**3. Coordination chemistry of boron (I) halides**

The boron monohalides used as ligands in organometallic complexes were examined in the context of theoretical approaches were reported since 1998 [15–17] (**Figure 1**). The bonding properties of isoelectronic species of CO, N2 and BX (X = F – I) molecules were studied in particular. Based on the various proportion of the FMOs and energies of these molecules, BX will be a better sigma donor than CO or N2, as well as a similar π-acceptor, although the unbound molecule should have a smaller HOMO–LUMO gap due to stronger localization in the donor atom. In line with the broader focus of the synthetic endeavour, the related ligands BO and BNH2 have undergone comparable computational research. It also has a set of non-degenerate p-type orbitals, which are similar to those found in the vinylidene (CCR2) ligand family. The exceptionally high energy of BO<sup>−</sup> means that it possesses no π acceptor characteristics, but it retains outstanding donor characteristics. It has been determined that BF and related metal complexes have very strong thermodynamic stability, despite heavily polar BF bonds and positive charge build-up at boron. Both methods have already been synthesised, and could be used to shield reactive boron centres: either by shielding with (NH2 groups) BNR2 or by incorporating haloborylene as a bridge between two metal sites [8–17]. In 2010, it was predicted that BF would adopt bridging modes of coordination (μ2 or μ3) in ruthenium bimetallic systems [45, 46]. Timms et al. synthesised and characterised a thermally unstable volatile complex [(F3P)4Fe(BF)] using IR and 19F NMR spectroscopy [39]. Aldridge and his co-workers successfully synthesised the first fluoroborylene ruthenium complex [Cp2Ru2(CO)4(μ-BF)] and characterised it by the X-ray diffraction method [47]. The earlier complex was synthesised using a stable source for the BF ligand (Et2O-BF3) and CpRu(CO)2 − (Cp = ɳ<sup>5</sup> - C5H5). The reaction with BX3 and [NaMn(CO)5] was yielded the haloborylene complexes (μ-BX)Mn2(CO)10 [48]. It is very similar to the ruthenium complexes (μ-BX)Mn2(CO)10. Several alkylborylene complexes were synthesised and characterised but studies on the haloborylene complexes were limited [49–51]. Several complexes synthesised with BX fragments attached with metal centres with additional Lewis base support structures [Cp\*Fe(CO)2{(4-pic)2BBr}]<sup>+</sup> Br<sup>−</sup> [52]. Hildendrand et al. synthesised bimetallic complex with manganese [{(η5-C5H5) Mn(CO)2}2(η:η:μ-B2Cl2)] [28]. The geometrical and bonding analysis of halo and alkylborylene complexes [(η<sup>5</sup> -C5H5)M(BX) (CO)2] (M = Mn and Re; R = Et, iPr, Me and tBu; X = F - I) were studied theoretically in 2011 by K. K. Pandey et al., [53]. The steric stability of terminal haloborylene complexes, as well as the pielectron contribution of the haloborylene ligands, play a significant role in their separation. A variety of alternative synthetic techniques have been developed in response to the shortage of group 13 monohalides, such as halide abstraction/ejection [54–56], metal–metal borylene transfer [57], borane dehydrogenation [58], and salt elimination [49, 50]. The strength of metal complexes depending on the

**Figure 1.** *Development of boron monohalides based complexes.*

coordination of the metal with highly reactive ligands. The difficult separation of metal complexes with highly reactive ligands such as BF drives a strong interest in synthetic approaches to these complexes [59–64]. According to recent studies, polar diatomic BF ligands bind to transition metal centres more effectively than CO ligands, with greater σ-donor and π-acceptor properties [17, 46, 65]. Due to the electron-withdrawing halogen atoms, the haloborylene ligands of metal complexes are particularly significant because they have potential π-acceptor characteristics [66]. The theoretical analysis will be the most appropriate way for understanding the bonding character of the metal borylene complexes [67–73]. The C3v point group of [(CO)3M-BX] complexes have been studied using DFT, as well as the interaction between CO and BX ligands, the covalent character of M-B bonds, and bonding donation to M-B bonds [18–21].
