**Abstract**

The chemistry of Group 13 Monohalide is of great interest due to its isoelectronic relationship with carbon monoxide and dinitrogen. In recent years, theoretical and experimental studies have been evolved on the group-13 atom-based diatomic molecules as a ligand. The synthetic, characterisation and reactivity of various metal complexes have been well discussed in recent reviews. The nature of the metal bonding of these ligands of various types has been explained in addition by the variety of theoretical studies (using DFT methods) such as FMO and EDA. This chapter has a comprehensive experimental and theoretical study of group 13 monohalides as a ligand in coordination chemistry.

**Keywords:** Group 13 Monohalides, Coordination Chemistry, DFT, FMO, BDE

### **1. Introduction**

The monohalides of group 13 elements (EX) are an isoelectronic relationship with molecules like CO and N2. The separation of metal complexes containing monohalides of group 13 elements as ligands is made possible by the recent developments in synthetic chemistry and with the investigations on electronic structural analysis and the reactivity of the coordinated diatomic group 13 monohalides. In general, the +3 oxidation states of group 13 elements have dominated the chemistry of their compounds. The applications of these compounds like catalysis, sensing, etc., are due to their inherent Lewis acidic behaviour [1–3]. Research in group 13 elements having lower oxidation states have been normally influenced by clusters of boranes. The availability of sub-valent systems reflects on the development of the ground-breaking synthetic approach in organic synthesis and the applications of some reagents specifically sub-valent indium compounds. For example, the study of Schnoeckel et al. approved to access metastable monohalides of aluminium and gallium, by utilising the entropic factor at a high temperature can be driven the equilibrium to the right and defined by Eq. (1) (E = B, Al, Ga, In and Tl) [4–14].

$$\text{EX}\_3(\text{g}) + 2\text{E}(\text{s}\,\text{or}\,\text{l}) = 3\text{EX}(\text{g})\tag{1}$$

Sub-valent aluminium and gallium compounds can be accessed by consequential entrapping and derivatization. The formation of sub-valent group 13 compounds is proposed by the theory to mimic the accession of a similar state of the elements [8–21]. For separating the discrete molecular systems, chemistry in solution has taken advantage of sterically bulkier groups (amino, guanidinate, β-diketiminate, pentamethyl cyclopentadienyl, terphenyl groups) as an approach [15–17]. Also, the competency of monovalent systems E(I) having such molecules to behave as ligands in complexes for transition metal atoms has been recognised extensively for B, Al and Ga. In organometallic chemistry, group 13 elements have been an unexplored area, even it has an isoelectronic relationship with well-known CO and N2 molecules. Even though the thorough comparative theoretical investigations on group 13 monohalides, there is no sufficient experimental data [15–17]. Such paucities naturally mimic the coordinated group 13 monohalide fragment's high polarity and low steric loading (as projected theoretically). This book chapter explores both synthetic and theoretical advances in the topic of group 13 monohalides in great depth.
