Recent studies on reactions between Mo$_x$O$_y$− cluster anions and H$_2$O/C$_2$H$_4$ mixtures revealed a complex web of addition, hydrogen evolution, and chemifragmentation reactions, with chemifragments unambiguously connected to cluster reactions with C$_2$H$_4$. To gain insight into the molecular-scale interactions along the chemifragmentation pathways, the anion photoelectron (PE) spectra of MoC$_2$H$_2$−, MoC4H4−, MoOC$_2$H$_2$−, and MoO$_2$C$_2$H$_2$− formed directly in Mo$_x$O$_y$− + C$_2$H$_4$ (x > 1; y ≥ x) reactions, along with supporting CCSD(T) and density functional theory calculations, are presented and analyzed. The complexes have spectra that are all consistent with η$^2$-acetylene complexes, though for all but MoC$_4$H$_4$−, the possibility that vinylidene complexes are also present cannot be definitively ruled out. Structures that are consistent with the PE spectrum of MoC$_2$H$_2$− differ from the lowest energy structure, suggesting that the fragment formation is under kinetic control. The PE spectrum of MoO$_2$C2H$_2$− additionally exhibits evidence that photodissociation to MoO$_2$− + C$_2$H$_2$ may be occurring. The results suggest that oxidative dehydrogenation of ethylene is initiated by Lewis acid/base interactions between the Mo centers in larger clusters and the π orbitals in ethylene.