D And F Block Elements

Given complex: [NiX₄]²⁻

Step 1: Oxidation state of Ni
The ligand X⁻ is uninegative and there are 4 of them. The overall charge on the complex is 2−.
Let oxidation state of Ni be +x:

Step 2: Electronic configuration of Ni²⁺
Ni atomic number = 28
Electronic configuration of Ni = [Ar] 4s² 3d⁸
Ni²⁺ = [Ar] 3d⁸

Step 3: Nature of ligand and hybridization
X⁻ is a weak field ligand (like Cl⁻, Br⁻, etc.), so it does not cause pairing of electrons.
Thus, the 3d⁸ configuration remains with two unpaired electrons.
Since the geometry is tetrahedral, the hybridization is sp³.

Answer: sp³ hybridization with two unpaired electrons.

Correct option: (A)

Let’s correctly determine the spin-only magnetic moment for the given complexes:

1. [FeCl₄]⁻:
   • Oxidation state of Fe: +3 (Fe → +3 because Cl⁻ × 4 = −4 and overall charge = −1 → Fe = +3)
   • Fe³⁺ → 3d⁵ configuration
   • Cl⁻ is a weak field ligand ⇒ high spin ⇒ 5 unpaired electrons
   • Spin-only magnetic moment:
   
2. [Fe(CN)₆]³⁻:
   • Oxidation state of Fe: +3 (same logic)
   • Fe³⁺ → 3d⁵ configuration
   • CN⁻ is a strong field ligand ⇒ low spin ⇒ 1 unpaired electron
   • Spin-only magnetic moment:
   

Correct answer: Option (A): 5.9 BM, 1.732 BM