Nucleophilic substitutions of iodide with bromides in the Fe(CO)(2)(PMe3)(2)MeI complex

Nucleophilic substitution of iodide in the complex Fe(CO)(2)(PMe3)(2)MeI ha s been studied in dichloromethane using a series of bromide salts with diff erent structure and solvation: Bu4NBr, PPNBr or KBr complexed with a molecu lar receptor (18-crown-6), as well as with a chloride salt (PPNCI) for comp arison purposes. The analysis of the rate constants suggests that the react ion takes place via two reaction paths. The principal path involves the fre e ion Br-. The other path involves the ion pair (A(+)Br(-)) and its rate ge nerally increases as the dissociation constant of the salt increases. The r ate of this reaction has been compared with that of the isotopomerisation a nd it results much faster. (C) 1999 Elsevier Science S.A. All rights reserv ed.