cis-1,4-Hexadiene can be synthesized from the cobalt-catalyzed reaction between butadiene and ethylene. The reaction of this hydrocarbon with Fe(CO)5 affords the trans-1,3-hexadiene-iron tricarbonyl complex (I), plus a small amount of the trans,trans-2,4-hexadiene complex(II). Mechanism of the conversion of cis-1,4-hexadiene to trans-2-methyl-1,3-pentadiene, via rearrangement in the presence of a nickel catalyst has been investigated.