This volume contains most of the invited lectures of the 2nd Structural Chemistry Indaba on "Molecular Interactions," held at Skukuza, Kruger Park, South Africa, August 3- 8, 1997. While the 1995 conference concentrated more on the principles underlying molecular modeling, like the existence of a molecular shape, this conference centers on molecular interactions or, more generally, on molecules in environments. Unfortunately, it was impossible, for various reasons, to unite all invited lectures in this volume, but nevertheless this collection contains contributions ranging from the fundamental quantum mechanical theory to recent research on organometallic crystals. For a summary, I would like to refer the reader to the introductory chapter by S.O. Sommerer, based on his concluding remarks at the conference. WemerGans for the editors v CONTENTS Intermolecular Interactions ...S. O. Sommerer Intermolecular Bonding ...3 1. C. A. Boeyens Chemical Reactions in the Framework of Single Quantum Systems ...9 A. Amann The Molecule and Its Environment ...25 ...B. T. Sutcliffe Dynamic Aspects of Intermolecular Interactions ...49 ...1. F. Ogilvie Atomic Interactions and the Charge Density ...5 7 ...T. Koritsanszky 71 Cyclometallation of Alkylphosphines M. T. Benson and T. R. Cundari C-H-. *0 Hydrogen Bonds in Organometallic Crystals 83 D. Braga and F. Grepioni The Importance of Intra-and Intermolecular Weak Bonds in Transition Metal Coordination Compounds ...97 P. Comba Relationships between Experiment and Theory in the Study of Intermolecular Interactions ...111 ...