X-Ray absorption fine structure study of the structural changes accompanying spin cross-over in the acetone solvates of [Fe(dppen)2X2][dppen =cis-1,2-bis(diphenylphosphino)ethylene; X = Cl or Br]

Iron and bromine K-edge X-ray absorption fine structure (XAFS) have been used to determine the changes in the local iron environment in [Fe(dppen)2Br2]·2Me2CO on going through the spin cross-over transition at 160–185 K. The mean Fe–P bond length decreases from the high- to the low-spin isomer by 0.29 Å[2.60(3) to 2.31(3)Å], whereas the mean Fe–Br bond length decreases by only ca. 0.02 Å[2.51(3) to 2.48(3), iron K-edge; 2.48(3) to 2.47(3)Å, bromine K-edge]. Iron K-edge XAFS for [Fe(dppen)2Cl2]·2Me2CO showed a reduction of 0.28 Å in the mean Fe–P distance [2.55(3) to 2.27(3)Å] and a reduction of 0.02 Å in the mean Fe–Cl bond length [2.33(3) to 2.31(3)Å] on going from the high- to the low-spin form, in good agreement with literature values, the spin transition being observed at 220–240 K.