Recently, Maitlis et al. [J. Catal. 167 (1997) 172] proposed an alternative reaction pathway for chain growth in the Fischer-Tropsch synthesis. In this mechanism, chain growth is assumed to occur by methylene insertion into a metal-vinyl bond, forming an allyl species that will subsequently isomerise to a vinyl species. Organo-metallic allyl complexes, Fe{[eta(5)-C5H5](CO)(2)CH2CH=CH2} and Fe{[eta(5)-C-5(CH3)(5)](CO)(2)CH2CH=CH2} were synthesised. Under thermal treatment, the decomposition of these complexes was observed, instead of the isomerisation. In a hydrogen atmosphere, the reduction of the iron-carbon bonds and the hydrogenation yielding iron-alkyl species was observed. This clearly shows that the proposed vinyl-allyl isomerisation is unlikely to occur in mono-nuclear iron complexes. Hence, it might be expected that the reaction mechanism proposed by Maitlis et al. [J. Catal. 167 (1997) 172] is unlikely to be the main route for chain growth in the Fischer-Tropsch synthesis.