Mixed-valence biferrocenes: Pronounced anion dependence of valence detrapping and effects of an asymmetric crystal lattice

The effect of changing the anion from I{sub 3}{sup {minus}} to either PF{sub 6}{sup {minus}} or SbF{sub 6}{sup {minus}} upon the rate of intramolecular electron transfer in salts of the mixed-valence 1{prime},1{prime}{double prime}-dibenzylbiferrocenium cation is investigated. Single-crystal X-ray structures are presented for 1{prime},1{prime}{double prime}-dibenzylbiferrocene (1), 1{prime},1{prime}{double prime}-dibenzylbiferrocenium hexafluorophosphate (2), and 1{prime},1{prime}{double prime}-dibenzylbiferrocenium hexafluoroantimonate (3). A center of symmetry (planar fulvalenide ligand) and a trans conformation characterize the Fe{sub 2}{sup II} molecule in 1. The PF{sub 6}{sup {minus}} anion was found to be disordered in two positions, one with an occupancy of 64% and the other 36%. The packing in complex 2 is that of layers of mixed-valence cations with the small PF{sub 6}{sup {minus}} anions sandwiched between the layers. The SbF{sub 6}{sup {minus}} anion in complex 3 was not found to be disordered at either temperature; however, at 298 K the thermal parameters for this anion are large.