The Importance of Intra- and Intermolecular Weak Bonds in Transition Metal Coordination Compounds

Abstract

There is an increasing interest in supramolecular systems involving inorganic and organic compounds with specific properties that may lead, eventually, to novel materials and devices and to challenging advances in materials science and biochemistry. Supramolecular chemistry is the “chemistry beyond the molecule”, i.e. the “chemistry controlled by non-covalent intermolecular forces”.1 Of specific interest are self-assembled arrays of molecules and selforganized supramolecular systems. The general textbook definition describes a molecule as a “well defined assembly of atoms bound to each other”, as the “smallest unit of a pure compound with the specific chemical properties of the corresponding bulk material”.2 These definitions of molecules and supramolecular systems are as intuitive as they are ambiguous. Is the array of water molecules in beautiful ice flowers a supramolecular assembly, is the crystallization of sodium chloride a self-organization process? Challenging and truly interesting supramolecular systems have properties that are different from those of the constituent “molecules”. Where then is the limit between a molecule and a supramolecular system, which of the interactions are intra- and which are intermolecular? When does an interaction cease to be a bond and start to be a nonbonded interaction? In order to circumvent problems of this kind, I will often use the term “system” instead of “molecule”, and I will not distinguish between inter-and intramolecular bonds and interactions. The aim of the present chapter is to demonstrate the importance of interactions in coordination compounds that are not always appreciated as bonds. The examples cover structural and thermodynamic aspects, as well as spectroscopy and reactivity. Since some of these studies are discussed with the help of molecular modeling, a few general and important aspects of modeling related to intermolecular interactions are presented first.