trans, trans-[RhL2(Ph)(CO)HCl] (2a) and trans,trans-[RhL2(Ph)Cl(CO)H] (2b) are known to form as major products in the reaction between benzene and a photoexcited state of trans-[RhL2(CO)CI] (1; L = PMe3). In the presence of carbon monoxide, these species are believed to ultimately lead to catalytic photocarbonylation of benzene. At the B3LYP level of theory the calculated free energy of activation for the concerted transition state (TS) of phenyl to CO migration in 2b (TSb) is 39 kcal/mol. In contrast, the barrier of insertion in the trans, trans-[RhL2(Ph)H(CO)CI] isomer (2c) which is 7.4 kcal/mol higher in energy than 2b is only 15 kcal/mol. The calculated geometries of the reactants and TSs indicate that the disparity in the given barriers arises from a combination of two factors. First, the orientation of H trans to the migrating Ph group in 2c favors the TS because it weakens the Rh Ph bond and thus makes it easier to begin migration of the phenyl group in comparison with 2b where the phenyl is trans to the chloride. Second, the orientation of the hydride trans to CO in 2b appears to strongly disfavor the reaction compared to 2c where CO is trans to Cl. Specifically, the calculations give strong evidence that the Rh H bond in 2b is substantially weakened in TSb, and this should add to the increased barrier of this isomer. These propositions are supported by calculations on the reaction of the cis-Rh(PMe3)(2) isomers of 2 and other related octahedral complexes.