Which of the following complex ions is not expected to absorb visible light?

Step-by-step solution

Key Idea For the absorption of visible light, presence of unpaired d-electrons is the necessity. (a) In [ Ni ( CN )₄ ]²⁻, Ni is present as Ni ²⁺ . Ni ²⁺=[ Ar ] 3 ~d ^8 4 ~s ^0 (Pairing occurs because CN ⁻ is a strong field ligand). Since, in [ Ni ( CN )₄ ]²⁻ , no unpaired electron is present in d-orbitals, it does not absorb visible light. (b) In [ Cr ( NH ₃ )₆ ]³⁺, Cr is present as Cr ³⁺ . Cr ³⁺=[ Ar ] 3 ~d ^3 4 ~s ^0 (Three unpaired electrons) (c) In [ Fe ( H ₂ O )₆ ]²⁺, Fe is present as Fe ²⁺ . Fe ²⁺=[ Ar ] 3 ~d