The mechanism and kinetic of a Cl/F-exchange SbF5-mediated reaction are studied for halocarbonyl COCl2 by means of theoretical calculations at the B3LYP level. Two energetic pathways are described in which antimony containing four-centre transition states are localized. The lowest energy barrier of 19.5 kcal/ mol corresponds to a transition state in which the formally 16-valence electron triatomic ClCO+ species is linked to hexacoordinated SbF5Cl anion. The rate constant for this Cl/F-exchange reaction is predicted by transition state theory and evaluated at k(TST)= 2.2 x 10(3) s(1) when T = 293 K. (c) 2008 Elsevier B. V. All rights reserved.