Diborane electron density

The usefulness of the bond density surface is more apparent in the following model o diborane. The surface shows that diborane is not flat. It also shows that there is relatively little electron density between the two borons. Apparently there is no boron-boron bonr in this molecule. This is information that we can extract from the bond density surfact model. We do not have to assume this information in order to construct a model. We would need it in order to construct a conventional model. 

Bond and size surfaces offer some significant advantages over conventional skeletal and space-filling models. Most important, bond surfaces may be applied to elucidate bonding and not only to portray known bonding. For example, the bond surface for diborane clearly shows a molecule with very little electron density concentrated between the two borons. 

The relief and contour electron density maps for diborane in the plane of the bridging H atoms are shown in Figures 13.15(a) and (b), respectively. One notable feature here Is that the atomic surface boundaries for the two bridging H atoms actually touch one another. Although there is no direct bond path between the two bridging H atoms, the larger value of the electron density in this plane can be used to explain the fact that DFT calculations indicate that the H H interaction... 

Carbon monoxide is an extremely weak Lewis base towards conventional Lewis acids. It does not complex with the boron trihalides, although it does yield a weak adduct H3B CO with diborane. On the other hand it forms numerous complexes with transition elements. The source of this difference is that in the latter complexes not only is the weakly donating a-orbital of CO involved, but also the 7C orbitals which can function as acceptors. The conventional explanation is that a synergic effect exists in which the n interaction removes electron density from the metal, allowing a donation from the ligand to be enhanced. 

This equilibrium lies very much to the side of diborane (B2Hg), leaving very little borane (BH3) present at equilibrium. It is possible to stabiHze BH3, thereby increasing its concentration at equihbrium, by using a solvent such as THF (tetrahydrofuran) which can donate electron density into the empty p orbital of boron ... 

Two views of the bonding in diborane. (a) The overall distribution of electron density in diborane (ft) the semitopo-logical diagram for diborane. 

A mixture of -propyldiboranes is obtained along with tri- -propyI-borane from the reaction of diborane with cyclopropane at about 95 C. Apparently the banana bonds of cyclopropane can provide sufficient electron density to form the expected pi-complex intermediate (124). 

Each topological feature of p(r), whether it be a maximum, a minimum, or a saddle, has associated with it a critical point (abbreviated to cp), a point denoted by the coordinate where Vp(rc) = 0. Such points are evident in the relief maps of the electron density given in Figures 1 and 2 for two planes of the diborane molecule. The behavior of the density in the neighborhood of a cp is obtained via a Taylor series expansion... 

Figure 1 Relief and contour maps of the electron density for diborane in the plane of the terminal hydrogen atoms. The density value is arbitrarily terminated at the positions of the boron nuclei. The lower maps illustrate the trajectories traced out by Vp, the gradient vectors of the density. The contour map is overlaid with the trajectories of Vp associated with the bond cps that define the bond paths and the intersection of the interatomic surfaces with the plane of the diagram. (Reproduced with permission of the American Chemical Society from J. Am. Chern. Soc., 1979, 101, 1389)...

The gradient vector field of the electron density is illustrated for diborane in Figures 1 and 2. The trajectory diagrams on... 

Figure 5 Space-filling displays of boron and hydrogen atoms in terms of the intersection of their interatomic surfaces with the 0.001 au envelope of the electron density, the van der Waals envelope (a) the boron atom in diborane with the surfaces for two terminal hydrogens on the LHS (b) and (c) a terminal and bridging hydrogen, respectively...

Based on its structure and valence electron count, draw a Lewis structure or series of Lewis structures for diborane Examine the bond density surface. Does it substantiate 01 refute your speculation ... 

Smith and Lipscomb established the crystal structure of the 3 phase of B2H6 [19] after earlier work by Stitt [20,21], Price [22,23] and Hedberg [24] on gas-phase diborane. Jones and Lipscomb [25,26] used structure factors from calculations to explain apparent discrepancies between bond lengths determined from x-ray and electron diffraction experiments. Several recent theoretical studies have considered diborane with correlated, ab initio methods [27-30] and density functionals [31,32]. Trinquier and Malrieu performed a general analysis of bonding in dibridged X2H6 compounds [33]. 

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