Paper: CSIR-NET June 2017 · Subject: Inorganic Chemistry · Chapter: Inorganic Spectroscopy · Topic: Inorganic Spectroscopy – General · Marks: 2 · Difficulty: Medium
Mössbauer spectrum of complex $[\mathrm{Fe(1,10\text{-}phenanthroline)_2(NCS)_2}]$ shows two lines at 300 K, four lines at 186 K, and again two lines at 77 K. This can be attributed to
\[ \begin{array}{l} \text{A. change in the coordination mode of NCS} \\ \text{B. change in the spin-state of iron} \\ \text{C. cis-trans isomerisation} \\ \text{D. change in metal-ligand bond distances} \end{array} \]
The correct statements are
(a)A and B
(b)B and C
(c)A and C
(d)B and D
Answer
Answer: B ✓ checked by 4AB · confidence high
The source book printed A; on checking, B is correct — see the explanation.
Printed A; the NCS coordination mode does not change, the spin state does.
Explanation
[Fe(phen)₂(NCS)₂] is the classic spin-crossover complex: high-spin at 300 K and low-spin at 77 K each give one quadrupole doublet; near the transition (186 K) both spin states coexist, giving four lines.