Bonding in the trihalides (X 3 ؊ ), mixed trihalides (X 2 Y ؊ ) and hydrogen bihalides (X 2 H ؊ ). The connection between hypervalent, electron-rich three-center, donor-acceptor and strong hydrogen bonding ‡

Carbon monoxide, CO, is a ubiquitous ligand in organometallic and coordination chemistry. In the present paper we investigate the neutral isoelectronic molecules AB = N2, CO, BF, and SiO and their coordination in the model complexes Fe(CO)4AB and Fe(AB)5, using nonlocal density functional theory and a large, polarized STO basis set of triple-ζ quality (NL-SCF/TZ(2P)). Our aim is to get more insight into the ligating properties of SiO and BF in comparison to CO and N2. The computed 298 K Fe(CO)4−AB bond dissociation enthalpies of C 3 v -symmetric Fe(CO)4AB are 18.1, 42.3, 67.9, and 35.6 kcal/mol for N2, CO, BF, and SiO, respectively; the corresponding values for C 2 v -symmetric Fe(CO)4AB are comparable:  19.0, 42.3, 66.7, and 39.7 kcal/mol. Good, balanced σ donation (through 5σ) and π acceptance (through 2π) are what makes CO a good donor, of course. The gap between these frontier orbitals (5σ and 2π) becomes even smaller in SiO and BF. The analysis of the bonding mechanism of the Fe−AB bond shows that SiO is a better σ donor but a worse π acceptor ligand than CO and that BF should be superior to CO in terms of both σ donor and π acceptor properties. However, these polar ligands are therefore also more reactive; and more sensitive, e.g. to nucleophilic attack, because of a low-energy 2π LUMO. Our results suggest that BF and SiO should, in principle, be excellent ligands. We also find interesting side-on and O-bound local minima, not very unstable, for SiO bound to an Fe(CO)4 fragment.

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A bromine ± alkyne p complex (l max 294 nm) of 1:1 stoichiometry has been observed for the first time in the course of the bromination of 1-phenylpropyne by means of a diode-array stopped-flow technique. ); this demonstrates that the complex is actually an essential intermediate on the reaction coordinate. The bromination of a series of nine alkynes has been studied. Bromination reactions with negative apparent activation parameters lead to mixtures of E and Z vinyl dibromides, whereas reactions with positive activation energy yield the E isomers exclusively. The reason for the difference in reactivity of these alkynes compared with structurally similar alkenes most likely lies in the stability of these 1:1 charge-transfer complexes. Usually open arylvinyl cations correspond to the energetically favored product-determining intermediates; bridged bromirenium ions are formed from deactivated alkynes and react to give E isomers. The kinetic effect of alkyl groups and of p-OCH 3 , p-CN, and p-NO 2 substituents at the aryl group on the bromination of arylalkylacetylenes is discussed. Density functional calculations provide insight into the geometries, energies, and bonding of the intermediate 1:1 and 2:1 Br 2 ± acetylene complexes involved. These theoretical investigations demonstrate that the most stable trimolecular Br 2 ± Br 2 ± acetylene adduct possesses a structure very similar to a crystallographically characterized Br 2 ± Br 2 ± alkene species, which can directly yield the ionic products, Br À 3 and vinyl cation, driven by the heterolytic action of a solvent.

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