The wavelength dependence of excimer laser photolysis of Fe(CO)5 in the gas phase. Transient infrared spectroscopy and kinetics of the Fe(CO)x (x=4,3,2) photofragments

Abstract

The transient infrared absorption spectra of the coordinatively unsaturated Fe(CO)_x species generated via excimer laser photolysis of gas phase Fe(CO)₅ are presented and discussed. The photofragments produced upon 351, 248, and 193 nm photolysis are characterized. Fe(CO)₃ and Fe(CO)₄ are produced upon 351 nm photolysis. In addition to these two fragments, Fe(CO)₂ is produced on 248 nm photolysis. The gas phase structures of these Fe(CO)_x fragments are observed to be compatible with those determined from condensed phase experiments. The coordinatively unsaturated photofragments are typically formed with significant amounts of internal excitation. The rate constants for reaction of Fe(CO)₄, Fe(CO)₃, and Fe(CO)₂ with CO are (3.5±0.9)×10¹⁰, (1.3±0.2)×10¹³, and (1.8±0.3)×10¹³ cm³ mol⁻¹ s⁻¹, respectively. The large difference in the magnitude of the rate constant for reaction of Fe(CO)₄ vs Fe(CO)₃ and Fe(CO)₂ with CO is rationalized in terms of the spin states of the reactants and products. Following 193 nm photolysis, a new product is observed which is tentatively assigned as an excited electronic state of Fe(CO)₃. A photochemical scheme which accounts for all observed products is presented.