Bonding chemistry, multicenter

One of the most important and exciting events in the chemistry of the twentieth century was a discovery of polyhedral boron hydrides at the end of 1950s. It was shown that boron atoms in boron hydrides are linked by unusual three-centered two-electron bonds or multicentered bonds. The establishment of three-centered two-electron bonds made a true revolution in the theory of chemical bonding and William Lipscomb was awarded the Nobel Prize in 1976 for his studies on the structure of boranes illuminating problems of chemical bonding [1]. 

Cluster modeling of possible chemisorption states and of possible intermediate states in surface reactions can to a first approximation be useful in guiding experiments or interpretations of experimental data for surface reactions (23-25). One important and enlightening result (6, 26, 27) in metal carbide cluster chemistry will be used here to illustrate this particular point because it bears directly on the importance of multicenter C-H-M bonding for hydrocarbon fragments in metal chemistry. 

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

The Lewis acid character of many boron compounds and the ability of boron to participate in multicenter bonds lead to a vast and complex chemistry. Boron exclusively forms covalent bonds in an irmnense variety of compounds, often... 

This article covers only a part of the chemistry of boron. Boron-carbon compounds are covered in other articles in this volume see Boron Organoboranes Boron Metallacarbaboranes, and Boron Polyhedral Carboranes). The main subject of the latter two articles, and the separate one on Boron Hydrides is the extensive chemistry of the multicenter bonded boron-hydride systems. This area has been a major focus of boron research for the past 60 years. There is some direct overlap between the two articles Borides Solid-state Chemistry and Borates Solid-state Chemistry, and this more general one covering the inorganic chemistry of boron. Boron-Nitrogen Compounds are also covered separately. These articles should be consulted for more detailed discussions of the structure, bonding, and properties of borides, solid-state borates, and boron-nitrogen compounds. 

If we skip even that, we speak of integral chemistry (1C). This is the realm of model building presented in this book. At this levelofchemistry, all multiplicities of covalent bonds are integers. However, there are phenomena in chemistry not compatible with this restriction (see Section 2.4). Mesomerism and multicentric bonds need a further generalization. We collect all these more general situations underthe notion of multicenter chemistry (NIC). 

The development of cluster chemistry has been influenced by possible, practical application of clusters in catalysis and organic synthesis. This development also comes about because of interest in metal-metal bonds and the theory of multicenter bonds, relationships between the structural theories of complexes and clusters, as well as analogies between clusters and metal surfaces. In clusters there are types of bonds which are not encountered in any other compounds, for example, bridges ... 

Many people make the mistake of equating the study of three-center bonds to the study of the boron hydrides. Certainly, the Colonel was very successful in applying 3-center bond concepts to boron hydrides. However, we should see the larger applicability of the general concept to noble gas compounds, localized orbitals for species such as COa, metals, metal- cluster compounds, etc. Someplace between the localized 2e-2c bonds beginning students (and some elderly chemists) love and cherish, and the fully-delocalized molecular orbitals we so often find useful, lies a localized multicenter bond. This approach to localized bonds looks like it will be very useful to chemists when it is more fiilly developed. They have some resemblance to the intuitive "best" valance structure selected from resonance hybrids. One should be able to do chemistry with these orbitals. Fractional bonds may be uncomfortable at first, but note that the octet rule applies to electrons, not orbitals. One of the most inqjortant contributions... 

Metallaborane and borylene complexes are related to some extent because both types possess direct metal-boron bonds however, the nature of these interactions is varied. In metallaborane clusters, the framework is made up by nonclassical, electron-deficient, multicenter two-electron bonds, while the borylene ligand (BR) is related to one or more transition metal centers by classicrJ, electron-precise, two-center two-electron (2c-2e) bonds [280 -283]. Since the discovery of various bridged and terminal borylene complexes by Braunschweig and Aldrige, the chemistry of this subarea of transition metal complexes of boron has received significant attention from both structure/bonding and... 

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