Electron deficient molecules Diborane

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is partiy responsible for the great interest surrounding borane chemistry and molecular stmcture. The stmcture of even the simplest boron hydride, diborane(6) [19287-45-7] 2 6 sufficientiy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged stmcture shown. 

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). 

Valence-bond theory is over 90% successful in explaining much of the descriptive chemistry of ground states. VB theory is therefore particularly popular among chemists, since it makes use of familiar concepts such as chemical bonds between atoms, resonance hybrids and the like. It can perhaps be characterized as a theory which explains but does not predict. Valence-bond theory fails to account for the triplet ground state of O2 or for the bonding in electron-deficient molecules such as diborane, B2H6. It is not very useful in consideration of excited states, hence for spectroscopy. Many of these deficiencies are remedied by molecular orbital theory, which we take up in the next two chapters. 

The H2 binds side-on (referred to as 172) to the metal primarily via donation of its two G-bonding electrons to a vacant metal orbital to form a stable H2 complex. It is remarkable that this already strongly bonded electron pair can be donated to a metal to form a nonclassical two-electron, three-center bond, as in other electron-deficient molecules such as diborane (B2H6). 

Electron-deficient molecules are those that have insufficient valence electrons to form the conventional two-centre, two-electron bonds required by the molecular structure. An example is diborane, B2H6, Structure 5.6. 

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. 

The boranes are electron-deficient compounds (Section 3.8) we cannot write valid Lewis structures for them, because too few electrons are available. For instance, there are 8 atoms in diborane, so we need at least 7 bonds however, there are only 12 valence electrons, and so we can form at most 6 electron-pair bonds. In molecular orbital theory, these electron pairs are regarded as delocalized over the entire molecule, and their bonding power is shared by several atoms. In diborane, for instance, a single electron pair is delocalized over a B—H—B unit. It binds all three atoms together with bond order of 4 for each of the B—H bridging bonds. The molecule has two such bridging three-center bonds (9). 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

A simple theoretical treatment that can be applied to electron-deficient substances in general can be given for the diborane molecule.81 Let us consider the various valence-bond structures that can be written for the molecule8 (with its known configuration) with use of the six... 

As a first step toward answering this question, it seems reasonable to investigate a high-symmetry polyatomic. BH3 is an obvious candidate, given its D3h equilibrium geometry (experimentally verified by photoelectron-spectroscopy studies of BH3 [38] and by gas-phase spectroscopic observations of the neutral molecule itself [39][40]). Besides, its small size makes all-electron OBS-GMCSC calculations on it easily feasible, nowadays, even on a run-of-the-mill Personal Computer. Moreover, the fact that BH3 is an electron-deficient system, and of course that it spontaneously dimerizes to the ever-intriguing diborane, implies that a study of its electronic structure may be of intrinsic interest. 

Borane has the same structure as a carbocation. The boron is sjr hybridized, with trigonal planar geometry, and has an empty p orbital. Although neutral, it is electron deficient because there are only six electrons around the boron. It is a strong Lewis acid. An electron-deficient compound often employs unusual bonding to alleviate somewhat its instability. In the case of borane, two molecules combine to form one molecule of diborane ... 

---