Interaction of H₂ and prototypical solvent molecules with Cr(CO)₅ in the gas phase

Transient infrared spectroscopy has been used to study reactions of Cr(CO)₅ generated by UV (351 or 355 nm) photolysis of Cr(CO)₆ in the gas phase. At 298 K Cr(CO)₅ reacts with H₂, C₂H₄, C₂F₄, and C₆H₆ with bimolecular rate constants of (9.0 ± 2.9), (19.0 ± 2.0), (1.8 ± 0.2), and (26.0 ± 1.0) × 10^-11 cm³ molecule^-1 s^-1, respectively, where the rate constants reported are for the high-pressure limit. Infrared absorptions attributed to Cr(CO)₅L where L = H₂, C₂H₄, C₂F₄, and C₆H₆ were observed. The rate of dissociative loss of L from the Cr(CO)₅L complexes was determined by the observation of the rate of regeneration of Cr(CO)₆ and/or the rate of disappearance of Cr(CO)₅L in a reaction mixture consisting of Cr(CO)₅, CO, and L. The rate of dissociative loss of a ligand can be directly related to the bond dissociation energy for the loss of L from Cr(CO)₅L which has been determined as 15.0 ± 1.3, 24.7 ± 2.4, 19.7 ± 1.4, and 13.7 ± 0.8 kcal/mol for H₂, C₂H₄, C₂F₄, and C₆H₆, respectively. Data were also obtained which pertain to the magnitudes of the bonding interaction between C₆F₆ and CH₄ and Cr(CO)₅. The nature of the bonding between Cr(CO)₅ and L is discussed. Implications of these measurements for reactions of coordinatively unsaturated species in solution are also considered.