intermediate carbanion [RfCFCF]− on the carbonylligand of Re(CO)5Hal. The involvement of [RfCFCF]− is demonstrated by their trapping with t-BuOH or CH-acid, which gives the protodehalogenated alkenes I-H and II-H, and suppresses the nucleophilic substitutionreaction leading to I-Fe, II-Fe or V-Cl. Arguments against a radical/SET mechanism for the substitutionreaction are also advanced.