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Clusters that are well approximated as tetrairidium were synthesized from Ir(C2H4)2 complexes anchored to the Zr6O8 nodes of the metal organic framework (MOF) UiO-67 by treatment in H2 at 353 K. The conversion taking place within the porous MOF structure was monitored by infrared and X-ray absorption spectroscopies, which provide evidence of tetrairidium clusters formed from the mononuclear precursors. The supported clusters were tested in a flow reactor as catalysts for ethylene conversion at 298 K and atmospheric pressure with a 1:1 M feed ratio of H2 to ethylene. The results show that the turnover frequency (per Ir atom) characterizing the clusters is twice that of the mononuclear iridium complexes, with both catalysts being active for hydrogenation and dimerization and the clusters being less selective than the complexes for dimerization. Density functional theory calculations of the reaction energetics are in good accord with experiment, showing that the rate-determining step for the hydrogenation on the isolated iridium complexes is the H2 activation on iridium, whereas the hydrogenation of an iridium-bound ethyl ligand is rate determining for the cluster.