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The series Structure and Bonding publishes critical reviews on topics of research concerned with chemical structure and bonding. The scope of the series spans the entire Periodic Table and addresses structure and bonding issues associated with all of the elements. It also focuses attention on new and developing areas of modern structural and theoretical chemistry such as nanostructures, molecular electronics, designed molecular solids, surfaces, metal clusters and supramolecular structures. Physical and spectroscopic techniques used to determine, examine and model structures fall within the purview of Structure and Bonding to the extent that the focus is on the scientific results obtained and not on specialist information concerning the techniques themselves. Issues associated with the development of bonding models and generalizations that illuminate the reactivity pathways and rates of chemical processes are also relevant. The individual volumes in the series are thematic. The goal of each volume is to give the reader, whether at a university or in industry, a comprehensive overview of an area where new insights are emerging that are of interest to a larger scientific audience. Thus each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole. The most significant developments of the last 5 to 10 years should be presented using selected examples to illustrate the principles discussed. A description of the physical basis of the experimental techniques that have been used to provide the primary data may also be appropriate, if it has not been covered in detail elsewhere. The coverage need not be exhaustive in data, but should rather be conceptual, concentrating on the new principles being developed that will allow the reader, who is not a specialist in the area covered, to understand the data presented. Discussion of possible future research directions in the area is welcomed.
Review articles for the individual volumes are invited by the volume editors



Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

The discovery of frustrated Lewis pairs (FLPs) was based on the reactions of combinations of sterically demanding phosphine donors and electrophilic boranes with dihydrogen. Since these early findings, the range of species that exhibit FLP chemistry has broadened dramatically to include a series of Lewis acids and bases. In this review, we describe a variety of such systems and the corresponding reactivity of FLPs derived from group 13–16 species.
Christopher B. Caputo, Douglas W. Stephan

Triphosphine Ligands: Coordination Chemistry and Recent Catalytic Applications

Phosphines are a long established class of ligand that are known to form a vast array of transition metal complexes. They behave as neutral electron pair donors, or Lewis bases, that alter the solubility and stereoelectronic properties of the metal centre. A key motivation for their continued development is for homogeneous catalysis. For over five decades, transition metal–phosphine complexes have been used for catalytic reactions, mainly exploiting monodentate or bidentate phosphine ligands. Multidentate phosphines by comparison have received much less attention in part because they tend to form more stable complexes with a saturated coordination environment around the metal centre. Recent developments in the areas of catalytic reduction of carboxylic acid derivatives using molecular hydrogen and in the field of biomass up-conversion have exploited catalysts based on tridentate phosphines. This chapter highlights the use of these multidentate phosphines for synthesis of coordination complexes and discusses some of their recent applications in homogeneous catalysis.
Andreas Phanopoulos, Nicholas J. Long, Philip W. Miller

Sigma Bonds as Ligand Donor Groups in Transition Metal Complexes

Covalent X–H bonds, particularly where X is H, C, B, and Si, can act as Lewis base ligands in forming metal complexes of general type L n M(H–X), where the M–H–X angle is strongly bent so that the coordination can best be considered as side-on. The binding is greatly enhanced by back donation from the metal into the X–H σ* orbital. This elongates and eventually breaks the X–H bond, leading to characteristic structural, physicochemical characteristics and alteration of the reactivity of the ligand. The requirement for back donation means that the appropriate L n M fragment must usually have appreciable π donor character. Since the binding of the X–H bond to the metal center is relatively weak, and the binding of the deprotonated X group left behind is much stronger, the binding also facilitates proton loss from the X–H bond. This electron redistribution results in reactivity differences which may be exploited. The case of H2 is treated in most detail in this review, because of its central place in the field.
Robert H. Crabtree

The Covalent Bond Classification Method and Its Application to Compounds That Feature 3-Center 2-Electron Bonds

This article provides a means to classify and represent compounds that feature 3-center 2-electron (3c–2e) interactions according to whether (1) the two electrons are provided by one or by two atoms; (2) the central bridging atom provides two, one, or zero electrons; and (3) the interaction is open or closed. Class I 3c–2e bonds are defined as those in which two atoms each contribute one electron to the 3-center orbital, while Class II 3c–2e bonds are defined as systems in which the pair of electrons are provided by a single atom. The use of appropriate structure-bonding representations enables the [ML l X x Z z ] covalent bond classification of the element of interest to be evaluated. This approach is of considerable benefit in predicting metal–metal bond orders that are in accord with theory for dimetallic compounds that feature bridging hydride and carbonyl ligands.
Malcolm L. H. Green, Gerard Parkin

Coordination of Lewis Acids to Transition Metals: Z-Type Ligands

This chapter provides a comprehensive review of M→Z complexes, that is to say complexes featuring Lewis acids coordinated as σ-acceptor ligands. The preparation, structure and bonding, as well as the characteristic features of M→Z complexes are discussed. Only Lewis acids derived from the p-block elements are considered, with a focus on two-center M→Z interactions supported by donor sidearms. The chapter is organized according to (1) the nature of the Lewis acid moiety (based on group 13, 14, 15 or 16 elements), (2) the way the M→Z interaction is formed (by B–H activation of pro-ligands or direct coordination of preformed ambiphilic ligands) and (3) the type of complexes (M→Z interactions supported by 3, 2 or 1 donor sidearms, unsupported M→Z interactions).
Ghenwa Bouhadir, Didier Bourissou


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