Q13 · CSIR-NET Chemistry, June 2021

Paper: CSIR-NET June 2021 · Subject: Inorganic Chemistry · Chapter: Coordination Chemistry · Topic: Term Symbols Spectra · Marks: 2 · Difficulty: Medium

The absorption spectrum of $\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$ in water shows two bands around 475 and 365 nm . The ground term and the spin-allowed transitions, respectively, are
(a)${ }^{4} \mathrm{F} ;{ }^{4} \mathrm{T}_{1 \mathrm{g}}(\mathrm{F}) \rightarrow{ }^{4} \mathrm{T}_{2 \mathrm{g}}$ and ${ }^{4} \mathrm{T}_{1 \mathrm{g}} \rightarrow{ }^{4} \mathrm{A}_{2 \mathrm{g}}$
(b)${ }^{4} \mathrm{F} ;{ }^{4} \mathrm{A}_{2 \mathrm{g}} \rightarrow{ }^{4} \mathrm{T}_{2 \mathrm{g}}$ and ${ }^{4} \mathrm{A}_{2 \mathrm{g}} \rightarrow{ }^{4} \mathrm{T}_{1 \mathrm{g}}(\mathrm{F})$
(c)${ }^{2} \mathrm{G} ;{ }^{2} \mathrm{E}_{\mathrm{g}} \rightarrow{ }^{2} \mathrm{T}_{1 \mathrm{g}}$ and ${ }^{2} \mathrm{E}_{\mathrm{g}} \rightarrow{ }^{2} \mathrm{T}_{1 \mathrm{g}}$
(d)${ }^{2} \mathrm{F} ;{ }^{4} \mathrm{A}_{2 \mathrm{g}} \rightarrow{ }^{2} \mathrm{T}_{2 \mathrm{g}}$ and ${ }^{2} \mathrm{A}_{2 \mathrm{g}} \rightarrow{ }^{2} \mathrm{T}_{1 \mathrm{g}}(\mathrm{F})$
Answer
Answer: B ✓ checked by 4AB · confidence high
Explanation
Cr³⁺ (d³) free-ion ground term ⁴F; the two bands are ⁴A₂g → ⁴T₂g and ⁴A₂g → ⁴T₁g(F) (b).

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