[en] The exchange of ¹³CO with Mn(CO)₅Br and Re(CO)₅Br in hexane has been measured using infrared spectral techniques. A complete accounting of the time dependencies of all isotopically labeled species and comparison with the results of phosphorus ligand substitution rates lead to the following conclusions. (a) The rate-determining process is CO dissociation. (b) The rate constant for cis CO dissociation is at least ten times that for trans. (c) The five-coordinate intermediate formed in the dissociation is fluxional. (d) The rate constant for cis CO dissociation in Mn(CO)₅Br is 2.8 x 10^-5 sec^-1 at 23⁰; for Re(CO)₅Br at 30⁰, 5.1 x 10^-7 sec^-1. The labilization of CO toward dissociation by cis ligands which are weaker π acceptors than CO seems general. (U.S.)