When one considers the overall representation of frontier orbital filling of hexacoordinate (Oh) and tetracoordinate (Td) inorganic and organo metallic complexes, it clearly appears that out of 26 cases covering both high spin and low spin situations, 21 represent paramagnetic species (K. Purcell, J. Kotz, "Inorganic Chemistry", Saunders, 1977, p561). This would suggest that, if there is a part in chemistry to illustrate the reactivity of radical species, this part certainly is inorganic organometallic chemistry. In contrast with these expectations, and whereas the standard Organic Chemistry textbook (J. March ,"Advanced Organic Chemistry", J. Wiley, N. Y. , 1985) has a specific chapter devoted to free radical reactivity, neither the inorganic standard (FA Cotton, G. Wilkinson, "Advanced Inorganic Chemistry", Wiley, 1988), nor the Organometallic one (J. P. Collman, L. S. Hegedus, J. R. Norton, R. G. Finke, "Principles and Applications of Organotransition Metal Chemistry", University Science Books Mill Valley C. A. , 1987) possess such a specific chapter. The balance is partly restored because the two last cited books have a more comprehensive treatment of electron transfer phenomena. These comparisons show unambiguously that the importance of paramagnetic species in chemical reactivity still lacks a consistent treatment transcending the artificial barriers between branches of Chemistry. This book, which brings together experimental facts and concepts originating from organometallic and organic reactivities, is a step in the direction of bridging this gap. The unifying thread which connects the 35 chapters throughout this book is Activation/Selectivity and Catalysis by means of radical chemistry.

Proceedings of the NATO Advanced Research Workshop, St. Maximin, France, October 4-9, 1987

An overview of modern organometallic thermochemistry, made by some of the most active scientists in the area, is offered in this book. The contents correspond to the seventeen lectures delivered at the NATO ASI Energetics of Organometallic Species (Curia, Portugal, September 1991), plus three other invited contributions from participants of that summer school. These papers reflect a variety of research interests, and discuss results obtained with several techniques. It is therefore considered appropriate to add a few preliminary words, attempting to bring some unity out of that diversity. In the first three chapters, results obtained by classical calorimetric methods are described. Modern organometallic thermochemistry started in Manchester, with Henry Skinner, and his pioneering work is briefly surveyed in the first chapter. The historical perspective is followed by a discussion of a very actual issue: the trends of stepwise bond dissociation enthalpies. Geoff Pilcher, another Manchester thermochemist, makes, in chapter 2, a comprehensive and authoritative survey of problems found in the most classical of thermochemical techniques - combustion calorimetr- applied to organometallic compounds. Finally, results from another classical technique, reaction-solution calorimetry, are reviewed in the third chapter, by Tobin Marks and coworkers. More than anybody else, Tobin Marks has used thermochemical values to define synthetic strategies for organometallic compounds, thus indicating an application of thermochemical data of which too little use has been made so far.

The use of electrochemical techniques by chemists, particularly those who regard themselves as "inorganic" coordination chemists, has undergone a very rapid growth in the last 15-20 years. The techniques, as dassically applied to inorganic species, had their origins in analytical chemistry, and the methodology had assumed, until the mid 60s, more importance than the chemiStry. However, the growth of interest in coordination compounds (including organometallic complexes) having unusually rich of electron-transfer in bio-inorganic redox properties, and in the understanding species, has propelfed electro-chemistry into the foreground of potentially readily available techniques for application to a very wide range of problems of interest to those chemists. This growth has been fuelled additionally by the availability of relatively cheap equipment of growing sophistication and by an increase in the "inorganic" chemists' general knowledge of physical electrochemistry. In particular, with increasing availability and sophistication of eqUipment, kinetic problems are now being addressed, and the range of electrode types and configuration and solvents has been greatly expanded. Furthermore, the rapid expansion of interest in biological problems has opened new avenues in functionalisation of electrodes, in the development of sensory devices and, in a sense, a return to the analytical base of the science, using novel and multi-disciplinary techniques drawing on synthesis chemistry of and electronic micro-engeneering. The drive towards increasing use microcomputer-controlled data analysis and the development of microeledrodes has opened exciting new avenues for the exploration of chemical reactions involving electron-transfer processes.

Free-radical reactions have been for long time the domain of the physical chemists and the basic chemical industry (polymerization of vinyl monomers, oxidation by molecular oxygen, chlorination of methane etc.), where the use of simple molecules and the possibility of partial conversions without heavy problems of product separations makes less dramatic the aspects of regio and chemoselectivity. As synonym of unselectivity, free radical reactions were considered of poor use in the synthesis of fine chemicals or sophisticated molecules, where a high selectivity is an essential condition for the success, or in the involvement of biological processes. Within the last 15 years, however, an authentic explosion of synthetic applications of free radical reactions occurred; they have gained a remarkable position among the selective methods of synthesis. At the same time the great importance of free radical reactions in fundamental biological processes and in the metabolism of drugs has been recognized. Thus a specialized meeting on these topics was generally felt appropriate. I had the honour and the onus to organize this workshop because for more than 30 years I have been involved in the research of free radical reactions. Thanks to a generous grant from NATO Scientific Affairs Division and the financial support of CNR and chemical industry (Montedison, Enichem, Zambon) such a meeting among almost 50 specialists and 15 observers of sufficient standard to take advantage of the discussion, became possible at Bardolino (Italy).

TEAN-LOUIS LUCHE A French poet of this eentury, Pierre Mae Orlan, wrote "Adventure does not exist, it is only in the mind of he who is pursuing it, and, as soon as it is at one's finger tips, it vanishes to come back to life, far away, in a different shape, at the frontiers of imagination". This sentence could be used to define the adventure that many sonochemists experienced. Most of them did not even suspect that the "laboratory trick" they were using was the first contact with a considerable amount of science. If a personal note is allowed here, it ean be interesting to mention the part played by chance in my involvement in sonochemistry. Almost 20 years ago, we had to perform an apparently simple Grignard reaetion with n-butylmagnesium bromide and geranial, but the results were repeatedly unsatisfactory. The one-pot Barbier technique was attempted, also without success. From my studies at the University, I imagined that the failure of the latter reaction could be caused by a common phenomenon known by solid state chemists as passivation, which in some cases can be overcome by ultrasonication. By chance, an ultrasonie bath was sitting on the next beneh, borrowed to clean some equipment. We clamped our reluctant reaction mixture into the bath, the reaction proceeded vigorously, and ... the adventure started. Without knowing anything about cavitation, high energies, ete., we had an illustration of Goethe's word "Am Anfang war die Tat" (at the Beginning was the Act).

Organometallic chemistry is based on the reactions and use of a class of compounds (R-M) that contain a covalent bond between carbon and metal. They are prepared either by direct reaction of the metal with an organic compound or by replacement of a metal from another organometallic substance. Research in organometallic chemistry is also conducted in the areas of cluster synthesis, main-group derivatives in unusual oxidation states, organometallic polymers, unstable organometallic compounds and intermediates in matrices, structure determination of organometallic compounds in the solid state [X-ray diffraction] and gaseous states [electron diffraction], and mechanisms of reactions of transient silylenes and related species. In addition to the traditional metals and semimetals, elements such as selenium, lithium and magnesium are considered to form organometallic compounds, e.g. organomagnesium compounds MeMgI, iodo(methyl)magnesium and diethylmagnesium which are Grignard reagents an organo-lithium compound BuLi butyllithium. Organometallic compounds often find practical use as catalysts, the processing of petroleum products and the production of organic polymers.