The reactions of butadiene and acetylene, both complexed to atomic metal cations M(+) (M Cr, Mn, Fe, Co, Cu), have been investigated using ion-beam four-sector and Fourier transform ion cyclotron resonance mass spectrometry. For all metal ions studied, with the exception of Cu+, the reaction proceeds as a formal [4 + 2] cycloaddition involving 1,4-cyclohexadiene/M(+) as an intermediate. This subsequently eliminates molecular hydrogen to generate the corresponding benzene/M(+) complexes or bare M(+) and C6H6. The Fe+-mediated reaction has been analyzed in detail, and isotopic labeling reveals that the cyclization step is rate-determining and that dehydrogenation of the six-membered ring occurs specifically from the C(1)/C(4)-positions of the butadiene building block. In conjunction with literature thermochemistry, qualitative potential-energy surfaces for the [4 + 2] cycloaddition are derived for M = Cr, Mn, Fe, and Co. The reactions are very efficient for Fe+ and Co+, while Cr+ and Mn+ are less capable of inducing C-C bond formation. Finally, Cu+ with its closed-shell s(0)d(10) electronic ground state does not mediate the [4 + 2] cycloaddition at all. These differences are explained in terms of a model which invokes the active participation of the transition metal's d orbitals (covalent assistance), rather than mere Lewis-acid catalysis, which is known to catalyze many Diels-Alder reactions in the condensed phase.