Discussion
Which of the following statements is true with respect to 57Fe Mossbauer spectra of (i) FeSO4·7H2O, (ii) K4[Fe(CN)6], (iii) Na2[Fe(CN)5NO]·2H2O ?
Answer: B
The spectra show a singlet in (ii) and doublets in (i) and (iii).
A Mossbauer doublet comes from quadrupole splitting, which needs an electric field gradient at the iron nucleus. In K4[Fe(CN)6] the iron is low-spin Fe(II), t2g6, a spherically symmetric configuration in a regular octahedron of identical ligands, so there is no field gradient and a single line appears.
FeSO4·7H2O contains high-spin Fe(II), t2g4eg2, whose unevenly filled t2g set produces a large field gradient and a wide doublet. Sodium nitroprusside is low-spin but its ligand set (five CN- and one NO+) is not cubic, so it too shows a doublet. That rules out every option giving (i) or (iii) as a singlet, or (ii) as a doublet.
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