Bonding Energetics of Organometallic Compounds

Abstract

AbstractThis contribution gives an overview of solution calorimetric studies carried out on organometallic complexes in the last decade. A special focus has been put on a close investigation of metal–ligand exchange reactions involving different ligand and metal systems. Ligand substitution reactions are at the heart of many catalytic transformations, since creation of vacant coordination sites often require liberation of one or more ligands from the coordination sphere of the metal. Detailed analyses of phosphine‐, phosphite‐, and NHC‐ (NHC = N‐heterocyclic carbene) ruthenium, rhodium, and platinum systems show that enthalpies of ligand substitution reactions are associated to electronic and/or steric variation of the ligands. They can be explained in terms of different amounts of these two contributions to the overall enthalpy. Extensive calorimetric studies on ligand substitutions showed that cis disposition of the ligands leads to an overwhelmingly steric contribution to the enthalpy of ligand substitution, whereas trans disposition of the ligands gives enthalpy values that are highly dependent on the electronic nature of the ligand systems studied. Thermodynamic studies of addition reactions of organic molecules to metal fragments are equally crucial in catalysis and gave valuable thermodynamic data for CO, H2, aldehyde, acyl chloride addition reactions to catalytically relevant rhodium systems. Thermodynamic data for different reductive cleavage reactions of organic fragments involving molybdenum systems are also presented.