Consider the following complexes. A: [ CoCl(NH 3) 5 ] 2+ , B: [ Co(CN) 6 ] 3- , C: [ Co(NH 3) 5(H 2O) ] 3+ , D: [ Cu(H 2O) 4 ] 2+ . The correct order of A , B , C and D in terms of wavenumber of light absorbed is :

Chemistry · Coordination Compounds

JEE Main 2024 — 6 April, Shift 1 — Question 68

Consider the following complexes.

\text{A: } [\mathrm{CoCl(NH_3)_5}]^{2+}, \quad \text{B: } [\mathrm{Co(CN)_6}]^{3-}, \quad \text{C: } [\mathrm{Co(NH_3)_5(H_2O)}]^{3+}, \quad \text{D: } [\mathrm{Cu(H_2O)_4}]^{2+}.

The correct order of $\mathrm{A}, \mathrm{B}, \mathrm{C}$ and D in terms of wavenumber of light absorbed is :

Step 1: Relation to ligand field splitting.

Wavenumber of absorption, $\nu \propto \Delta$ , where $\Delta$ is the ligand field splitting.

Step 2: Estimate $\Delta$ for each complex using spectrochemical series:

\mathrm{H_2O} < \mathrm{NH_3} < \mathrm{Cl^-} < \mathrm{CN^-}.

D: Cu $^{2+}$ + H $_2$ O → weak field → smallest Δ → smallest ν
A: Co $^{3+}$ + 5 NH $_3$ + 1 Cl⁻ → moderate Δ → ν larger than D
C: Co $^{3+}$ + 5 NH $_3$ + 1 H $_2$ O → Δ slightly larger than A → ν larger than A
B: Co $^{3+}$ + 6 CN⁻ → strong field → largest Δ → largest ν

Step 3: Arrange in increasing wavenumber:

\text{D < A < C < B}.

Answer key and solution verified before publishing.

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