Boron valency

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

Boron trifluoride has a trigonal-planar structure. Formulate the bonding in terms of molecular orbitals for the Dsjj symmetry. In addition, construct wave functions for three equivalent sp2 hybrid orbitals, using the 2px, 2p, and 2s boron valence orbitals, which may be used to form three localized bonds with the three fluorines. Compare and contrast the molecular-orbital and the hybrid-orbital descriptions. 

The short B-F distance in BF3 (130 pm compared with 143 pm in BFj") and the large B-F bond energy are suggestive of partial double bond character. BF3 is a weaker Lewis acid than the other boron trihalides, which may indicate that the four boron valence orbitals in BF3 are more fully engaged than in BC13 etc. Molecules of the type R2BNR2 are believed to have substantial p -p bonding ... 

Boron Trichloride, BCI3. Here the molecule is planar, with the boron atom at the centre of an equilateral triangle of chlorine atoms (Fig. 46). The valence state must be described in terms of three similar hybrid AO s pointing towards the comers of the triangle. Such orbitals can be formed by mixing 2s and two 2p AO s, 2p and 2p say they lie in the plane of the latter and are precisely equivalent (Fig. 47). If the so-called trigonal hybrids are denoted by h, hg and hg, the appropriate boron valence state must be B(ls2 h h2 hg ). The hybrid AO s overlap chlorine 3p AO s, directed towards the boron atom, to form localised MO s similar to those in beryllium chloride. 

The B6H62 ion is a useful example a convenient set of coordinate axes for its boron atoms is shown in Figure 13-9. Each boron has four valence orbitals (s, ft, py, and pz), a total of 24 boron valence orbitals for the cluster. It is convenient to assign the z axis of each boron to point toward the center of the octahedron, with the x and y axes oriented as shown. 

Each boron atom possesses at least one terminal hydrogen atom, and a B—H fragment contributes two electrons to framework bonding one boron valence electron participates in the covalent bond to hydrogen. The framework contribution is 11 X 2 = 22 electrons. 

The molecular-orbital energy-level scheme for BF3 is shown in Fig, 4-7. The fluorine valence orbitals are more stable than the boron valence orbitals, and so electrons in bonding molecular orbitals spend more time in the domain of the fluorine nuclei. The a- and (Tj molecular orbitals are degenerate in trigonal-planar molecules such as BF. Since this is by no means obvious from Eqs. (4-3), (4-4), (4-5), and (4-6), we shall devote a short section to an explanation. 

Tricyclooctylborane, though, forms B-alkyl-9-boratricyclo[3.3.1]nonane (LV) very readily. This reaction can be carried out in the presence of a trialkylborane, so that the third boron valence is saturated. 

A semiconductor is a material having electrical conductivity between a conductor and a nonconductor. Silicon, which is in the fourth column of the periodic table (valence = 4), is normally a nonconductor. It may be converted to a semiconductor by diffusing a small amount ( 1 part in 10 ) of boron (valence = 3) or phosphorous (valence 5) throughout its structure. This is called doping. When boron is the dopant, it is called a positive type (p-type) semiconductor but when phosphorous is the dopant it is called an n-type semiconductor. When a potential difference (voltage) is applied across an n-type semiconductor, the unattached electrons where phosphorous atoms are located move toward the positive terminal. When a boron atom is in a p-type semiconductor, there is an unfilled bond site called a hole. Holes tend to act as positively charged particles and move toward the negative terminal when a potential difference is applied. 

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

The electron counts of nitrogen in ammonium ion and boron in borohydride ion are both 4 (half of eight electrons in covalent bonds) Because a neutral nitrogen has five electrons in its valence shell an electron count of 4 gives it a formal charge of +1 A neutral boron has three valence electrons so that an electron count of 4 in borohydride ion corresponds to a formal charge of -1... 

When a sibcon crystal is doped with atoms of elements having a valence of less than four, eg, boron or gallium (valence = 3), only three of the four covalent bonds of the adjacent sibcon atoms are occupied. The vacancy at an unoccupied covalent bond constitutes a hole. Dopants that contribute holes, which in turn act like positive charge carriers, are acceptor dopants and the resulting crystal is -type (positive) sibcon (Fig. Id). 

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is partiy responsible for the great interest surrounding borane chemistry and molecular stmcture. The stmcture of even the simplest boron hydride, diborane(6) [19287-45-7] 2 6 sufficientiy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged stmcture shown. 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

The possible number of valence stmctures for a given boron hydride has been defined exacdy using three general equations of balance. For a borane... 

Sihcon carbide can be doped using boron [7440-42-8] to provide acceptor levels within the band gap (0.3 eV above the valence band), thus making it a -type conductor, or nitrogen can be added to provide donor levels and n-ty e conduction (0.07 eV) below the conduction band. 

Another type of anion, confined for practical purposes to boron compounds, has no unshared electrons at the anionic site, and must be thought of as being formed by addition of hydride to a boron atom (or other atom with an incomplete valence shell). Such structures were not anticipated at the time general heterocyclic nomenclature was developed, and they are only recently being fitted into systematic nomenclature (lUPAC Provisional Recommendation 83.2). Proposals for a suffix to indicate such structures are under consideration (1982). 

A second doping method is the substitution of an impurity atom with a different valence state for a carbon atom on the surface of a fullerene molecule. Because of the small carbon-carbon distance in fullerenes (1.44A), the only species that can be expected to substitute for a carbon atom in the cage is boron. There has also been some discussion of the possibility of nitrogen doping, which might be facilitated by the curvature of the fullerene shell. However, substitutional doping has not been widely used in practice [21]. 

Boron is a unique and exciting element. Over the years it has proved a constant challenge and stimulus not only to preparative chemists and theoreticians, but also to industrial chemists and technologists. It is the only non-metal in Group 13 of the periodic table and shows many similarities to its neighbour, carbon, and its diagonal relative, silicon. Thus, like C and Si, it shows a marked propensity to form covalent, molecular compounds, but it differs sharply from them in having one less valence electron than the number of valence orbitals, a situation sometimes referred to as electron deficiency . This has a dominant effect on its chemistry. 

The structure requires 160 valence electrons per unit cell computed as follows internal bonding within the 4 icosahedra (4 x 26 = 104) external bonds for the 4 icosahedra (4x12 = 48) bonds shared by the atoms in 2(b) positions (2x4 = 8). However, 50 B atoms have only 150 valence electrons and even with the maximum possible excess of boron in the unit cell (0.75 B) this rises to only 152 electrons. The required extra 8 or 10 electrons are now supplied by 2C or 2N though the detailed description of the bonding is more intricate than this simple numerology implies. 

It is also noteworthy that Alfred Stock, who is universally acclaimed as the discoverer of the boron hydrides (1912). " was also the first to propose the use of the term "ligand (in a lecture in Berlin on 27 November 1916). Both events essentially predate the formulation by G. N. Lewis of the electronic theory of valency (1916). It is therefore felicitous that, albeit some 20 years after Stock s death in 1946, two such apparently disparate aspects of his work should be connected in the emerging concept of boranes as ligands . 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

Notice that the beryllium atom has no unpaired electrons, the boron atom has one, and the carbon atom two. Simple valence bond theory would predict that Be, like He, should not form covalent bonds. A boron atom should form one bond, carbon two. Experience tells us that these predictions are wrong. Beryllium forms two bonds in BeF2 boron forms three bonds in BF3. Carbon ordinarily forms four bonds, not two. 

Explain why chemists say that boron has three valence electrons and that chlorine has seven. How many valence electrons has fluorine Oxygen Nitrogen ... 

Explain the magnitudes in terms of the electron configurations of boron and deduce the number of valence electrons of boron. 

Therefore we should expect in the gaseous state to find molecules such as BeH2 and BeF2. These molecules have been detected. On the other hand, beryllium has the trouble boron has, only in a double dose. It has two vacant valence orbitals. As a result, BeH2 and BeF2 molecules, as such, are obtained only at extremely high temperatures (say, above 1000°K). At lower temperatures these vacant valence orbitals cause a condensation to a solid in which these orbitals can participate in bonding. We shall discuss these solids in the next chapter. 

By the middle of the nineteenth century more than 60 elements were known with new ones continuing to be discovered. For each of these elements, chemists attempted to determine its atomic weight, density, specific heat, and other properties. The result was a collection of facts, which lacked rational order, Mendeleev noticed that if the elements were arranged by their atomic weights, then valencies and other properties tended to recur periodically. However, there were gaps in the pattern and in a paper of 1871 Mendeleev asserted that these corresponded to elements that existed but had not yet been discovered. He named three of these elements eka-aluminium, eka-boron and eka-silicon and gave detailed descriptions of their properties. The reaction of the scientific world was sceptical. But then in 1874 Lecoq de Boisbaudran found an... 

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