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

Abstract

Abstract The Mossbauer spectroscopic parameters of three dibromoboryl ferrocenes and of a related dichloroboryl ferrocene are reported. The results are discussed in relation to the known crystal structures, the Mossbauer parameters and the correlation of the latter with Hammet substituent constants. These data are also compared with those of other ferrocenes that form ferrocenyl carbocations (–CR 2 + is isoelectronic with –BX 2 where X=halogen) as well as protonated ferrocenyl ketones. It is concluded that normal ferrocenyl carbocations, though nominally isoelectronic with the similarly substituted dibromoboryl ferrocenes, bond in a very different manner involving a change in structure where the two cyclopentadienyl rings are no longer parallel. However, the protonated ferrocenyl ketones show Mossbauer quadrupole splittings that are similar to those of the dibromoboryl ferrocenes; this is interpreted to suggest that the former cations have significant carbocationic character and parallel cyclopentadienyl rings. The nature of the Fe–B interactions are discussed in terms of the overall C–B bonding, particularly in the case where four dihaloboryl groups are present.