DV X-ALPHA 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]+ (C6[1 symmetry), [(C6H6)2Ga]+ (£>«,), [(C6H6)2Ga]+ (C2t,), and [(C6H6)2Ga(GaCl4)] (C2l)) 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 (C2l)). 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. © 1990, American Chemical Society. All rights reserved.