The thermal chemistry of 1,6-diiodohexane, 6-bromo-1-hexene, 1,5-hexadiene, methyl cyclopentane, methylene cyclopentane, and 1-methyl-1-cyclopentene on Ni(100) surfaces has been studied under ultrahigh vacuum conditions by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The thermal activation of the diiodo alkane leads to the initial scission of the C-I bonds around 160 K, the same as with other iodoalkanes, and presumably results in the formation of a surface metallacyclic intermediate. Further heating of that system induces the desorption of hexene, hexane, iodohexane, methylene cyclopentane, benzene, and cyclohexene. The formation of the latter two cyclic products through 5-hexen-1-yl, methyl cyclopentane, methylene cyclopentane, or l-methyl-l-cyclopentene intermediates was ruled out in this case because direct activation of those compounds does nor lead to the desorption of any C-6-cyclic molecules at all. TPD experiments with 1,5-hexadiene, on the other hand, did show the formation of benzene, suggesting that such a molecule could be involved in the conversion of the diiodohexane. Additional results from studies with cyclohexane, iodocyclohexane, and cyclohexene indicate that the first cyclic intermediate from the reaction of the C-6-Ni metallacycle is likely to be cyclohexene, since both cyclohexane and cyclohexyl moieties yield much more cyclohexane than the diiodo compound. On the basis of these data, a mechanism is proposed for the cyclization reaction of nickelacycloheptane where two initial beta-hydride elimination steps at both ends of the hydrocarbon moiety result in the formation of adsorbed 1,5-hexadiene and where that is followed by insertion of one of the double bonds into the metal-carbon bond at the other end to yield cyclohexene.