Diborane description

It must be emphasized that the formulation of three-center T-bonds provides a qualitative (not merely incremental) improvement in the accuracy of the natural Lewis-structure description of diborane. Because a three-center orbital is intrinsically more mathematically flexible than a two-center orbital, the description of any molecule is seemingly improved by employing three-center in place of two-center NBOs. However, for most non-boron molecules this improvement would be quite negligible (e.g., less than 0.1% for ethane, whose two-center Lewis-structure... 

The equilibrium geometry and three-center NBOs of the protonated ethylene model are displayed in Fig 3.97. The qualitative similarities to the diborane bridge bond (Fig. 3.93) are evident. Thus, the 7t—s model (cf. Fig. 3.91) may be considered a useful descriptive picture of three-center T-bond formation in diborane. 

Fig. 16.40 Qualitative description of atomic orbitals (loft). resulting three-center molecular orbitals (right), and the approximate energy level diagram (center) for one B—H—B bridge in diborane...

The addition of a gas to a reaction mixture (commonly the hydrogen halides, fluorine, chlorine, phosgene, boron trifluoride, carbon dioxide, ammonia, gaseous unsaturated hydrocarbons, ethylene oxide) requires the provision of safety precautions which may not be immediately apparent. Some of these gases may be generated in situ (e.g. diborane in hydroboration reactions), some may be commercially available in cylinders, and some may be generated by chemical or other means (e.g. carbon dioxide, ozone). An individual description of the convenient sources of these gases will be found under Section 4.2. 

Comparison of the bonding in diborane and ethylene is illuminating. In the usual description, each C atom of ethylene has planar sp orbitals, two directed toward H and the third directed toward the other C atom. Overlap of the two sp orbitals directed toward each other, and of the remaining ir-orbital normal to the plane of sp hybridization with the corresponding orbitals of the other C atom, give the two molecular orbitals of the double bond. If we call these molecular orbitals and we... 

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. 

Three-center Three-center bonds are necessary for the description of some molecules. Bridge bonds in bonding diborane are of the three-center two-electron type, whereas three-center four-electron bonding provides an explanation of some hypervalent molecules. 

A) Gas-phase structure of diborane (B2H ) with structural parameters determined by electron diffraction B) representation by Longuet-Higgins and Lipscomb of three-center, two-electron bonds C) molecular orbital description of three-center, two-electron bonding... 

Out of a welter of contradictory statements about the pyrochemical behavior of the boranes, only two will be quoted Diborane and the two pentaboranes ignite in air the other boranes, if they are pure, are not self-flammable and from a description of Borane Pilot Plants Solid boranes in presence of lower hydrides or in mixture are unpredictable and may ignite from moisture. ... 

Ion implantation [15] forces ions of dopant into the silicon using a stream of high energy ions produced in a machine similar to a mass spectrophotometer. Sources of dopants for this process are elemental arsenic and phosphorus, arsine, phosphine, diborane, and some boron halides. A more detailed description of doping procedures is beyond the scope of this text and can be found elsewhere [8]. 

BHB bonds in diborane (symmetry D2h) are labeled as in the figure on the right. Give a molecular orbital description of the bonding. 

B Without reference to the text, write a one-paragraph description, accompanied by well-labeled diagrams, of what is meant by a multicenter bond as found in diborane. Give several other examples. 

Even if a novice NBO user were unaware of the conceptual problems presented by hypovalency, there would be ample numerical evidence that something is drastically wrong with a conventional (lc,2c) Lewis structure description of diborane or other electron-deficient species. Suppose a default NBO analysis is attempted for the diborane input geometry file and atom numbering shown in 1/0-4.11 below. 

Note that despite the superficial resemblance to the H-bridging geometry of diborane (B2H6, Section 4.4), the optimal NBO description of AI2CI6 reflects a quite different bonding pattern, with no appreciable three-center hypovalent bond character in the latter case. Thus, AI2CI6 provides a fairly direct intramolecular comparison between two-center cr bonds of distinct covalent versus coordinate character, with the latter bonds responsible for the unusual four-membered ring motif. 

Boron Hydrides The molecule BH3 (borane) may exist as a reaction intermediate, but it has not been isolated as a stable compound. The B atom in BH3 lacks a complete octet—it has only six electrons in its valence shell. The simplest boron hydride that has been isolated is diborane, B2H6, but this molecule defies simple description. In the following structural formula, what holds the two borane units together ... 

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