Mononuclear [Ir{ArNC(NR2)NAr}(C8H12)] complexes (where R = Me or Et; Ar = Ph, 4-MeC6H4, or 2,6-Me2C6H3; and C8H12 = 1,5-cyclooctadiene) were synthesized from the neutral N,N-dialkyl-N',N ''-diarylguanidines via deprotonation and trans metalation. As confirmed by single-crystal structure determinations, the guanidinato(1-) ligands coordinate the low-valent d(8) Ir-I center in an N,N'-chelating binding mode, and the C-13 NMR chemical shifts of the alkene carbon atoms establish that these ligands function as stronger donors than related monoanionic, bidentate nitrogen-based ligands. In the reactions of the complexes with O-2, the observed reactivity trends correlate with the electronic and steric influences of the substituents of the guanidinato ligands.