THERMOCHEMISTRY AND STRUCTURES OF COC3H6- METALLACYCLE AND METAL-ALKENE ISOMERS( )

The thermochemistry for the metallacyclic and metal-alkene isomers of CoC3H6+ is studied in a guided-ion-beam mass spectrometer. A flow-tube source is used to produce thermalized CoC3H6+ ions formed by condensa tion of Co+ with propene and cyclopropane, and by the decarbonylation of cyclobutanone by Co+ These ions are then studied by threshold colli sional activation and the results compared with the bimolecular reacti vity of Co+ with cyclopropane and propene. The results are interpreted to indicate that CoC3H6+ ions formed in these three methods have both the Co+-propene and cobaltacyclobutane ion structures in ratios of 10 0:0, 85:15, and 40:60, respectively. Some evidence for a third isomer, tentatively identified as the cobalt pi-allyl hydride ion complex, is also obtained. Reactivity and thermochemical differences of the ions produced in these three different ways are discussed and compared to p revious work by other investigators and to work on these isomers of Fe C3H6+. Analysis of the kinetic energy dependence of the cross sections allows the determination of the 0 K bond dissociation energies: 1.87 +/- 0.07 eV for Co+-propene and 0.93 +/- 0.07 eV for cobaltacyclobutan e to dissociate and yield Co+ + propene. We also determine 0 K bond di ssociation energies for Co+-C3H5, Co+-C3H4 and Co+-C2H3 to be 2.01 +/- 0.17, 0.81 +/- 0.09, and 2.10 +/- 0.08 eV, respectively, and revise o ur reported 0 K bond energy for Co+-CH2 to 3.24 +/- 0.06 eV.