Studies in Modern Chemistry – serie

Students embarking upon a colour chemistry course usually approach it by way of a general introduction and proceed to more detailed treatment of the subject when they have acquired some knowledge of its character and scope. This book has been written with the twofold purpose of serving as a guide to such students during the introductory part of their course and of supplying the needs in this field of others whose main interest is in a related branch of technology or pure chemistry. An attempt has been made to present the main features of the subject in an easily assimilable form. The great amount of published information renders the choice of material for a short book somewhat difficult, and I am keenly conscious of topics that might be thought worthy of more extensive treatment. However, a concise account cannot be comprehensive, and suggestions for further reading are provided at the end of the book. The chemistry of colouring matters can be regarded as a branch of pure chemistry, but the development of knowledge in this field has followed a course determined chiefly by the applications of dyes and pigments. It has therefore appeared appropriate to treat the subject here as a branch of technology.

This book is about compounds such as the boron hydrides and associated metal hydrides and alkyls which acquired the label 'electron deficient' when they were thought to contain too few valence electrons to hold together. Though they are now recognized as containing the numbers of bonding electrons appropriate for their structures, the term 'electron deficient' is still commonly applied to many substances that contain too few valence electrons to provide a pair for every pair of atoms close enough to be regarded as covalently bonded. The study of such substances has contributed much to chemistry. Techniques for the vacuum manipulation of volatile substances were devised specifically for their study; developments in valence theory resulted from considerations of their bonding; and the reactivity of several (for example, diborane and complex metal hydrides, lithium and aluminium alkyls) has made them valuable reagents. The purpose of this book is to provide an introduction to the chemistry of these fascinating compounds. The experimental and spectroscopic methods by which they can be studied are outlined, the various types of structure they adopt are described and profusely illustrated, and the relative merits of extended valence bond and simple molecular orbital treatments of their bonding are discussed, with as liberal use of diagrams and as limited recourse to the Greek alphabet as possible. A recurring theme is the importance attached to considerations of molecular sym­ metry. Their reactions are treated in sufficient detail to show whether these reflect any deficiency of electrons.

Carbenes, nitrenes, and arynes are of great interest to chemists because of the central role they play, as the key intermediates, in a wide range of chemical reactions. Their study has illuminated many aspects of synthetic and mechanistic chemistry. Their organic chemistry has developed enormously over the last ten to fifteen years so that now a fairly clear pattern of reactions and reactivity, strikingly similar for all three, has emerged. Partly because of this, but also because of the many unsolved problems and gaps in our present knowledge, it now seems appropriate to present a comparative account of their chemistry. Our approach is a simple, descriptive one that nevertheless attempts to bring the reader as nearly up-to-date as possible; there is considerable emphasis on recent work in the text and the problems. We have been selective rather than comprehensive, have concentrated on underlying principles, and have tried to emphasize what can, and what cannot, reasonably be claimed on the basis of the available experimental evidence. (The development of this area of organic chemistry could weil serve as a cauti'onary tale in this respect. ) We have written the book primarily at the advanced student level but it should also be useful to postgraduates students and to more experienced chemists. Some of the problems are at a rather more advanced level, the aim being to extend the reader's knowledge and understanding and to direct him to some recent originalliterature.