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 H-2, 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) X 10(-11) cm(3) molecule(-1) s(-1), respectively, where the rate constants reported are for the high-pressure limit. Infrared absorptions attributed to Cr(CO)(5)L where L = H-2, C2H4, C2F4, and C6H6 were observed. The rate of dissociative loss of L from the Cr(CO)(5)L 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)(5)L 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 H-2,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.