • F^- is a weak field ligand, leading to incomplete electron pairing.
   • This configuration yields 4 unpaired electrons.
4. Complex: [Mn(CN)_6]^{3-}
   • Oxidation state of Mn is +3.
   • Electronic configuration of Mn: [Ar] 3d^5 4s^2
   • Mn³⁺ ion: [Ar] 3d^4
   • CN^- is a strong field ligand, promoting electron pairing.
   • This results in 2 unpaired electrons.

Based on the preceding analysis, the number of unpaired electrons responsible for the paramagnetic nature of each complex species is, in order: 1, 5, 4, 2.