Supported iridium complexes were synthesized by the reaction of Ir(CO)(2) (acac) (acac = acetylacetonato) with SiO2, gamma-Al2O3, and MgO. Extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectra demonstrate that the iridium was present as atomically dispersed species anchored to each support. The samples were treated in flowing H-2 at 673 K to form supported iridium clusters. EXAFS spectra and highangle annular dark-field scanning transmission electron microscopy images demonstrate that the average diameter of the iridium clusters on each support was approximately 1.2 nm. The catalysts before and after cluster formation were coated with each of a family of 1,3-dialkylimidazolium ionic liquids (ILs) having varying electron-donor tendencies probed by their nu(C2H) frequencies determined by IR spectroscopy. The coated and uncoated samples were tested as catalysts for partial hydrogenation of 1,3-butadiene in a flow reactor at 333 K, with turnover frequencies determined from differential conversions. The individual influences of the IL coatings and supports on the catalyst performance were found to depend strongly on whether the iridium was site-isolated complexes or present in clusters. The IL coatings as ligands exerted dominant effects on the clusters as catalysts, whereas the supports exerted dominant effects on the isolated iridium atoms. The results indicate how to tune the effects of metal nuclearity, IL coatings, and supports on the electronic environments and catalytic properties of the metals. (C) 2020 Elsevier Inc. All rights reserved.