The boron atom

This is a unique feature of boron chemistry it is the only non-metal or semi-metal that cannot gain an octet solely by using its outer electrons to form shared electron-pair bonds. Much boron chemistry can be regarded as processes in which the atom gains a share in a greater number of electrons. Many of these reactions of boron can be classified as interactions between a Lewis acid and a Lewis base. 

---

This compound, which contains atoms arranged tetrahedrally around the boron atom, can readily be isolated from a mixture of dimethyl ether and boron trichloride. On occasions a chlorine atom, in spite of its high election affinity, will donate an electron pair, an example being found in the dimerisation of gaseous monomeric aluminium chloride to give the more stable Al2Clg in which each aluminium has a tetrahedral configuration ... 

Substituents containing boron are of interest because of the possibility which the boron atom offers of conjugation of a vacant orbital with the 77-electrons of the benzene ring (—717). The case of phenylboronic acid has been discussed ( 5.3.4). 

Figure 8.16 shows the B l spectmm of the B5FI9 molecule. The boron atoms are situated at the comers of a square pyramid. There are four B-FI-B bridging hydrogen atoms and... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

Adducts of BF and some organic compounds having labile hydrogen atoms in the vicinity of the atom bonding to the boron atom of BF may form a derivative of BF by splitting out HF. For example, P-diketones such as acetylacetone or benzoylacetone react with BF in benzene (38) ... 

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

The triaLkoxy(aryloxy)boranes are typically monomeric, soluble in most organic solvents, and dissolve in water with hydrolysis to form boric acid and the corresponding alcohol and phenol. Although the rate of hydrolysis is usually very fast, it is dependent on the bulk of the alkyl or aryl substituent groups bonded to the boron atom. Secondary and tertiary alkyl esters are generally more stable than the primary alkyl esters. The boron atom in these compounds is in a trigonal coplanar state with bond hybridization. A vacantp orbital exists along the threefold axis perpendicular to the BO plane. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

The boron atoms ia resonance stmctures A and B possess a formal negative charge. The simplest representation for these compounds is... 

The boron atom arrangement in Each boron atom... 

Similar possibilities arise for 10-atom clusters. Thus, dimerization of the c/oso-CtBj claster l,5-Me2C2B3Et3 (56) by means of K metal then I2 in thf yields the classical adaniantane derivative Me4C4B6Et6 (f) when this is heated to 160° the mdd-tetracaibadecaborane cluster (g) is obtained rapidly and quantitatively. It will be noted that in (f) all four C atoms are 4-coordinate and all six B atoms are 3-coordinate, whereas in (g) the three C atoms in the C3 triangular face are 5-coordinate while the boron atoms are variously 4, 5 or 6 coordinate. 

The di- and triborolyl ligands tend to i -coordination in sandwichforming reactions. There is a clear-cut tendency for stacking processes followed by the formation of multidecker species and often stabilization of the unusual oxidation states of the transition metals. The route to the linked sandwich and multidecker complexes is attractive for materials chemistry. Thia- and azaborolyl organome-tallic chemistry follows the same trends, although in the azaborolyl complexes the i -rather than i -coordination is sometimes realized. Moreover, coordination via the boron atom is known. In the B, N, Si-heterocycles, the heteroring is j " -coordinated. 

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. 

To show how orbital diagrams are obtained from electron configurations, consider the boron atom (Z = 5). Its electron configuration is ls22s22p1. The pair of electrons in the Is orbital must have opposed spins (+j, or f j). The same is true of the two electrons in the 2s orbital. There are three orbitals in the 2p sublevel. The single 2p electron in boron could be in any one of these orbitals. Its spin could be either up or down. The orbital diagram is ordinarily written... 

If you look carefully at Figure 6.15, you will note a few exceptions to the general trends referred to above and illustrated in Example 6.11. For example, the ionization energy of B (801 kj/mol) is less than that of Be (900 kj/mol). This happens because the electron removed from the boron atom comes from the 2p as opposed to the 2s sublevel for beryllium. Because 2p is higher in energy than 2s, it is not too surprising that less energy is required to remove an electron from that sublevel. 

Two electron pairs are as far apart as possible when they are directed at 180° to one another. This gives BeF2 a linear structure. The three electron pairs around the boron atom in BF3 are directed toward the comers of an equilateral triangle the bond angles are 120°. We describe this geometry as trigonal planar. 

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. 

There is no more room in the 2s orbital for a fifth electron, which appears when we move on to the boron atom. However, another orbital with principal quantum number 2 is available. A 2p orbital accepts the fifth electron, giving the configuration Is ls-lfi. Continuing this process, we obtain the following configurations ... 

The boron atom presents the same sort of option in orbital occupancy as does carbon ... 

The electron configuration (41) is somewhat higher in energy than (40). It is necessary to promote a 2s electron to the 2p state to obtain (41). In return, however, the boron atom gains bonding capacity. Whereas a boron atom can form only one covalent bond in configuration... 

The boron atom in BFa uses the 2s and two 2p orbitals in bonding. Therefore the bonding is... 

---