Complete geometry optimizations were carried out using density functional theory to study potential energy surfaces of the transition-metal complex insertions into carbon-halogen bonds. The trans-Ir(Cl)(PH3)(2) + CX4 (X = F, Cl, Br, I) systems are the subject of the present study. Three different reaction mechanisms are proposed and an as follows: (I) oxidative insertion (OxIn) of trans-Ir(Cl)(PH3)(2) into the C-X bond, (ii) radical mechanisms proceeding via single electron transfer (SET), and (iii) backside S(N)2 substitution mechanisms. The results of B3LYP/LANL2DZ calculations suggest the following. (a) For oxidative addition of 14-electron T-shaped ML3 complexes to saturated C-X bonds the order of reactivity is I > Br > Cl much greater than F, whether collision conditions exist or not. (b) The ease of oxidative insertion increases with increasing halogen electronegativity. For the heavier halogens, especially idoine, OxIn and SET reaction pathways are in competition. (c) In the competition of the S(N)2 path with OxIn and SET processes, the former has the highest energy requirement and is therefore the least energetically favorable path in all cases in the gas phase. Further, the reaction pathway cannot be determined for the singlet transition states. The problem has been solved by computing the intrinsic reaction coordinate (IRC). The IRC results have demonstrated that the transition state corresponds to a CX3 fragment abstraction, rather than the backside S(N)2 substitution. Furthermore, a configuration mixing model based on the work of Press and Shaik is used to rationalize the computational results. It is demonstrated that both the sigma(C-X) --> sigma*(C-X) triplet excitation energy of halocarbons and the halogen lone-pair repulsions play a decisive role in determining the dominant reaction pathways (i.e., OxIn or SET).