Question

Match the following.

• A. A-IV, B-I, C-III, D-II
• B. A-I, B-II, C-III, D-IV
• C. A-IV, B-III, C-I, D-II
• D. A-I, B-III, C-II, D-IV

Hint:

The spectrochemical series helps determine the relative strengths of ligands and their effect on the color of complexes.

Answer 1

The Correct Option is C

To match the complex ions in List I with their corresponding colors in List II, we need to rely on knowledge of coordination chemistry and the typical colors associated with nickel(II) complexes based on their ligands and geometry. Nickel(II) (Ni²⁺) typically forms complexes with a d⁸ configuration, and the color of these complexes depends on the ligand field strength, geometry (e.g., octahedral, tetrahedral, or square planar), and the resulting d-d transitions.

• A. [Ni(en)₃]²⁺: The ligand "en" stands for ethylenediamine, a bidentate ligand. With three en ligands, this forms an octahedral complex (since 3 bidentate ligands occupy 6 coordination sites). Nickel(II) octahedral complexes with strong-field ligands like ethylenediamine typically exhibit a violet or purple color due to the absorption of light in the yellow-green region of the spectrum.
  Match: IV. Violet
• B. [Ni(H₂O)₄(en)]²⁺: Here, there are 4 water (H₂O) ligands and 1 ethylenediamine (en) ligand. Since en is bidentate, this also forms an octahedral complex (4 H₂O + 2 sites from en = 6 coordination sites). Water is a weaker field ligand compared to en, but the presence of en strengthens the overall ligand field compared to a pure [Ni(H₂O)₆]²⁺ complex. The [Ni(H₂O)₆]²⁺ complex is typically green, but substituting with a stronger ligand like en shifts the absorption slightly, often resulting in a bluish or pale blue color.
  Match: III. Pale blue
• C. [Ni(H₂O)₆]²⁺: This is the hexaaqua nickel(II) complex, a classic octahedral complex with 6 water ligands. It is well-known in coordination chemistry for its green color, caused by the absorption of light in the red region of the spectrum due to d-d transitions in the octahedral field.
  Match: I. Green
• D. [Ni(H₂O)₂(en)₂]²⁺: This complex has 2 water ligands and 2 ethylenediamine ligands. Each en occupies 2 coordination sites, so 2 en ligands occupy 4 sites, plus 2 H₂O ligands, making it octahedral (4 + 2 = 6). With more en ligands than in B, the ligand field is stronger, pushing the color further toward the blue region compared to the hexaaqua complex. This often results in a blue color for such mixed-ligand complexes.
  Match: II. Blue

Final Matches:
- A → IV (Violet)
- B → III (Pale blue)
- C → I (Green)
- D → II (Blue)