The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory [1]. In this talk [2], we follow the evolution of Rh1 adatoms and minority Rh2 dimers on Fe3O4(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh1 sites results exclusively in stable Rh1CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh1(CO)2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh2 dimers via an unstable Rh2(CO)3 intermediate. The results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis. In the second part of the presentation, I will focus on our recent studies of Rh and Pt atoms on Fe2O3, and show that stability is achieved through a distortion of the support lattice. The reactivity of the resulting species to small molecules will be discussed. References [1] J.Hulva, M.Meier,…..G.S.Parkinson., Science 371 (2023) 375. [2] C.Wang, P.Sombut, L.Puntscher, Z.Jakub, M.Meier, J.Pavelec, R.Bliem, M.Schmid, U.Diebold, C.Franchini, G.S.Parkinson, Angewandte Chemie International Edition (2024) e202317347.