The preparation of a new family of iridium(I) complexes with the mixed-donor bidentate ligands o-Ph2PC6H4CH=NR (PNalkyl) (R = Et, Pr(i), Bu(t)) is described. In these complexes, the iridium center is coordinated by two PNalkyl ligands in a square-planar environment. All[(PNalkyl)2Ir]+ compounds undergo reversible electron transfer to the Ir(0) oxidation state as well as irreversible oxidation to Ir(III). Only for R = Bu(t) is the lr(II) oxidation state accessible through a reversible oxidation process. For comparative purposes, the electrochemical behavior of the Rh(I) congeners [(PNalkyl)2Rh]PF6 has been investigated. Illustrative examples of the paramagnetic Ir(0),Rh(0), Ir(II), and Rh(II) derivatives have been studied by ESR spectroscopy, which confirms that the unpaired electron is mainly localized on the metal center. The Ir(I) complexes [(PNalkyl)2Ir]Y (Y = BPh4-, PF6-, Cl-; alkyl = Et, Pr(i)) react with O2, inorganic solvent solutions at room temperature, to form peroxo complexes of formula [(PNalkyl)2Ir(O2)]Y. The molecular structure of the PNEt derivative [(PNEt)2(O2)]PF6 has been determined by X-ray diffraction methods : monoclinic P2(1)/a, a = 17.426(4) angstrom, b = 14.685(3) angstrom, c = 16.131(4) angstrom, alpha = 90.00(0)degrees, beta = 98.07(2)degrees, gamma = 90.00(0)degrees, Z = 4, R = 0.053. The O-O bond distance is 1.47(1) angstrom. The coordination of the metal may alternatively be described as distorted trigonal bipyramidal or distorted octahedral, according to whether the dioxygen molecule is treated as occupying one or two equatorial sites, respectively. The steric hindrance of the alkyl substituent on the imino donors appears as a limiting factor for the dioxygen uptake, as shown by the fact that [(PNBu(t))2Ir]+ does not react with O2. Unlike the rhodium congeners, the Ir(I) complexes do not behave as oxygen carriers, the O2 molecule being irreversibly coordinated to the metal center even at high temperature. In contrast, reversible square-pyramidal adducts form upon reaction of the PNEt and PNPr(i) Ir(I) complexes with CO. No reaction occurs with the PNBu(t) derivative, confirming that the size of the imino substituent controls the access of incoming reagents at the metal center.