Synchrotron x-ray diffraction experiments reveal the adsorption of iron pentacarbonyl [Fe(CO)5] on graphite basal-plane surfaces to be an extreme example of incomplete wetting in which only one solid layer of film coexists with bulk particles. Upon heating through the monolayer melting point, the amount of coexisting bulk solid first increases and then decreases. This anomalous layering behavior has not been observed in other systems exhibiting incomplete wetting, and we suggest that it is related to the steric properties of the Fe(CO)5 molecule.