Studies of the bonding in iron(II) cyclopentadienyl and arene sandwich compounds. Part 5. An interpretation of the Fe-57 Mossbauer spectroscopic dataof dibromoborylferrocenes, and related molecules

The Mossbauer spectroscopic parameters of three dibromoboryl ferrocenes and of a related dichloroboryl ferrocene are reported. The results are discuss ed in relation to the known crystal structures, the Mossbauer parameters an d the correlation of the latter with Hammet substituent constants. These da ta are also compared with those of other ferrocenes that form ferrocenyl ca rbocations (-CR2+ is isoelectronic with -BX2 where X = halogen) as well as protonated ferrocenyl ketones. It is concluded that normal ferrocenyl carbo cations, though nominally isoelectronic with the similarly substituted dibr omoboryl ferrocenes, bond in a very different manner involving a change in structure where the two cyclopentadienyl rings are no longer parallel. Howe ver, the protonated ferrocenyl ketones show Mossbauer quadrupole splittings that are similar to those of the dibromoboryl ferrocenes; this is interpre ted to suggest that the former cations have significant carbocationic chara cter and parallel cyclopentadienyl rings. The nature of the Fe-B interactio ns are discussed in terms of the overall C-B bonding, particularly in the c ase where four dihaloboryl groups are present. (C) 1999 Elsevier Science S. A. All rights reserved.