Boron, electron-deficient molecular compounds

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Boron is a unique and exciting element. Over the years it has proved a constant challenge and stimulus not only to preparative chemists and theoreticians, but also to industrial chemists and technologists. It is the only non-metal in Group 13 of the periodic table and shows many similarities to its neighbour, carbon, and its diagonal relative, silicon. Thus, like C and Si, it shows a marked propensity to form covalent, molecular compounds, but it differs sharply from them in having one less valence electron than the number of valence orbitals, a situation sometimes referred to as electron deficiency . This has a dominant effect on its chemistry. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

The group 3A elements—B, Al, Ga, In, and T1—are metals except for boron, which is a semimetal. Boron is a semiconductor and forms molecular compounds. Boranes, such as diborane (B2H6), are electron-deficient molecules that contain three-center, two-electron bonds (B-H-B). 

The boron hydrides are called electron-deficient compounds because they are easily reduced by hydrogen. Incorrect boron hydrides are electron-deficient because they lack the electrons required to fill the bonding and non-bonding molecular orbitals. 

Moffitt and Ballhausen,41 and the application of the molecular orbital theory to electron-deficient compounds, particularly the hydrides of boron, by Eberhardt, Crawford, and Lipscomb has been fully described by these authors in their original papers.14 The present review will, therefore, be devoted entirely to those developments in the molecular orbital theory which have been associated with its application to the electronic spectra of unsaturated hydrocarbons. These developments form in themselves a relatively coherent story, the main lines of which are now clear, and it seems a suitable moment at which to put the history of the subject into perspective. Before doing this, however, it will be convenient to outline the general premises of the molecular orbital theory. 

Lewis Acids with Electron-Deficient Atoms Some molecular Lewis acids contain a central atom that is electron deficient, one surrounded by fewer than eight valence electrons. The most important of these acids are covalent compounds of the Group 3A(I3) elements boron and aluminum. As noted in Chapters 10 and 14, these compounds react to complete their octet. For example, boron trifluoride accepts an electron pair from ammonia to form a covalent bond in a gaseous Lewis acid-base reaction ... 

Boron.— The remarkable structural behaviour of boron, which tends to confound the newcomer, continues to receive considerable attention, although the extent of the coverage of boron compounds is perhaps reduced by the difficulties of synthesis which often exist. Since it is immediately to the left of carbon in the periodic table and only sli tly less electronegative, covalent behaviour predominates, but the electron-deficiency of simple tervalent-boron compounds leads to an extensive variety of molecules in which boron acquires electrons in various ways, particularly from nearby bonds or lone pairs. Interpretation of the structures observed frequently involves molecular-orbital calculations, which emphasize the multicentred bonding so often found. 

Despite these spectacular successes, the molecular structures of the boranes were entirely unknown and basically unknowable at that time. Lacking modem tools such as NMR and single-crystal X-ray diffi-action techniques (5), and with ideas of covalence in that era dominated by Lewis-style electron pair bonding as found in organic compounds. Stock and his contemporaries could only speculate about structure. For lack of a better idea, they assumed that the boron hydrides, notwithstanding an apparent deficiency of electrons, must adopt hydrocarbon-like chain structures such as the examples shown in Chart 1, for which both non-ionic and ionic models were suggested (4). 

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