A large number of five-coordinate metal catecholate complexes of the general formula [(triphos)M(Cat)]Y have been synthesized and characterized by chemical, spectroscopic and electrochemical techniques (MCo, Rh, Ir; Cat=9,10-phenathrenecatecholate, 1,2-naphthalencatecholate, 3,5-di-tert-butylcatecholate, 4-methylcatecholate, 4- carboxycatecholate-ethylester, tetrachlorocatecholate; YBPh 4, PF 6; triphos=MeC(CH 2PPh 2) 3). All of the compounds undergo electron-transfer reactions that encompass the M(III), M(II) and M(I) oxidation states of the metal, and the catecholate, semiquinone and quinone oxidation levels of the quinoid ligand. Paramagnetic Ir(III) semiquinonate complexes, [(triphos)Ir(SQ)] 2+, and Ir(II) catecholates, [(triphos)Ir(Cat)], have been characterized by X-band ESR spectroscopy. The reactions of the metal catecholates in non-aqueous media with dioxygen have been investigated. With very few exceptions, all of the compounds react with O 2 to give adducts of the general formula [(triphos) M(O O)(S Q)]Y. An X-ray analysis has been carried out on [(triphos) Ir(O O)(P henSQ)]BPh 4, (Phen=9, 10-phenanthrenesemiquinonate). In the complex cation, the metal is octahedrally coordinated by the three phosphorus atoms of triphos and by three oxygen atoms, one from O 2 and the other two from the catecholate ligand that has attained a semiquinoid character. The electrochemical behavior of the dioxygen adducts has been studied in detail. Depending on the E°′ values relative to the M III(SQ)/M III(Cat) couples of the parent metal catecholates, the dioxygen adducts undergo either a one-electron oxidation to give o-quinone complexes [(triphos) M(Q)] 3+ and superoxide ion (O 2−) or a two-electron oxidation to give [(triphos)M(Q)]3+ and O 2. Several factors have been found to affect the O 2 uptake by metal catecholates. Of particular importance are: (i) the coordination number of the metal; (ii) the basicity of either the catecholate ligand or metal; (iii) the temperature; (iv) the pressure of dioxygen. The role of each factor has been analyzed and rationalized. The transport of dioxygen from one metal catecholate to another has been studied. A mechanistic interpretation for the formation of the [(triphos) M(O O)(S Q)] + complexes is proposed in light of a large crop of experimental data and molecular orbital considerations.