Halogen cleavage reactions of cis -( threo -PhCHDCHD)Mn(CO) 4, PEt 3 ( 3 ) are studied in detail and are found to proceed either predominantly with retention or inversion, or non-stereospecifically, depending on reaction conditions. Reaction mechanisms involving S E2 (inversion), S E2 (retention) and S E (oxidative) processes are proposed, demonstrating that halogen cleavage reactions of 3 straddle the mechanistic borderline between electrophilic substitution and oxidation processes of transition metal alkyl compounds.