Bonding studies of compounds of boron and elements of groups 3–5. Part 16.—Ab initio SCFMO calculations and He(I) photoelectron spectra of halogen-bridged dimeric group 3 metal halides and methylmetal halides

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He(I) photoelectron spectra are reported for the halogen-bridged dimeric species Al2Cl6, Al2Br6, Ga2Cl6 and (Me2AlX)2(X = Cl, Br, or I). The electronic structures of monomeric and dimeric AlCl3 and Me2AlCl are discussed using the results of all electron ab initio SCFMO calculations in a variety of bases of Gaussian type functions. Assignments of the spectra are made with the aid of these calculations, and by correlation with the corresponding monomers. Values for enthalpies of dimerisation imply a more significant contribution by the Al 3d-orbitals to bonding in the dimer than in the monomer, while Cl 3d-orbitals act only as polarisation functions. For all dimers, calculation and experiment show that the predominantly bridging halogen orbitals ionise at higher ionisation potential (i.p.) than in the respective monomers, in accordance with loss of electron density at the bridging halogen atom. For (AlCl3)2 the lowest i.p.s correspond to ionisation from the terminal chlorine lone pair orbitals, and the highest observed i.p.s. to the Al–Cl bridging orbitals.