The local coordination geometry of single-atom catalysts (SACs) is a key determinant of their stability and reactivity. Using scanning tunneling microscopy, x-ray photoelectron spectroscopy, and an extensive structure search based on density functional theory, we show that Pt atoms deposited on the (1-102) facet of α-Fe₂O₃ spontaneously restructure the support to adopt a two-fold linear O–Pt–O coordination.1 This configuration is 0.84 eV more stable than the optimal site on the unreconstructed surface and offers a favorable balance between thermal stability and reactivity. STM imaging confirms the presence of isolated adatoms at room temperature and reveals dynamic adsorption behavior linked to the residual gas. In this presentation, I will also address the thermal stability of these Pt₁ species. While they sinter into metallic nanoparticles upon annealing in UHV, exposure to oxidizing conditions instead leads to the formation of two-dimensional PtOₓ rafts, underscoring the crucial role of the environment in determining structural evolution. For comparison, I will discuss the contrasting behavior of Rh, which sinters immediately after deposition under UHV but can be stabilized as a square-planar species by coadsorbed water. These observations highlight the interplay between adatom-support interaction, adsorbates, and thermal treatments in defining the stability window of SACs.