**2. Free molecules of boron (I) halides**

In 1935, the properties of gaseous boron monofluoride synthesised from elemental boron and CaF using techniques such as high-temperature in situs method were reported. Since then, it has been studied using several spectral and thermal analyses [22–36]. BF was first synthesised on a preparative scale in 1967, based on Peter Timms' pioneering work to develop a proportionation pathway from BF3 and elemental boron [37–39]. The boron trifluoride gas is passed through elemental boron at very high temperature (2000°C) and very low pressure (1 mmHg) and in a specially designed reactor [37]. In addition, while BF molecule in the vapour phase maintains its diatomic nature. A variety of BF-containing molecules (such as B8F12, B3F5 and B2F4) [37, 40, 41] are then formed with the proportion based on the amount of BF3 co-condensate. BF insertion into BF3 produces B2F4, and BF insertion into B2F4 produces B3F5 [37]. Then, Timms reported how metastable boron monochloride can be prepared by cracking B2Cl4 (which produces BCl3 and BCl) or by reacting elemental boron with BCl3 at a very high temperature [39, 42]. A laser-ablated boron atom reacts with its respective X2 molecule, resulting in combinations of BX3, BX2, and BX. Matrix isolation offers a feasible way to capturing and interrogating diatomics. Microwave spectroscopy [35] produced a bond length of 1.26267 Å, as well as values for Do 757 kJ mol−1 [36], ωe (1765 cm−1) [33], and the first ionisation energy 11.115 eV [29]. A similar approach was used to determine bond lengths for the BF, BCl and BI 1.715, 1.888, 2.131 Å respectively [1, 2]. Infrared spectroscopy reveals vibrational frequencies of 564 cm−1 (11BI), 667/666 (11B79Br/11B81Br), 815/810 (11B35Cl/11B37Cl), and 1374 (11BF) for BX molecules separated in solid argon atmosphere [43]. Research with C2F4 and SiF4 has shown that even though the BF insertion is simple for B-F bonds, similar procedures have yielded traces of products with other E–F couplings. When BF is combined with soft donors (such as CO, AsH3, SMe2, PH3, PF3 and PCl3) leads to the formation of (F2B)3B-L (general formula). Based on the crystallographic description of BCl is very similar to carbonyl complexes [42, 44]. In the presence of acetylene or propene, BF produced acyclic or cyclic products containing BF or BF2 moieties [38], whereas when BF was combined with gaseous BCl, only cyclic compounds were produced.

*Application of Density Functional Theory in Coordination Chemistry: A Case Study of Group 13… DOI: http://dx.doi.org/10.5772/intechopen.99790*
