The correct order of intensity of the d-d transitions in the complexes of a 3d-transition metal ion · ABC26IN1120

Subject: Inorganic Chemistry · Chapter: Coordination Chemistry · Topic: Coordination Chemistry – General · Exam: CSIR NET DEC 2018 · Marks: 2 · Difficulty: Hard

The correct order of intensity of the d-d transitions in the complexes of a 3d-transition metal ion
(a)cis-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$ $>$ trans-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$ $>$ $\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(b)$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ $>$ cis-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$ $>$ trans-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$
(c)trans-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$ $>$ cis-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$ $>$ $\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(d)$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ $>$ cis-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$ $>$ trans-$\left[\mathrm{M}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]$
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
d–d bands are Laporte-forbidden in centrosymmetric complexes. [M(H₂O)₆]²⁺ and trans-[M(H₂O)₄Cl₂] have an inversion centre (weak), cis-[M(H₂O)₄Cl₂] does not (strongest); the trans isomer gains a little from vibronic/asymmetric-field mixing.

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