We report the synthesis, characterization, and reactivity of [LFe3O((R)ArIm)(3)Fe][OTf](2), the first Hammett series of a site-differentiated cluster. The cluster reduction potentials and CO stretching frequencies shift as expected on the basis of the electronic properties of the ligand: electron-donating substituents result in more reducing clusters and weaker C-O bonds. However, unusual trends in the energetics of their two sequential CO binding events with the substituent (sigma)p parameters are observed. Specifically, introduction of electron donating substituents suppresses the first CO binding event (Delta Delta H by as much as 7.9 kcal mol(-1)) but enhances the second (Delta Delta H by as much as 1.9 kcal mol(-1)). X-ray crystallography, including multiple-wavelength anomalous diffraction, Mossbauer spectroscopy, and SQUID magnetometry, reveal that these substituent effects result from changes in the energetic penalty associated with electronic redistribution within the cluster, which occurs during the CO binding event.