Boron trihalides bonding

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

The structure of the complex (96) between benzaldehyde (95) and boron trifluoride (equation 15) was investigated by X-ray crystallography169. In 96, BF3 is in the anti position to the phenyl ring and this geometry remains also in solutions, as tested by the 19F-NMR spectrum in CD2CI2. An ab-initio study170 on interactions between formaldehyde and boron trihalides showed that these complexes (mainly donor-acceptor complexes) affect spectroscopic properties and the reactivity of the carbonyl group the polarization of the C=0 bond favours the attack of nucleophiles. 

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-sulfur bonds, addition, to alkynes, 10, 778 Boron trihalides, in boron compound synthesis, 9, 146 Boron-zinc exchange and copper-catalyzed substitutions, 9, 518 for organozinc halide preparation, 9, 89 Borostannylation, enynes, 10, 334... 

Exchange involving boron-halogen bonds and boron-oxygen bonds has been observed in the synthetically important reactions for the preparation of alkyldihaloboranes from alkylboroxines and boron trihalides (165, 167, 169). 

Lewis acids such as boron trihalides will also form stronger bonds with fluoride than with the other halogens, and can be used to abstract fluoride selectively from halogenated compounds. Some examples of Friedel-Crafts alkylations with fluoro-haloalkanes in which only fluoride is displaced are sketched in Scheme 4.14. As shown by the last example, however, hydride migrations can readily occur under such strongly acidic reaction conditions. 

The boron trihalides hydrolyze quickly the bromide and chloride yield boric acid and the halogen acid but the fluoride retains the B—F bonds, forming the addition compound F3B OH2, a strong acid ... 

From the above illustration, we expect boron to form three covalent bonds, which are equal in energy and directed 120° from each other. Accordingly, the boron trihalides, BX3, have the following trigonal planar structure (D3h symmetry) ... 

If the donor-acceptor bond is weak, two different boron trihalide adducts of the same donor will exchange halogen to form the mixed-halogen adducts. The weaker the donor-acceptor bond, the more rapidly the mixed adducts form, indicating that free boron trihalide formed by dissociation is an active species in halogen exchange. 

The requirement for an initial dissociation step implies that halogen redistribution rates should parallel the ease of dissociation of one of the halogens, and this expectation is confirmed in fluorine - heavier halogen redistributions where the rates change from slow to very fast over the series Cl, Br, I as the B—X bond becomes weaker. Rates should also be related to stabilization of the residual boron trihalide formed on dissociation. The boron trihalide with the greatest number... 

Pairwise interaction parameters can perhaps be interpreted in chemical bonding terms (76, 80,123, 182). Pairwise terms involving fluorine are often anomalous and this may be related to some surviving multiple bonding in the boron-fluorine bonds in the adducts (20). Pairwise interaction parameters of the Me3N mixed trihalide adduct system are reported to correlate consistently for 41, 13C, 19F, and nB chemical shifts and for nB—19F coupling constants (123). It is perhaps surprising that even the proton shifts can be correlated in this way. 

The mixed boron trihalide adducts hold few surprises in terms of their donor-acceptor bond behavior, but provide striking examples of dependence of halogen redistribution behavior on the nonhalogen substituent. The simplicity and accessibility of these systems suggests their use as model compounds in the study of ligand redistribution reactions. Many of the features complicating ligand redistribution in, for example, metal carbonyl systems (46) are simplified or absent here. 

Tris(pentafluorophenyl)borane, a compound first made by Stone et al. in 1963, has in recent years become one of the most widely applied boron reagents, as a highly effective activator for metallocene-based olefin polymerisation catalysts. There are several reasons for this it is a strong Lewis acid, unlike boron trihalides it is resistant to hydrolysis, and it possesses strong B-C and C-F bonds which make it essentially inert to chemical attack for example, it can be recovered unchanged from neat elemental bromine. 

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