A TIME-RESOLVED INFRARED STUDY OF THE GAS-PHASE REACTIONS OF 1,3-PENTADIENE AND 1,4-PENTADIENE WITH FE(CO)3 AND FE(CO)4

Time-resolved infrared (TRIR) spectroscopy has been used to study the addition of trans-1,3-pentadiene (1,3-PD) and 1,4-pentadiene (1,4-PD) to Fe(CO)3 in the gas phase. The addition of either diene to Fe(CO)3 i nitially involves the formation of an excited-state triplet species, [ (PD)Fe(CO)3]dagger. This excited-state intermediate can then either re lax to form the ground-state singlet product, (eta4-PD)Fe(CO)3, dissoc iate to regenerate the Fe(CO)3 and PD reactants, or rearrange to form the unsaturated intermediate, (eta2-PD)Fe(CO)3. The rate constant for addition of either diene to Fe(CO)3 in the high-pressure limit is (1.7 +/- 0.2) X 10(14) CM3/(Mol s), which approaches the gas-kinetic cross section for this process. The only significant difference between the reactive behavior of 1,3-PD and 1,4-PD with Fe(CO)3 is that addition of 1,4-PD leads to the formation of a small amount of an isomer of (et a2-1,4-PD)Fe(CO)3. This species is generated from triplet [(PD)Fe(CO)3 ]t with an activation energy and preexponential factor estimated to be 12 kcal/mol and 4.5 X 10(11) s-1, respectively. It is most likely tha t this isomer is (eta2:CH-1,4-PD)Fe(CO)3, Which contains an agostic M- H-C bond. The rates for addition of pentadiene to Fe(CO)4 have also be en measured and are (7.0 +/- 0.4) X 10(11) and (3.5 +/- 0. 1) X 10(11) CM3/(Mol s) for 1,3-PD and 1,4-PD, respectively. The reaction mechani sm for Fe(CO)3 + PD is compared to that for the Cr(CO)4 + PD system, w hich has been the subject of a prior study. Conclusions for the two sy stems are (1) neither the Cr(CO)4 + PD nor the Fe(CO)3 + PD mechanism is dominated by effects due to pentadiene conjugation, (2) the Cr(CO)4 mechanism is more influenced by geometric differences between 1,3-PD and 1,4-PD than is Fe(CO)3, and (3) the Cr(CO)4 mechanism occurs on on e singlet potential surface whereas the Fe(CO)3 mechanism involves bot h triplet and singlet potential energy surfaces.