Iridacyclohexadienone complexes, a new class of unsaturated six-membered metallacycles, have been synthesized, and their reactions with acids have been investigated. Treatment of CH=C(Me)-CH=C (Me)-CH=Ir(PMe3)(3) (''iridabenzene'' 1) with nitrous oxide produces the iridacyclohexadienone complex CH=C(Me)-CH=C- (Me)-C(O)-Ir(PMe3)(3)(H) (2), which upon stirring in diethyl ether solution slowly isomerizes to (1,2,5-eta-2,4-dimethylpenta-1,3-dien-5-oyl)Ir(PMe3)(3) (3). Treatment of 2 with methyl trifluoromethanesulfonate leads to removal of the metal hydride and production of CH=C(Me)-CH=C(Me)-C(O)-Ir(PMe3)(3)(O3SCF3) (4). In acetone solution, 4 forms an equilibrium mixture with a ring-contracted iridacyclopentadiene isomer, [CH=C(Me)-CH=C(Me)-Ir(PMe3)(3)(CO)]+O3SCF3- (5). Treatment of 4 with trimethylphosphine generates the tetrakis(trimethylphosphine)iridacyclohexadienone complex [CH=C(Me)-CH=C(Me)-C(O)-Ir(PMe3)(4)]+O3SCF3- (6). Iridacyclohexadienone 2 reacts with trifluoromethanesulfonic acid to produce the iridacyclopentene complex [CH2-C(Me)=CH-CH(Me)-Ir(P Me-3)(3)(CO)]+O3SCF3- (7). In contrast, treatment of iridacyclohexadienones 4 and 6 with trifluoromethanesulfonic acid leads to protonation of oxygen and production of the corresponding ''iridaphenols'' 8 and 10. When 4 is treated with trifluoroacetic acid, an iridaphenol product is again generated (compound 9), but in this case the phenol proton forms an intramolecular hydrogen bond with the carbonyl oxygen of the trifluoroacetate ligand. The ring protons in compounds 8, 9, and 10 exhibit downfield H-1 NMR chemical shifts consistent with aromatic character. Compounds 3, 4, 6, 7, 8, and 9 have been structurally characterized by single-crystal X-ray diffraction.