A series of seven [Ir{ArNC(NR2)NAr}(cod)) complexes (1a-1g; where R = Me or Et; Ar = Ph, 4-MeC6H4, 4-MeOC6H4, 2,6-Me2C6H3, or 2,6-(Pr2C6H3)-Pr-i; and cod = 1,5-cyclooctadiene) were synthesized by two different methods from the neutral guanidines, ArN=C(NR2)NHAr, using either MeLi and [{Ir(cod)}(2)(mu-Cl)(2)] or [{Ir(cod)}(2)(mu-OMe)(2)]. Reaction of 1a-1g with CO produced the corresponding [Ir{ArNC(NR2)NAr}(CO)(2)] complexes (2a-2g), which were characterized by NMR and solution- and solid-state IR spectroscopy. Complexes 1b (R = Et, Ar = Ph), Id (R = Et, Ar = 4-MeC6H4), If (R = Me, Ar = 2,6-Me2C6H3), and 2b (R = Et, Ar = Ph) were characterized by X-ray crystallography as mononuclear complexes with a guanidinato-kappa N-2,N' ligand and a cod or two CO ligands coordinated to the Ir center in a distorted square-planar environment. On the basis of the CO stretching frequencies of 2a-2g [avg. nu(co) (n-pentane) = 2016-2019 cm(-1)] and the alkene C-13 chemical shifts of 1a-1g [delta(C-13(C=C)) = 58.7-61.0 ppm], the donor strength of the guanidinato ligands was evaluated and compared to that of related monoanionic ligands. Reaction of la lg in solution with O-2 at 20 degrees C afforded (alkene)peroxoiridium(III) intermediates, [Ir{ArNC(NR2)NAr}(cod)(O-2)) (3). The steric properties of the supporting ligand play a decisive role in O-2 binding in that complexes without ortho substituents react largely irreversibly with O-2 (1a-1e; where Ar = Ph, 4-MeC6H4 or 4-MeOC6H4), whereas complexes with ortho substituents exhibit fully reversible O-2 binding (If and 1g; where Ar = 2,6-Me2C6H3 or 2,6-(Pr2C6H3)-Pr-i). Complexes 3a-3f were characterized by H-1 NMR and IR spectroscopy (nu(OO) = 857-872 cm(-1)). Decay of the new intermediates and subsequent reaction with cod produced 4-cycloocten-1-one and the respective precursor.