Reactions of metal carbonyl derivatives. Part III. The mechanism of halogenation of tetracarbonylbis-π-cyclopentadienyldi-iron and its derivatives

Abstract

A study of the halogenation of [{Fe(π-RC₅H₄)(CO)₂}₂](R = H or Me) has revealed that under the appropriate experimental conditions, products other than [Fe(π-RC₅H₄)(CO)₂X]viz.[{Fe(π-RC₅H₄)(CO)₂}₂X]X and [Fe(π-RC₅H₄)(CO)₃]X (X = Cl, Br, or I), are also formed. it is further shown that the derivative [{Fe(π-RC₅H₄)(CO)₂}₂X]X containing a bridging halogen group is an intermediate in the formation of [Fe(π-RC₅H₄)(CO)₂X] and that the yield of [Fe(π-RC₅H₄)(CO)₃]X increases along the series I < Br < Cl. The experimental results are interpreted in terms of a mechanism involving electrophilic attack of the halogen on [{Fe(π-RC₅H₄)(CO)₂}₂] followed by (I) asymmetric cleavage of the bridging carbonyl groups of the parent dimer to yield [Fe(π-RC₅H₄)(CO)₃]X and/or (II) symmetric cleavage of these carbonyl groups to afford [{Fe(π-RC₅H₄)(CO)₂}₂X]X. Subsequent nucleophilic attack of halide ions on this latter derivative produces the neutral compound [Fe(π-RC₅H₄)(CO)₂X]. The proposed mechanism is compared with that well established for the electrophilic addition of halogens to olefins.