Boron Tetrahalides

The staggered rotamer is predicted to be more stable, in agreement with experiment for B2C14 but not for B2F4, but in the latter case the error in the barrier is only ca. 3 kJ, so it is possible that a more extended basis set would correct this. The wave functions were analysed in detail and used to interpret the photoelectron spectrum of B4C14. 

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Boron Tetrahalides, BjX,. Although these molecules have been extensively investigated by semi-empirical methods, only recently have ab initio calculations been performed. Using minimal basis sets for the core and s-orbitals, but DZ for the valence p-orbitals, SCF calculations on B2CI4 and B2F4 (as well as on B4CI4 and B4F4) were carried out, and the results analysed in terms of localized molecular orbitals. 

Phosphorus pentahalide complexes contain phosphonium cations and boron tetrahalide anions, for example, PCI4 BCI4 (Chapter 2.6). Phosphonium borohydrides, R4P BjH., can be prepared from phosphonium halides and sodium borohydride. On hydrolysis they give phosphine boranes. Salts with phosphonium cations and boron-containing anions are formed in other reactions (6.379) and (6.531). 

Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Halogens react directly to form tetrahalides. 

Addition of a boron-boron bond across a carbon-carbon triple bond is known for some 40 years since the finding that diboron tetrahalides add to alkenes and alkynes in the absence of catalysts.36 Although the reaction seemed to be potentially attractive, the instability of diboron tetrahalides was the critical drawback for the practical use in synthesis. In 1993, much more stable pinacol ester derivative of diboron was found to add to alkynes in the presence of platinum catalysts such as Pt(PPh3)4, Pt(CH2=CH2)(PPh3)2, and Pt(CO)2(PPh3)2 (Figure 1, Scheme 2).37,38 Other... 

It is worth recalling that the phenomenon of co-catalysis (now more consistently renamed co-initiation ) was found first with boron trifluoride and with the tetrahalides of titanium and tin. It is well known that it can be interpreted by the reaction scheme (1)... 

Other Lewis acids that complex with thietane are titanium tetrachloride or bromide,boron trifluoride,trimethylaluminum, and tin tetrachloride. The enthalpies of formation of the aluminum complex (— 16.04 kcal/mole) and the tin complex (— 14.2 kcal/mole) and the wavelength of the charge transfer band of the tin complex (270 nm) have been determined. The titanium tetrahalides form both a 1 1 and a 1 2 adduct (titanium halide thietane). °° Treatment of 3-methyl-thietane with aluminum chloride or tin tetrachloride yields a rubbery white soUd." "... 

Some of the more active catalysts for the polymerization of styrene are shown in Figure 22.17. Monocyclopentadienyltitanium halides, such as CpTiXj, CpTiXj, and CpTiXj, in combination with MAO, were more active catalysts than simple tetrahalides. Of these Cp-ligated trihalide complexes, the fluoride complexes were most active, followed by alkoxides and then chlorides. Moreover, cyclopentadienyl-ligated alkyltitanium compounds, such as Cp TiRj (R = hydrocarbyl) activated by boron compounds B(C Fj)j... 

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