Other Chemistry of the Group 13 Elements

The boron trihalides, BXj, are Lewis acids (Chapter 6). These compounds are monomeric and planar—unlike diborane, B2H5, and the aluminum halides, AI2X5 (Section 3.1.4). As Lewis acids, boron trihalides can accept an electron pair from a halide to form tetrahalobo-rate ions, BX4. Boron halide catalysts act as halide ion acceptors, as in the Friedel-Crafts alkylation of aromatic hydrocarbons (in margin). 

Parallels between main group and organic chemistry can be instructive. One of these parallels is between the organic molecule benzene and the isoelectronic borazine ( inorganic benzene ), B3N3H6. Some of the striking similarities in physical properties between these two are in Table 8.6. 

Despite these parallels, the chemistry of these two compounds is quite different. In borazine, the difference in electronegativity between boron (2.051) and nitrogen (3.066) adds considerable polarity to the B—N bonds and makes the molecule much more susceptible to attack by nucleophiles (at the more positive boron) and electrophiles (at the more negative nitrogen) than benzene. 

Source Data from N. N. Greenwood and A. Earnshaw, Chemistry ofthe dements, Pergamon Press, Elmsford, NY, 1984, p. 238. 

Are multiple bonds between Group 13 elements possible For example, can the simple neutral diborene, HB=BH, be synthesized Diborene is predicted to be extremely reactive because it exhibits two one-electron tt bonds (similar to B2 as described in Section 5.2.3). However, a suitable Lewis base can be envisioned to potentially stabilize this L species (in margin). A sterically demanding and strongly donating N-hetOTOcycUc carbene (Fig- 

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