Properties of dirhodium catalysts with cyclometalated aryl phosphine ligands have been studied. We report here the study of the acid-base reaction of Rh-2(RCO2)(2)(PC)(2)(H2O)(2) catalysts (PC = cyclometalated aryl phosphine) with different Lewis bases. The determination of the equilibrium constants of these reactions can be used to study to which extent the properties of the axial coordination site of the catalyst, considered the active site, are affected by modification of the metalated phosphines, the carboxylate ligands, or the incoming axial ligand. The trends in the computational density functional theory interaction energies show good agreement with the major trends in the equilibrium constants, thus enabling a further study of the influence of the modification of the ligand core.