The VCN in $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}(\mathrm{A})$ and $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}(\mathrm{B})$ and $\mathrm{V}_{\mathrm{CO}}$ in $\left[\mathrm{Cr}(\mathrm{CO})_{3}\left(\mathrm{NH}_{3}\right)_{3}\right](\mathrm{C})$ and $\left[\mathrm{Cr}(\mathrm{CO})_{6}\right](\mathrm{D})$ are compared below. The pair with correct order is
(a)A $>$ B ; C $>$ D
(b)A $>$ B $;$ C $<$ D
(c)A $<$ B ; C $>$ D
(d)A $<$ B $;$ C $<$ D
Answer
Answer: B ✓ checked by 4AB · confidence high
Explanation
Fe(III) back-donates less than Fe(II), so ν(CN) of [Fe(CN)₆]³⁻ > [Fe(CN)₆]⁴⁻ (A > B). NH₃ donors enrich the metal and lower ν(CO): [Cr(CO)₃(NH₃)₃] < [Cr(CO)₆] (C < D).