Lower halides of boron

B2CI4 was the first compound in this series to be prepared and is the most studied it is best made by subjecting BCI3 vapour to an electrical discharge between mercury or copper electrodes  

The reaction probably proceeds by formation of a BCl intermediate which then inserts into a B-Cl bond of BCI3 to give the product directly. 

Another route is via the more stable B2(NMe2)4 (see reaction scheme). Thermal stabilities of these compounds parallel the expected sequence of Pjr-Pjr bonding between the substituent and B  

The halides are much less stable than the corresponding BX3, the most stable member B2F4 decomposing at the rate of about 8% per day at room temperature. B2Br4 disproportionates so rapidly at room temperature that it is difficult to purify  

The compounds B2X4 are spontaneously flammable in air and react with H2 to give BHX2, B2H6 and related hydrohalides they form adducts with Lewis bases (B2CI4L2 more stable than B2F4L2) and add across C-C multiple bonds, e.g. 

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If one of the Y atoms of a planar XY3 molecule is replaced by a Z atom, the symmetry is lowered to C2U. If two of the Y atoms are replaced by two different atoms, W and Z, the symmetry is lowered 10 C. As a result, the selection rules are changed, as already shown in Table 1-12. In both cases, all six vibrations become active in infrared and Raman spectra. Table II-4c lists the vibrational frequencies of planar ZXYj and ZXYW molecules. Although not listed in this table, the infrared spectra of binary mixed halides of boron" and aluminum " have been measured. The frequencies listed for ihe formate and acetate ions were obtained in aqueous solution. These frequencies are important when we discuss the vibrational spectra of metal salts of these anions (Sec. HI-7). 

The first moderate-yield process for the formation of B—H bonds from the reaction of Hj with boron halides utilizes passage of mixtures of and BClj through an electrical discharge between Cu electrodes. When H BClj is 10 at 2.67 X 10 Pa, yields of < 60% B H, based on BClj consumed in 2 h, are available. This approach is more convenient when BBrj is used instead of BClj. The lower volatility of the bro-mo species makes the process simpler and separation of products more convenient. This discharge process proceeds in steps ... 

Croft explains the results in terms of the acceptor property of the boron atoms. They tend to take an electron pair from a donor molecule to form sp bonds. This is why only halides in lower oxidation states react with boron nitride (e.g., CuCl and SbCls but not Cuds or SbCls) This is different from the case of graphite in which the carbon planes are a source of electrons. The intercalation of aluminum and ferric chlorides is attributed by Croft to bonding by the electron pairs of the nitrogen atoms of boron nitride. It is surprising, however, that boron trifluoride and trichloride, which are particularly good electron acceptors, appear unable to become intercalated. 

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

Boron forms two simple series of lower halides, BX3 and B2X4(X2B—BX2). The highly reactive trihalides are volatile covalent compounds whose molecules have the expected trigonal planar geometry. The molecules of the other series are also planar in the solid state however, in the gaseous state, the chloride and the bromide... 

Lower LD Boron halides. In M. Grayson (ed) Kirk-Othmer Concise Encyclopedia of Chemical Technology, p 180. New York, Wiley-Interscience, 1985... 

Boron trichloride undergoes several exchange reactions with dihorane, other boron halides and trialkyl-, triaryl-, or triaryloxy boranes (Lower, L.D. 1978. Boron compounds (halides). In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed. pp 129-135. New York Wiley Interscience). The reactions occur at ambient or lower temperatures ... 

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