Dv Χα molecular orbital studies of the bonding in benzene complexes of univalent gallium salts

The electronic structures of the benzene/gallium(I) model complexes [(C₆H₆)Ga]⁺ (C_6[1 symmetry), [(C₆H₆)₂Ga]⁺ (£>«,), [(C₆H₆)₂Ga]⁺ (C_2t,), and [(C₆H₆)₂Ga(GaCl₄)] (C_2l)) have been calculated by using the discrete variation Χα (DV Χα) molecular orbital method. The results show that a weak dative bond is formed between the C₆H₆ molecule and the Ga¹ atom in these species, electronic charge being transferred from the π orbitals of the benzene molecule to the three 4p orbitale of the Ga¹ atom. The total amount of charge transfer to the Ga¹ atom increases from 0.13e to 0.24e in this series, but the amount of charge transfer from a single C₆H₆ molecule decreases from 0.13e to 0.04e. These amounts are comparable to those estimated for charge-transfer complexes of benzene and substituted benzenes with other electron acceptors. The results for [(C₆He)₂Ga]⁺ suggest that there is a slight strengthening of the CgHg-Ga¹ dative bond in going from the “flat sandwich” structure (Dy,) to the “bent sandwich structure (C_2l)). However, the results for [(C₆H₆)₂Ga(GaCl₄)] indicate that there is also a significant covalent interaction between the Ga¹ center and the [GaCl₄]~ counterion. This is therefore also expected to be a factor in determining the frequently observed “bent sandwich” arrangement in complexes of benzene and substituted benzenes with GatGaCy. Although K⁺ has an ionic radius similar to that of Ga⁺, there is no charge-transfer interaction between C₆H₆ and K⁺, due to the fact that the potassium 4s and 4p orbitals are too high in energy to allow a significant bonding interaction with benzene. This agrees with the experimental observation that potassium salts do not form stable complexes with benzene.