AIIMS 2005 Chemistry Magnetic Properties MCQ Question

To determine the diamagnetic species among the given options, we need to analyze the magnetic properties of the coordination complexes based on their electronic configurations and the presence or absence of unpaired electrons.

Key Concepts

Diamagnetism: A substance is said to be diamagnetic if all its electrons are paired, resulting in a net magnetic moment of zero. This means that a diamagnetic substance will not be attracted to a magnetic field.

Paramagnetism: Conversely, a paramagnetic substance contains unpaired electrons, leading to a net magnetic moment and attraction to a magnetic field.

Analyzing Each Option

1. (a) [Ni(CN)₄]²⁻:

   • Nickel (Ni) has an atomic number of 28 and an electronic configuration of $[Ar] 3d^8 4s^2$. In the oxidation state of +2, Ni will have the configuration of $[Ar] 3d^8$.
   • CN⁻ is a strong field ligand and causes pairing of the electrons in the 3d subshell.
   • In [Ni(CN)₄]²⁻, the 3d orbitals will be fully paired: $3d^{10}$, leading to zero unpaired electrons.
   • Conclusion: [Ni(CN)₄]²⁻ is diamagnetic.

2. (b) [NiCl₄]²⁻:

   • In this case, Cl⁻ is a weak field ligand, which does not cause electron pairing.
   • The electronic configuration remains as $3d^8$, with two unpaired electrons.
   • Conclusion: [NiCl₄]²⁻ is paramagnetic.

3. (c) [CoF₆]³⁻:

   • Cobalt (Co) has an atomic number of 27 and an electronic configuration of $[Ar] 3d^7 4s^2$. In the +3 oxidation state, Co will have the configuration of $[Ar] 3d^6$.
   • F⁻ is also a weak field ligand, leading to unpaired electrons.
   • In [CoF₆]³⁻, the 3d electrons will not be fully paired, resulting in unpaired electrons.
   • Conclusion: [CoF₆]³⁻ is paramagnetic.

4. (d) [CoF₆]²⁻:

   • In [CoF₆]²⁻, Co is in the +2 oxidation state, giving it a configuration of $[Ar] 3d^7$.
   • Similar to [CoF₆]³⁻, the weak field nature of F⁻ does not cause electron pairing, leading to unpaired electrons.
   • Conclusion: [CoF₆]²⁻ is paramagnetic.

Summary

• The only diamagnetic species among the options provided is [Ni(CN)₄]²⁻ due to all electrons being paired.
• The other options contain unpaired electrons and are thus paramagnetic.

Final Answer

The correct answer is (a) [Ni(CN)₄]²⁻.