Identify the homoleptic complex(es) that is/are low spin.
(A) [Fe(CN)₅ NO]²⁻
(B) [CoF₆]³⁻
(C) [Fe(CN)₆]⁴⁻
(D) [Co(NH₃)₆]³⁺
(E) [Cr(H₂ O)₆]²⁺
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Apply two filters: keep only same-ligand complexes, then keep those whose d electrons pair up.

1. Homoleptic first. A homoleptic complex has only one kind of ligand around the metal. [Fe(CN)₅NO]²⁻ has both CN⁻ and NO, so it is heteroleptic and is out before spin is even checked.
2. Then spin. Find the metal's oxidation state and d-electron count. A strong-field ligand makes Δₒ larger than the pairing energy, so electrons pair up in t_(2g) and the complex is low spin. A higher metal charge (+3) also raises Δₒ.

   Complex	Homoleptic?	Metal ion	Ligand and configuration	Spin
   (A) [Fe(CN)₅NO]²⁻	No, CN⁻ and NO	Fe²⁺, d⁶	strong field, t_(2g)⁶	low, but not homoleptic
   (B) [CoF₆]³⁻	Yes	Co³⁺, d⁶	F⁻ weak, t_(2g)⁴e_g²	high (4 unpaired)
   (C) [Fe(CN)₆]⁴⁻	Yes	Fe²⁺, d⁶	CN⁻ strong, t_(2g)⁶	low (0 unpaired)
   (D) [Co(NH₃)₆]³⁺	Yes	Co³⁺, d⁶	NH₃ with Co³⁺, t_(2g)⁶	low (0 unpaired)
   (E) [Cr(H₂O)₆]²⁺	Yes	Cr²⁺, d⁴	H₂O weak, t_(2g)³e_g¹	high (4 unpaired)

3. So. Only (C) and (D) are both homoleptic and low spin, so the answer is (a).

The idea to keep. Read the question's filters one at a time. For spin, CN⁻ always pairs electrons, F⁻ and H₂O usually do not, and NH₃ with a +3 metal like Co³⁺ gives low spin.

Answer from the NTA final answer key. Solution by PrepWiser.