Basicities of the series of complexes CpIr(CO)(PR(3)) [PR(3) = P(p-C6H4CF3)(3), P(p-C6H4F)(3), P(p-C6H4Cl)(3), PPh(3), P(p-C6H4CH3)(3), P(p-C6H4OCH3)(3), PPh(2)Me, PPhMe(2), PMe(2), PEt(3), PCy(3)] have been measured by the heat evolved (Delta H-HM) when the complex is protonated by CF3SO3H in 1,2-dichloroethane (DCE) at 25.0 degrees C. The -Delta H-HM values range from 28.0 kcal/mol for CpIr(CO)[P(p-C6H4CF3)(3)] to 33.2 kcal/mol for CpIr(CO)(PMe(3)) and are directly related to the basicities of the PR(3) ligands in the complexes. For the more basic pentamethylcyclopentadienyl analogs, the -Delta H-HM values range from 33.8 kcal/mol for the weakest base Cp*Ir(CO)[P(p-C6H4CF3)(3)] to 38.0 kcal/mol for the strongest Cp*Ir(CO)(PMe(3)). The nucleophilicities of the Cp ’ Ir(CO)(PR(3)) complexes were established from second-order rate constants (k) for their reactions with CH3I to give [Cp ’ Ir(CO)(PR(3))(CH3)]I-+(-) CD2Cl2 at 25.0 degrees C. There is an excellent linear correlation between the basicities (Delta H-HM) and nucleophilicities (log k) of the CpIr(CO)(PR(3)) complexes. Only the complex CpIr(CO)(PCy(3)) with the bulky tricyclohexylphosphine ligand deviates dramatically from the trend. In general, the pentamethylcyclopentadienyl complexes react 40 times faster than the cyclopentadienyl analogs. However, they do not react as fast as predicted from electronic properties of the complexes, which suggests that the steric size of the Cp* ligand reduces the nucleophilicities of the Cp*Ir(CO)(PR(3)) complexes. In addition, heats of protonation (Delta H-HP) of tris(2-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, and tris(2,4,6-trimethylphenyl)phosphine were measured and used to estimate pK(a) values for these highly basic phosphines.