Understanding how the local environment of a "single-atom" catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper(1), silver(1), gold(1), nickel(1), palladium(1), platinum(1), rhodium(1), and iridium(1) species on Fe3O4(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag-1-CO) or weaken it (as for Ni-1-CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.