Photochemical experiments with Fe(CO)(3)(eta(4)- 1,5-cyclooctadiene) (1) in a (CO)-C-13 matrix at 10 K, monitored by means of IR spectroscopy, indicate the generation of stereoselectively labeled Fe(CO)(2)((CO)-C-13)-(eta(4)-1,5-cyclooctadiene) (1-1a), with (CO)-C-13 in the apical position of the square-pyramidal coordination geometry. The spectral changes occurring upon annealing the matrix to 28 K. reveal the thermally activated conversion into a mixture of the two possible stereoisotopomers, the species with (CO)-C-13 in a basal position (l-lb) becoming predominant. These findings characterize the carbonyl ligand site exchange in complex 1 as a chemical reaction involving a very small barrier. The variable-temperature IR spectra of 1 in hydrocarbon solution exhibit broadening and coalescence of bands in the nu(CO) region, which is interpreted in terms of a CO site exchange occurring in the picosecond time domain. The theoretical approach to the simulation of these spectral changes involves a transfer of transition dipole moment between the nu(CO) modes. On the basis of this approach, the rates of CO site exchange at the various temperatures could be evaluated by line shape simulation. They were found to range from 0.15 x 10(12) s(-1) at 133 K to 1.54 x 10(12) s(-1) at 293 K, yielding Delta H double dagger = 0.7 kcal.mol(-1) (Eyring plot) and E-a = 1.1 kcal.mol(-1) (Arrhenius plot) for the activation barrier of the underlying process.