For diborane

Bond density surface for diborane locates bonds. 

What happens if there are not enough electrons to form conventional two-electron bonds Diborane (B2H6 provides a good example. Were the molecule to look like ethane, how many valence electrons would be required tc hold it together How many valence electrons does diborane possess Examine the actual structure for diborane. 

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 ... 

Using essentially the same reduction technique, 2-methoxyethyl esters have been converted into aldehydes [13] via the dioxalonium cations (Scheme 11.2) in a manner superior to earlier recorded procedures. Although there is a potential for diborane to be formed in the last step, no subsequent reactions involving this species have been observed. 

The 2003 ACGIH threshold limit value-time-weighted average (TLV-TWA) for diborane is 0.1 ppm (0.11 mg/m ). 

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. 

Diborane. As shown in Table I, the B-H terminal stretching modes of diborane occur at an average frequency of 2563 cm-1, whereas the average of the bridging B-H stretches v% y6, 3, and vyi) occurs 28% lower, at 1847 cm-1. The existing normal coordinate vibrational analyses display some abnormalities, such as a near zero Hb-B-Hb bending force constant, but the general features of the force field for diborane are reasonably well understood. The vibrational analyses indicate that the B-Hb force constant is about half of that for B-Ht (i, 2,3). This... 

The method of averaging for all valence-bond structures, asTdescribed above for diborane, is extremely laborious for any except very simple molecules. A statistical theory of resonating valence bonds that can be easily applied to complex as well as simple molecules has been developed.87 It can be illustrated by application to B6H9. Let us begin by assigning the probability 1 to the nonbridging B—II bonds and to the other bonds in the molecule ... 

Isotope patterns. Referring to Exercise 22-C, predict the relative abundances of l[Pg.498]

PROBLEM 19.4 Why is an ethane-like structure unlikely for diborane ... 

For diborane, also reactive towards surface hydroxyl groups, and because of its electron deficient character reactive towards the oxygen bridges -Si-O-Si- in the zeolite structure, the primary and secondary reactions on H-zeolites can be summarized as ... 

Figure 10.9 Ratio of evolved H2 and the integration of the silane band as a function of reaction time for diborane-treatment silica gel.

This representation is compatible with a total coordination of 1 (1/2 for each a) for each hydrogen atom. Furthermore, for diborane(6), which had been nomenclated in Chapter 2 as (EaHa)2, attention is directed to the observation that each boron atom is singly-bonded to two hydrogen atoms and alpha-bonded to two other hydrogen atoms thereby yielding the traditional valence of 3 for each boron atom. 

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. 

Exposure limits OSH A has set a limit of 0.1 ppm for diborane in workplace air for an 8-hour workday (TWA). The revised immediately dangerous to life or health (IDLH) concentrations for diborane are set at 15 ppm. ... 

Thus, in carboranes, there are fewer electrons than would be required for a classical Lewis structure in which the bonding can be described in terms of two-electron two-center bonds. Therefore, some type of multicenter bonding, such as shown in Figure 1, for diborane is required. 

Diborane is not stable at room temperature and is usually generated in situ. It is available from Alfa Inorganics but only in a very dilute solution in THF. Adams et al. report that dimethyl sulfide -borane provides a useful substitute for diborane. It is less reactive than diborane, but can be used for most hydroboration and reduction applications. 

The concepts discussed above, permit proposals to be made concerning the structure of other boron hydrides which have not been subjected to such an extensive experimental investigation as has diborane. The proposed formulae fall into two groups, that favouring the resonance of structures analogous to those represented schematically for diborane in IV to VIII (Syrkin and Dyatkina ) and that employing the concept of the protonated double bond (Pitzer ). ... 

If you have access to ab initio software such as GAMESS, you might find it amusing to compute which of following possible stmctures for diborane B2H6 is the more stable ... 

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