Diborane structures

Not more than four alkyl groups can be accommodated in the diborane structure, thus preserving the essential hydrogen bridge. At low temperatures, the exchange rate is sufficiently slow as to allow isolation of the various species by fractional condensation. 

However, Dilthey s hypothesis was soon forgotten, and the idea of similarity with ethane prevailed. Only in the 40s the arguments concerning the diborane structure renewed. 

The structure of diborane B2H6 is considered later (p. 145). Here we may note that "BHj and "AlHj will be acceptor molecules since there are only six valency electrons around the B or A1 atom and a vacant orbital exists. Both in fact can accept the electron pair from a hydride ion thus ... 

Diborane has a qeometric structure similar to that of dimeric aluminium chloride, namely... 

This is known as a hydrogen-bridge structure. There are not enough electrons to make all the dotted-line bonds electron-pairs and hence it is an example of an electron-deficient compound. The structure of diborane may be alternatively shown as drawn in... 

Aluminium tetrahydridoborate is a volatile liquid. It is the most volatile aluminium compound known. It is covalent and does not contain ions but has a hydrogen-bridge structure like that of diborane, i.e. each boron atom is attached to the aluminium by two hydrogen bridges ... 

Bell and Hall have incorporated an organometallic unit into a crown by using the ferrocenyl unit as part of the ring or as a third strand. The unit is incorporated either as the 1,1 -diformylferrocene or the corresponding acid. In the former case, the bis-imine is prepared and reduced to give the saturated crown (see structure 24). In the latter case, the acid is converted into its corresponding chloride and thence into the diamide by reaction with a diamine. Diborane reduction affords the saturated amino-crown. Structure 24 could be prepared by either of these methods but the dialdehyde approach was reported to be poor compared to the amide approach which afforded the product in ca. 60% yield . 

The yellow compound BsF 2 appears to have a diborane-like structure (112) and this readily undergoes symmetrical cleavage with a variety of ligands such as CO, PF3, PCI3, PH3, ASH3 and SMe2 to give adducts L.B(BF2)3 which are stable at room temperature in the absence of air or moisture. 

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 ... 

You might wish to predict the structure of diborane (which is now known) but do not be discouraged if you are not able to. Its structure, once elucidated, came as quite a surprise to even the most sophisticated chemists. The explanation of the structure is, even today, composed of a large proportion of words and a small proportion of understanding. 

Lewis s theory also fails to account for the compound diborane, B2H6, a colorless gas that bursts into flame on contact with air. The problem is that diborane has only 12 valence electrons (three from each B atom, one from each H atom) but, for a Lewis structure, it needs at least seven bonds, and therefore 14 electrons, to bind the eight atoms together Diborane is an example of an electron-deficient compound, a compound with too few valence electrons to be assigned a valid Lewis structure. Valence-bond theory can account for the structures of electron-deficient compounds in terms of resonance, but the explanation is not straightforward. 

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). 

X 14.111 (a) Examine the structures of diborane, B2H6, and AI2CI6(g), which can be found on the Web site for this book. Compare the bonding in these two je compounds. How are they similar (b) What are the differences, if any, in the types of bonds formed (c) What is the hybridization of the Group 13/III element (d) Are the molecules planar If not, describe their shapes. 

I began graduate studies at the University of Chicago in 1936. At that time, the structure of diborane was a subject of considerable study and... 

However, in this structure, boron does not have an octet. It has an empty p orbital, (very similar to a carbocation, except there is no positive charge here). Therefore, borane is very reactive. In fact, it reacts with itself to give dimeric structures, called diborane ... 

Table 4.3 provides some data on the regioselectivity of addition of diborane and several of its derivatives to representative alkenes. Table 4.3 includes data for some mono- and dialkylboranes that show even higher regioselectivity than diborane itself. These derivatives are widely used in synthesis and are frequently referred to by the shortened names shown with the structures. 

In accord with the resonance structure drawn, there is little B=B bonding in this diborane(4) derivative and the B—B distance is found to be 1.859 A,68 which is considerably longer than even the normal expected value of 1.7 A for a boron-boron single bond.67 This is consistent with the normal repulsion of negative charges on adjacent atoms not stabilized by 7r-bonding. 

Shore, S. G., and R. W. Parry Chemical Evidence for the Structure of the Diammoniate of Diborane , III. The Reactions of Borohydride Salts with Eithium Halides and Aluminium Chloride. J. Amer. chem. Soc. 80, 12 (1958). 

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.

In this chapter many of the basic principles related to structure and bonding in molecules have already been illustrated. However, there is another type of compound that is not satisfactorily described by the principles illustrated so far. The simplest molecule of this type is diborane, B2H6. The problem is that there are only 10 valence shell electrons available for use in describing the bonding in this molecule. 

As we shall see later, borides (as well as oxides, nitrides, carbides, etc.) react with water to produce a hydrogen compound of the nonmetal. Thus, the reaction of magnesium boride with water might be expected to produce BH3, borane, but instead the product is B2ff6, diborane (m.p. -165.5 °C, b.p. -92.5 °C). This interesting covalent hydride has the structure... 

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