Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate mu-eta(1):eta(1)-peroxo ligand. We observe markedly enhanced intensity at 788 cm(-1) in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon O-18 labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH2 substituents directly attached to P(Ph)(2) groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)(3)(OCH2PPh2) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O-2 also led to the growing in of a Raman band at 788 cm(-1), similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H-2, (ii) O-2, (iii) H-2, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O-2, and mass spectra of the effluent gas show that CO2 formed by oxidation of one terminal CO ligand per cluster as H-2 (and not H2O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O-2 reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.