The Very Low Barrier of CO Site Exchange in Tricarbonyl(η4-1,5-cyclooctadiene)iron: Picosecond Kinetics in Solution Investigated by Line Shape Simulation of the ν(CO) IR Bands and Complementary Evidence from the Course of 13CO Incorporation in a Low-Temperature Matrix

Abstract

Photochemical experiments with Fe(CO)3(η4-1,5-cyclooctadiene) (1) in a 13CO matrix at 10 K, monitored by means of IR spectroscopy, indicate the generation of stereoselectively labeled Fe(CO)2(13CO)(η4-1,5-cyclooctadiene) (1-1a), with 13CO 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 13CO in a basal position (1-1b) 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 ν(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 t...