The Boron Halides

Aside from the well-known trihalides, BXg, the structures of only B2F4 (71), B2CI4 (S), and B4CI4 (1,2) are established. At least three more halides (7S, 72) (BClo. ), soluble in organic solvents (BClo.g), insoluble in solvents and a probably two-dimensional polymer, (BCl) , have been 

In deriving the principles of boron chloride structures, we shall make the following assumptions  

Note that the assumption of no bridge Cl atoms and no BCI2 groups at this stage requires that s = = 0. The above two equations give t = p, from which we find y = —q/2 from either of the above equations. Values for s, t, y, and x are thus 0, p, —q/2 and 0. But y 0, and hence 9 0 moreover, y is integral, so that 9 is an even negative number or zero. 

Suppose we illustrate these principles with a discussion of a molecule 

The molecule (BClo.g) (where x 10) is currently under investigation by X-ray diffraction methods 29). 

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These molecules exist in the solid halides, explaining the low melting points of these halides, and also in the vapour phase at temperatures not too far above the boiling point. At higher temperatures, however, dissociation into trigonal planar monomers, analogous to the boron halides, occurs. 

The boron halides are made either by direct reaction of the elements at a high temperature or from boron oxide. The most important is boron trifluoride, BF, an industrial catalyst produced by the reaction between boron oxide, calcium fluoride, and sulfuric acid ... 

The observed order of reactivity for the boron halides is BF3 < BCI3 < BB F3 < BI3 From electronegativity considerations, we might expect the opposite trend. The electronegativity difference between boron ( = 2.0) and fluorine ( () = 4.0) is 2, whereas boron and iodine ()Y — 2.5) differ by only 0.5. Thus, fluorine atoms... 

P. F. Strehle, and P. A. Tierney Singly-bridged Compounds of the Boron Halides and Boron Hydrides. J. Amer. chem. Soc. 79, 2020 (1957). 

The boron halides react violently with ammonia. 

As has already been mentioned, boron halides are electron-deficient molecules. As a result, they tend to act as strong Lewis acids by accepting electron pairs from many types of Lewis bases to form stable acid-base adducts. Electron donors such as ammonia, pyridine, amines, ethers, and many other types of compounds form stable adducts. In behaving as strong Lewis acids, the boron halides act as acid catalysts for several important types of organic reactions (see Chapter 9). 

In addition to functioning as Lewis acids, boron halides undergo many other types of reactions. As is typical of most compounds containing covalent bonds between a nonmetal and a halogen, the boron halides react vigorously with water to yield boric acid and the corresponding hydrogen halide. 

Boron, which occurs in the second period of the periodic system as a tervalent ion, is naturally very small (see Table F), and, in the compounds which it forms with the halogens, the boron ion will be completely shielded by the three halogen ions, so that the boron halides are thus compounds with low boiling points. Of the halogen ions, iodine, which contains the most electrons, is the largest. In aluminium iodide the three iodine ions will shield the aluminium ion, as also do the somewhat smaller bromine or chlorine ions in... 

The boron halides again can form sextets only, e.g. 

With increasing reactivity of the boron halide, however, trans addition can also be observed BI3 yields a 1 1 mixture of Z and E compounds in the addition to phenylacetylene.469 However, the tram isomer is formed when BBr3 adds to acetylene 468... 

The first borinate-transition metal complex to be prepared was actually the first known derivative of borin. Bis(cyclopentadienide)cobalt (94) reacts with organic halides and was analogously found to react with boron halides in a redox reaction to give (95), followed by an insertion to yield (cyclopentadienide)(borinato)cobalt (97) (72CB3413). The product composition depends on the ratio of reactants. Compound (97) is the main product (80% yield when R = Ph, X = Br) when the molar ratio between (94) and the boron halide is 2.5 1. A second and slower insertion occurs to give (28) when (97) is treated with another equivalent of the boron halide (Scheme 13). Compounds (28), (29) and (97) have one electron more than predicted by the 187r-electron rule for transition metal complexes. They are red in colour and, of course, paramagnetic. The mixed complexes (97) are thermally labile, in contrast to (28) and (29), which can be heated to 180 °C and sublimed at 90 °C. Their ionization potentials are low and the complexes are sensitive to air. 

The boron halides are Lewis acids because they have a vacant 2p orbital. 

Among the boron halides, B4CI4 is of interest as both B4H4 and B411 do not exist. Figure 13.3.7 shows the molecular structure of B4CI4, which has a tetrahedral B4 core consolidated by four terminal B-Cl 2c-2e bonds and four BBB 3c-2e bonds on four faces. This structure is further stabilized by a-n interactions between the lonepairs of Cl atoms and BBB 3c-2< bonds. The... 

Boron. Boron hydride, dl-, penta- and deca-, and the boron halides, trichloride, trifluoride, are the chief compounds of interest. No adequately documented collection method exists for any of these compounds, and most recent work in the area has been futile. Boron oxide is the only compound that can be analyzed with certainty by P CAM 173. This compound is a nuisance dust like aluminum oxide, and the AAS method provides a means of identifying the compound. 

Because they are strong Lewis acids, the boron halides act as acid catalysts for several important organic reactions. 

In the very stable compounds like BH3.COand BH3.N(CH3)2 the boron sextet has been completed with the lone pairs of the other components. Also B(CH3)3, though it does exist as such contrary to BH3, readily adds compounds with lone pairs such as NH3. This also occurs with the boron halides, especially with BF3, e.g. BF3.NH3 (p. 166). With the other halides an octet can also be attained by resonance with configurations as... 

In a study of the B2Cl4-trichloroethylene system, no isolable adduct was obtainable and all the haloolefin could be recovered upon treatment of the reaction mixture with excess ammonia to remove the boron halide 35). It was reported that peaks attributable to an addition product and to an intermediate compound retaining the double bond were observed in the proton NMR spectrum, but later workers 28, 84, 117) were unable to verify this finding. 

Boron and aluminium give trihalides. BF3 is most conveniently made by heating BgO with concentrated H2SO4 and a fluoride. It reacts with aluminium chloride and bromide to produce involatile AIF3 and volatile BCI3. The volatility of the boron halides decreases with molecular weight. 

Born tribromide is used as received. It is photosensitive, but can be purified by storage over elemental mercury, followed by storage in the dark. The use of the boron halides BF, and BCl, results in much lower yields and dilTiculties with purification, and they are therefore considered to be unsatisfactory for this reaction. 

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