MOBILITY OF TRANSIENT OLEFINIC INTERMEDIATES DURING EXCHANGE-REACTIONS OF PARAFFINS WITH DEUTERIUM ON METAL-CATALYSTS

The mechanisms of exchange reactions of paraffins with deuterium on tr ansition metal catalysts are briefly reviewed with an emphasis on thos e of ethane and cyclopentane on platinum. Convincing arguments are pre sented to show that the reactive sites are individual coordinatively h ighly unsaturated metal atoms such as those at comers, edges and on te rraces, and thus relatively few in numbers. A key intermediate for mul tiple exchange formed on those sites is pi-bonded alkene which exhibit s a considerable degree of mobility including roll-over and inter-site transfer subsequent to the rupture of the weak metal-olefin pi-bond. Mobility of transients is therefore a complicating factor which helps to explain the variety of features noted in initial deuteroisomer dist ributions, without requiring more than two or three fundamentally dist inct types of single atom sites. An additional variable governing the relative rates of the various steps, including interconversion of sigm a-bonded alkyl and pi-bonded olefin, is the electron density at each t ype of site. This can be varied for a given metal by dispersion, suppo rt effects, alloying and dopants.