We investigate the binding thermodynamics of a series of phosphorus ligands to a model compound, PdCl2(solv)(2), where solv refers to a molecule of solvent, using isothermal titration calorimetry (ITC). ITC allows for the quantification of the equilibrium binding constant, the binding enthalpy, and the binding stoichiometry all in a single experiment. For systems in which two equivalents of ligand were able to bind to the Pd center, the binding sites on each Pd center in solution showed a different thermodynamic affinity for the same ligand. Changes in binding modes between different phosphorus ligands were due to steric bulk and poor electron-donating ability of such ligands. Our results demonstrate ligand binding was strongly enthalpy-driven due to solvent reorganization, which is the rearrangement of solvent molecules in the bulk solvent and the solvent molecules surrounding the solvated species. (c) 2013 Elsevier Inc. All rights reserved.