We present a thermodynamic study of the chelate effect for a series of P-P, N-N, and P-N bidentate ligands binding to PdCl2(MeCN)(2) in acetonitrile (MeCN), using isothermal titration calorimetry (ITC) to measure the solution-phase binding thermodynamics. The chelate effect is considered to be an entropic effect, that is, the enthalpic contributions attributed to binding for comparable monodentate and bidentate ligands are nearly identical, meaning that changes in AG are due to favorable changes in AS upon displacement of bound solvent molecules. However, our results demonstrate that enhanced enthalpic contributions (i.e. large and exothermic) control the stability of the chelated complexes and generally are accompanied by large entropic penalties. We discuss the contribution of solvent reorganization and its role in enthalpy-entropy compensation for the binding equilibria studied. (C) 2013 Elsevier Inc. All rights reserved.