THE (MU-OXO)BIS[TRICHLOROFERRATE(III)] DIANION REVISITED

From 11 published crystal structure determinations of salts containing the (mu-oxo)bis[trichloroferrate(III)] dianion the following structural features of the anion in the solid state are evident: (i) The structure is dependent on the nature of the respective countercation. (ii) The Fe-O-Fe bond angle spans a range from 146.5 to 170.8-degrees; a linear Fe-O-Fe unit has not been experimentally verified. (iii) The Fe-O(oxo) bond distances are within experimental error identical (1.756 angstrom). (iv) The geometry of the Cl3FeO unit is approximately tetrahedral, but the symmetry is C(s); description in C3, symmetry is in many instances not appropriate. (v) The two corner-sharing tetrahedra adopt different conformations depending on the nature of the countercation. The crystal structure of [Ph4P]2[Fe2Cl6O] has been determined: space group P1BAR, a = 9.847(3) angstrom, b = 12.953(5) angstrom, c = 21.099(5) angstrom, a = 85.12(3)-degrees, beta = 76.45(2)-degrees, gamma = 67.70(3)-degrees, and Z = 2. Five crystallographically well-characterized salts with Fe-O-Fe angles ranging from 148.1 to 170.8-degrees have been investigated by infrared and Raman spectroscopy, and their magnetic susceptibilities have been measured in the temperature range 2.7-295 K. Antiferromagnetic exchange coupling between the ferric ions has been established in all cases. The coupling constant J was found to be 112 +/- 5 cm-1 (H = -2JS1.S2; S1 = S2 = 5/2) irrespective of the actual structure of the [Fe2Cl6O]2- anion. An angle dependence of the intramolecular antiferromagnetic coupling has not been detected within the experimental accuracy of our measurements.