Transient infrared spectroscopy has been used to study reactions of Cr(C0)s generated by UV (35 1 or 355 nm) photolysis of Cr(C0)6 in the gas phase. At 298 K Cr(C0)5 reacts with H2, C2H4, CzF4, and C6H6 with bimolecular rate constants of (9.0 f 2.9), (19.0 f 2.0), (1.8 f 0.2), and (26.0 f 1.0) X 10-l' cm3 molecule-I s-l, respectively, where the rate constants reported are for the high-pressure limit. Infrared absorptions attributed to Cr(CO)5L where L = Hz, C2H4, CzF4, 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(C0)sL in a reaction mixture consisting of Cr(C0)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(C0)sL which has been determined as 15.0 f 1.3, 24.7 f 2.4, 19.7 f 1.4, and 13.7 f 0.8 kcal/mol for H2, C2H.4, C2F4, and C6H6, respectively. Data were also obtained which pertain to the magnitudes of the bonding interaction between C6F6 and CH4 and Cr(C0)s. The nature of the bonding between Cr(C0)5 and L is discussed. Implications of these measurements for reactions of coordinatively unsaturated species in solution are also considered.

Time-resolved IR spectroscopy has been used to study the oxidative addition of H2 to Fe(CO)4 and its reverse reaction, the reductive elimination of H2 from Fe(CO)4H2, in the gas phase. The rate constant for oxidative addition of H2 shows little temperature dependence, indicating that if there is an activation barrier for this process it is small (<4 kcal mol-1). The activation barrier for the reductive elimination of H2 is 20.5 ± 2.1 kcal mol-1. From these measurements, the average of the dissociation energies for the two Fe−H bonds in Fe(CO)4H2 is calculated to be 62 ± 2 kcal mol-1. Kinetic measurements employing D2 indicate small kinetic isotope effects for both the oxidative addition and reductive elimination reactions.

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The bond dissociation energy for loss of C 2 H 4 from Fe(CO) 3 (C 2 H 4 ) 2 , produced by the reaction of C 2 H 4 + Fe(CO) 3 (C 2 H 4 ), has been determined as 21.3 ( 2.0 kcal/mol. An estimate is made for a lower limit for the bond dissociation energy of Fe(CO) 4 (C 2 H 4 ), which can be formed by reaction of CO + Fe(CO) 3 (C 2 H 4 ) or Fe (CO) 4 + C 2 H 4 with rate constants of (4.3 ( 0.8) × 10 -12 and (1.7 ( 0.2) × 10 -13 cm 3 /(molecule s) at 24 °C, respectively. The values for these bond dissociation energies are compared with those determined in prior studies of these systems. A new compound with infrared absorptions at 2147, 2091, and 2068 cm -1 is best assigned as Fe(CO) 3 (C 2 F 4 ) 2 . A rate constant of (5.4 ( 1.7) × 10 -12 cm 3 /(molecule s) at 24 °C is reported for the reaction of C 2 F 4 with Fe(CO) 3 (C 2 F 4 ) to form Fe(CO) 3 (C 2 F 4 ) 2 . Fe(CO) 4 (C 2 F 4 ) can be formed by reaction of C 2 F 4 and Fe(CO) 4 , with a rate constant of (1.8 ( 0.4) × 10 -14 cm 3 /(molecule s) at 24 °C. Infrared absorptions observed at 2135, 2074, and 2043 cm -1 are assigned to this species. The relative stabilities of the mono-and bisethylene and perfluoroethylene compounds of iron are compared. Where possible, they are also compared to the corresponding chromium compounds and are discussed in the context of current concepts regarding metal-olefin bonding.

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