Influences of the substituents on the M–M bonding in Cp4Al4 and Cp2M2X2 (M = B, Al, Ga; Cp = C5H5, X = halogen)

Abstract

Although the geometries of Cp^*₄Al₄ (Cp* = C₅Me₅) and Cp₄Al₄ (Cp = C₅H₅) are similar, Cp^*₄Al₄ is more stable than Cp₄Al₄. Cp^*₂Al₂I₂ is the first complex involving an Al(II)–Al(II) bond to be supported by Cp-type ligands. In this work, the stability of Cp^*₄Al₄ and Cp₄Al₄ (Cp = C₅H₅), the nature of M–M bonding in Cp₂M₂X₂ (M = B, Al, and Ga), and the influences of the X atom on the M–M bonds have been analyzed and compared within the framework of the atoms in molecules (AIM) theory, electron localization function (ELF), energy decomposition analysis (EDA), and natural bond orbital (NBO) analysis. The calculated results show that Cp^*₄Al₄ is more stable than Cp₄Al₄ because of H⋯H interactions between the methyl groups on the same and different Cp rings and not because of the Al–Al bonds. In Cp₂M₂X₂, the B–B bond is stronger than the Al–Al and Ga–Ga bonds. The B–B bond is most consistent with covalent bonding, whereas the Al–Al and Ga–Ga bonds are more consistent with metallic bonding. The strengths of the B–B bond increase in the sequence X = F, Cl, Br, and I, whereas the Al–Al and Ga–Ga bonds decrease in the sequence X = F, Cl, Br, and I. The different change tendencies arise from the different M–M bonds and the orbital interactions between atoms X and M.