wide variety of reagents (q. amine bases, alcohols, protonic acids) which may be present as the solvent, or a component of the solvent, or as a so-called co-~atalyst.~*7 If such reagents are absent, a hydrido-complex may form spontaneously by intramolecular reaction of, say, a triphenylphosphine ligand with the transition meta1.8*9Consequently, to observe isomerisation vin x-allylic intermediates [equation (2)] it is necessary to choose for study systems in which these possibilities of hydridocomplex formation do not readily arise. The solvent must be inert, co-catalysts avoided, and complexes chosen which do not contain substituted phosphines as ligands. Two systems that apparently meet these requirements are Fe,(C0)12 in benzene, and PdCI,(C,H,CN), in benzene. Deuteriopent-l-ene isomerisation catalysed by each system is reported here.The isomerisation of hexenes,1° oct-l-ene,ll and undec-l-ene l2 catalysed by Fe,(CO),, has been known for some time. Although equations ( 1) and (2) were appreciated to be alternative mechanisms by which isomerisation might occur, the former was favoured on the grounds that iron carbonyl hydrides are easily formed,6 are reactive even in minute concentrations,6 and such hydrides are known to catalyse olefin isomerisation via the formation of alkyl intermediates.13 How-
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