KINETIC-ENERGY RELEASE DISTRIBUTIONS AS A PROBE OF LIGATION EFFECTS ON POTENTIAL-ENERGY SURFACES IN ORGANOMETALLIC REACTIONS - REVERSIBLE DEHYDROGENATION OF CYCLOALKENES BY FE+

The kinetic energy release distributions associated with dehydrogenation and double-dehydrogenation processes for Fc+ reacting with cyclopentene, cyclohexene, cyclopentane, and cyclohexane have been obtained. Previously, dehydrogenation of alkanes and alkenes by Co+ and Ni+ in the gas phase has been characterized by release of more energy into product translation than can be accounted for by statistical theory. This observation is not general, however, since here we show dehydrogenations of cyclopentene and cyclohexene by Fe+ are well described by statistical phase-space theory with the best fit of theory to experiment yielding D0°(Fe+-C5H6) = 50 ± 5 keal/mol and D0°(Fe+-C6H8) = 66 ± 5 kcal/mol. For statistical theory to beapplicable, it is required that there be no barrier for the reverse association reaction. The absence of a barrier in these systems is consistent with studies that indicate H/D exchange for Fe(C5H6)+ is reversible and occurs in the presence of excess D2 at about 5% of the Langcvin collision rate. The product kinetic energy release distributions measured for the final hydrogen loss in the double dehydrogenations of cyclopentane and cyclohexane by Fe+ are remarkably similar to those obtained for single dehydrogenations of cyclopenteneandcyclohexene. This similarity is explained by participation of electronically excited Fe+ in the first step of the double-dehydrogenation processes, which supplies the additional energy required to observe the second H2 loss as a metastable process. © 1990, American Chemical Society. All rights reserved.