NEET 2025 Chemistry Question 57 Solution

When a coordination entity absorbs a photon, an electron is promoted across the crystal field splitting gap (d-d transition):

• • A larger crystal field splitting energy corresponds to a shorter absorbed wavelength.

Analyze central metal oxidation states and ligand field strengths from the spectrochemical series:

• • Complex B [Co(CN)₆]³⁻: Co³⁺ with the extremely strong field ligand CN⁻ produces the highest splitting (absorbs UV/violet, λ ≈ 310 nm).
• • Complex A [Co(NH₃)₆]³⁺: Co³⁺ with strong field ligand NH₃ has large Δ_o (absorbs blue-green, λ ≈ 475 nm).
• • Complex D [Ti(H₂O)₆]³⁺: Ti³⁺ (+3 state) with H₂O has Δ_o = 20,300 cm⁻¹ (absorbs yellow-green, λ ≈ 498 nm).
• • Complex C [Cu(H₂O)₄]²⁺: Cu²⁺ (+2 state) with H₂O has much lower splitting (absorbs red/near-IR, λ ≈ 600–800 nm, transmitting blue).

Since absorbed wavelength is inversely proportional to splitting energy:

• • Thus the correct order of absorbed wavelength is B < A < D < C, matching Option (2).