Diborane, model structure

Figure 8.7 Diborane, BaH. (a) Contour map of pb in the plane of the terminal hydrogens, (b) Contour map of pb in the plane of the bridging hydrogens, (c) Calculated geometry, (d) Experimental geometry. (e) Interatomic H-H distances, (f) Ionic model, (g) Resonance structures, (h) Protonated doublebond model, (i) VSEPR domain model showing the two three-center, two-electron bridging domains, (j) Hybrid orbital model.

Besides specific surface area, silicas are also characterised by their porosity. Most of the silica s are made out of dense spherical amorphous particles linked together in a three dimensional network, this crosslinked network building up the porosity of the silica. Where the reactivity of diborane towards the silica surface has been profoundly investigated, little attention has been paid to the effect of those reactions on the pore structure. However different methods are developed to define the porosity and physisorption measurements to characterise the porosity parameters are well established. Adsorption isotherms give the specific surface area using the BET model, while the analysis desorption hysteresis yields the pore size distribution. 

One early and insightful model for diborane is the protonated double-bond model shown in Figure 1.12. Symmetrical protonation of the it bond of ethylene above and below the molecular plane leads to the structure of diborane. Replacement of each C in ethylene with [B] leads to [B2H4]2. In fact Li2B2R4, where R is a bulky substituent, has been structurally characterized and shown to possess B-B multiple-bond character. 

While the four-center transition structure for BH3 addition is a widely used model, other reaction pathways have also been considered. In a synthesis of optically active (-)-l-butanol-l-d, Streitwieser and co-workers used the optically active borane formed from diborane and (-l- )-a-pinene (R2BH) to carry out the hydroboration-oxidation of (Z)-l-butene-l-d. To explain the observed stereochemistry of the reduction, they proposed that hydroboration involves a n complex between R2BH and the alkene (Figure 9.41). Note the close resemblance of such a complex to the cyclic structures... 

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