Question

Consider the following complex compounds (A) \([Ni(NH_3)_6]^{2+}\) (B) \([NiCl_4]^{2-}\) (C) \([Ni(en)_3]^{2+}\) The number of unpaired electrons in A, B and C respectively and order of frequency of absorbed radiation among A, B and C is:

• A. \(2, 2, 2\) and \(C > A > B\)
• B. \(0, 2, 2\) and \(A > B > C\)
• C. \(2, 2, 0\) and \(A > C > B\)
• D. \(2, 2, 2\) and \(C > B > A\)

Answer 1

The Correct Option is A

The question involves the spectrochemical series and crystal field theory to determine the number of unpaired electrons and the order of frequency of absorbed radiation for the given complex compounds: \([Ni(NH_3)_6]^{2+}\), \([NiCl_4]^{2-}\), and \([Ni(en)_3]^{2+}\).

1. Firstly, consider the oxidation state and electronic configuration of Nickel: Nickel (Ni) in its elemental form is \([Ar] 3d^{8} 4s^{2}\). For the \([Ni(NH_3)_6]^{2+}\), \([NiCl_4]^{2-}\), and \([Ni(en)_3]^{2+}\) complexes, nickel is in the +2 oxidation state, having a configuration of \([Ar] 3d^{8}\).
2. Next, determine the ligand field strengths:
   • Spectrochemical Series Order (from strong field to weak field ligands): \(en > NH_3 > Cl^-\)
3. Evaluate the complexes:
   • \([Ni(NH_3)_6]^{2+}\): Ammonia \((NH_3)\) is a moderate field ligand. This complex forms an octahedral configuration. The d-orbitals split into \(t_{2g}\) and \(e_g\) with 2 unpaired electrons.
   • \([NiCl_4]^{2-}\): Chloride \((Cl^-)\) is a weak field ligand, resulting in the formation of a tetrahedral complex with 2 unpaired electrons.
   • \([Ni(en)_3]^{2+}\): Ethylenediamine (en) is a strong field ligand. Despite forming an octahedral complex, it still results in 2 unpaired electrons due to insufficient splitting to pair all d electrons.
4. The order of unpaired electrons is \(2, 2, 2\) for \([Ni(NH_3)_6]^{2+}\), \([NiCl_4]^{2-}\), and \([Ni(en)_3]^{2+}\) respectively.
5. To determine the order of frequency of absorbed light: The frequency of absorbed radiation relates directly to the ligand field strength. The stronger the field, the greater the energy difference (\(\Delta\)) between the split d-orbitals:
   • Since ethylenediamine \((en)\) > ammonia \((NH_3)\) > chloride \((Cl^-)\) in field strength, the order of the energies follows similarly: \([Ni(en)_3]^{2+}\) > \([Ni(NH_3)_6]^{2+}\) > \([NiCl_4]^{2-}\).

Therefore, the correct answer is: number of unpaired electrons is \(2, 2, 2\) and the order of frequency of absorbed radiation is \(C > A > B\).