Boron electronegativity

Elemental boron has a diverse and complex chemistry, primarily influenced by three circumstances. Eirst, boron has a high ionization energy, 8.296 eV, 23.98 eV, and 37.75 eV for first, second, and third ionization potentials, respectively. Second, boron has a small size. Third, the electronegativities of boron (2.0), carbon (2.5), and hydrogen (2.1) are all very similar resulting in extensive and unusual covalent chemistry. 

Boron forms B—N compounds that are isoelectronic with graphite (see Boron compounds, refractoryboron compounds). The small size also has a significant role in the interstitial alloy-type metal borides boron forms. Boron forms borides with metals that are less electronegative than itself including titanium, zirconium, and hafnium. 

The mles can readily be extended to isoelectronic anions and carbaboranes (BH=B =C) and also to metalloboranes (p. 174), metallocarbaboranes (p. 194) and even to metal clusters themselves, though they become less reliable the further one moves away from boron in atomic size, ionization energy, electronegativity, etc. 

The values of f (l) given in the table for electronegative atoms are their normal covalent single-bond radii28 (except for boron, discussed below). The possibility that the radius 0.74 A. of Schomaker and Stevenson29 should be used for nitrogen in the metallic nitrides should be borne in mind. 

Increasing content of the large M E(MRE)-metals as well as increasing electronegativity differences reduce the formation of boron-boron aggregates, with... 

The designation of hard acids is not restricted to metal cations. For example, in BF3 the small boron atom in its +3 oxidation state is bonded to three highly electronegative fluorine atoms. All the B—F bonds are polarized away from a boron center that is already electron-deficient. Boron trifiuoride is a hard Lewis 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... 

Third, substituents raising the effective electronegativity of boron will be expected to enhance B—C 7r-overlap (cf. 25a and 26). Finally, there can be competition between linear conjugation (36) and cross-conjugation (24). 

Even with the diborylacetylenes illustrated by structures 39a-c, XRD data again show a shortening of the B—C bond as more electronegative groups are affixed to the boron centers but only a modest contraction of the C=C bond36 ... 

The heightened reactivity of these neutral boraethenes undoubtedly stems from the available boron 2p-orbital and the greater electronegativity... 

In contrast to the highly reactive organoboranes, borabenzene metal complexes are surprisingly inert toward nucleophiles. However, cationic complexes may undergo nucleophilic addition reactions, and nucleophilic substitution has been observed with compounds having a hydrogen or an electronegative substituent at boron. 

The strength of a Lewis acid is a measure of its ability to attract a pair of electrons on a molecule that is behaving as a Lewis base. Fluorine is more electronegative than chlorine, so it appears that three fluorine atoms should withdraw electron density from the boron atom, leaving it more positive. This would also happen to some extent when the peripheral atoms are chlorine, but chlorine is less electronegative than fluorine. On this basis, we would expect BF3 to be a stronger Lewis acid. However, in the BF3 molecule, the boron atom uses sp2 hybrid orbitals, which leaves one empty 2p orbital that is perpendicular to the plane of the molecule. The fluorine atoms have filled 2p orbitals that can overlap with the empty 2p orbital on the boron atom to give some double bond character to the B-F bonds. 

Under high pressure and temperature, boron nitride can be converted to a cubic form. The cubic form of (BN) is known as borazon, and it has a structure similar to that of diamond. Its hardness is similar to that of diamond, and it is stable to higher temperatures. The extreme hardness results from the fact that the B-N bonds possess not only the covalent strength comparable to C-C bonds, but also some ionic stabilization due to the difference in electronegativity between B and N. 

The chlorine atoms are bonded to boron atoms as expected on the basis of the difference in electronegativity. 

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