**4. Free molecules of the heavier group 13 metal monohalides**

Aluminium and gallium monohalides belong to two groups: indium and thallium monohalides (excluding InF), which can be disproportionated into metal and metal trihalide under ordinary conditions, and indium and thallium trihalides, which are stable and commercially available [1, 2]. High temperature/low pressure give rise to entropically favoured Al (I) halides in the gas phase, which could be captured by inert gas matrices. In the gas phase, spectroscopic analysis of the AlX (X = F-I) molecule show bond distances of 2.537, 2.295, 2.130 and 1.654 A for Al-F to Al-I respectively [1, 2, 74–76], whereas in low-temperature matrices, symmetrical bridging halides, Al(μ-Cl)2Al and Al(μ-F)2Al predominate [77, 78]. The bond lengths of the gaseous GaX molecules corresponding to aluminium (1.774, 2.202, 2.352, and 2.575 A, respectively) varied slightly, which is consistent with Ga(I) having a greater covalent radius [1, 2, 75, 76]. The metastable AlX solutions can be synthesised by using mixtures of AlX molecules and donor solvents like toluene [5–7, 59, 79, 80]. In the solid-state, there are no completely stable Ga (I) halides. GaI was first reported in 1955, and it was synthesised by vacuum heating Ga and I. Green reported a new synthesis of GaI in 1990, employing an ultrasonically activated Ga with iodine in C6H5CH3 [81], but later Raman spectroscopy investigation indicated that it was made up of a mixture of Ga subhalides and its valence salt [Ga]2[Ga2I6] [82]. This green's reagent acts as an accessible and versatile reagent for the synthesis of various monovalent gallium compounds, as well as a possible source of gallium iodide in addition to procedures involving different chemical bonds [81, 83, 84]. And also acts as an extreme source of the gallium iodide fragment in one newly described compound including GaI diatomic fragments [85–87]. In contrast to AlX and GaX, InX (X = Cl-I) and TlX (X = F-I) halides are stable to disproportionation in the solid-state at room temperatures, making them a potential candidate for low oxidation state In/Tl compounds and, as a result, their wider use as reagents in organic synthesis [1, 2, 4, 88–90]. Bond distances of 2.754 (I), 2.543 (Br), 2.401 (Cl) and 1.985 A (F), for the monovalent indium diatomics, have been determined in the vapour phase, with equivalent distances of 2.814, 2.618, 2.485 and 2.084 Å observed for the corresponding Tl molecules [1, 2, 4, 75]. In contrast, in an argon atmosphere, the reactivity of singlets (1S) and triplets (3P) of monovalent indium chloride towards HX (X = H, Cl, or OH) has been investigated [91–93]. Tuck previously reported that In(I) halides can be treated with Lewis bases at low temperatures to create insoluble complexes that are disproportionable. So, below 20°C, InBr (16 mM) solutions in toluene/TMEDA mixtures are stable, the crystalline complex InBr(TMEDA) was separated from the same solution, indicating long-range In/In interactions (3.7 Å) [94, 95].

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