Excited-state properties of fac-[Re(dmb)(CO)(3)(CH3CN)]PF6, [Re(dmb)(CO)(3)](2) (where dmb = 4,4'-dimethyl-2,2'-bipyridine), and other tricarbonyl rhenium(l) complexes were investigated by transient FTIR and UV-vis spectroscopy in CH3CN or THF. The one-electron reduced monomer, Re(dmb)(CO)(3)S (S = CH3CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)(CO)(3)(CH3CN)]PF6 or by homolysis of [Re(dmb)(CO)(3)](2). In the reduced monomer's ground state, the odd electron resides on the dmb ligand rather than on the metal center. Re(dmb)(CO)(3)S dimerizes slowly in THF, k(d) = 40 +/- 5 M-1 s(-1). This rate constant is much smaller than those of other organometallic radicals which are typically 10(9) M-1 s(-1). The slower rate suggests that the equilibrium between the ligand-centered and metal-centered radicals is very unfavorable (K approximate to 10(-4)). The reaction of Re(dmb)(CO)(3)S with CO2 is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO)(3)](2) in the presence of CO2 produces CO with a 25-50% yield based on [Re]. A CO2 bridged dimer, (CO)(3)(dmb)Re-CO(O)-Re(dmb)(CO)(3) is identified as an intermediate. Both [Re(dmb)(CO)(3)](2)(OCO2) and Re(dmb)(CO)(3)(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.