A comparative synthetic, structural, and thermochemical study on a series of chelate complexes containing the fragment (eta(5)-C5Me5)Ir [(eta(5)-C5Me5)Ir(TsNCH2CH2NTs) (1), (eta(5)-C5Me5)Ir(TsNCH2CO2) (2), (eta(5)-C5Me5)Ir(CO2CO2) (3)] was performed to clarify the roles of carboxylato and sulfonamido ligands. Whereas 1 and 2 are monomeric in solution and in the solid state, 3 appears to exist as an oligomer or polymer, (3)(n), which can be broken up by addition of a Ligand L such as a phosphine, CO, or 2-methoxypyridine to form (eta(5)-C5Me5)Ir(L)(CO2CO2) (6). The synthesis of (3), from [(eta(5)-C5Me5)IrCl(mu-Cl)](2) required the use of silver oxalate in CH3CN, but if other solvents were used, the bridging oxalato complex (eta(5)-C5Me5)IrCl(mu-eta(2)-eta(2)-C2O4)ClIr(eta(5)-C5Me5) (7) was obtained and identified by X-ray diffraction. Enthalpies for reaction of THF-soluble monomers 1 and 2 with PMe3 were determined to be -28.7(0.5) and -28.5(0.4) kcal mol(-1), respectively. The oligomerization behavior of 3 may be a result of reduced sigma- or pi-donation of carboxylato ligands compared to N-tosylamido ligands, because the values for nu(CO) in oxalato and bissulfonamido complexes 6-CO and (eta(5)-C5Me5)Ir(CO)(TsNCH2CH2NTs) (4-CO) were 2064 and 2042 cm(-1), respectively.