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The extent of Mössbauer quadrupole splitting of iron follows the order

  • A.FeCl\(_{2}\)•4H\(_{2}\)O \(\gt\) K\(_{2}\)[Fe(CN)\(_{5}\)(NO)] \(\gt\) FeCl\(_{3}\)•6H\(_{2}\)O
  • B.K\(_{2}\)[Fe(CN)\(_{5}\)(NO)] \(\gt\) FeCl\(_{2}\)•4H\(_{2}\)O \(\gt\) FeCl\(_{3}\)•6H\(_{2}\)O
  • C.FeCl\(_{3}\)•6H\(_{2}\)O \(\gt\) K\(_{2}\)[Fe(CN)\(_{5}\)(NO)] \(\gt\) FeCl\(_{2}\)•4H\(_{2}\)O
  • D.FeCl\(_{2}\)•4H\(_{2}\)O \(\gt\) FeCl\(_{3}\)•6H\(_{2}\)O \(\gt\) K\(_{2}\)[Fe(CN)\(_{5}\)(NO)]

Answer: A

The order is FeCl2•4H2O > K2[Fe(CN)5(NO)] > FeCl3•6H2O.

Quadrupole splitting measures the electric field gradient at the iron nucleus, which comes mainly from an unsymmetrical distribution of the iron's own d electrons, with a smaller contribution from unequal ligands.

  • FeCl2•4H2O: high-spin Fe(II), d6. The sixth electron sits alone in the t2g set, a strongly asymmetric distribution, giving a large splitting (about 3 mm/s).
  • Nitroprusside: low-spin Fe(II), a symmetric t2g6 shell, but one NO+ ligand differs sharply from the five cyanides, so the ligand field gradient gives a moderate splitting (about 1.7 mm/s).
  • FeCl3•6H2O: high-spin Fe(III), d5, a spherical half-filled shell, so the splitting is small.

The other orders either put the spherical d5 ion above another or rank the ligand-only gradient of nitroprusside above the high-spin Fe(II) salt.

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