Theoretical studies on all-metal binuclear sandwich-like complexes M ₂(η⁴-E₄)₂ (M=Al, Ga, In; E=Sb, Bi)

A series of all-metal binuclear sandwich-like complexes with the formula M₂(η⁴-E₄)₂ (M=Al, Ga, In; E=Sb, Bi) was studied by density functional theory (DFT). The most stable conformer for each of the M₂(η⁴-E₄)₂ species is the staggered one with D_4d symmetry. The centred metal-metal bond in each M₂(η⁴-E₄) ₂ species is a covalent single bond, with the main contributors to these covalent bonds being the a₁ and e orbitals. For all these species, the interactions between the centred metal atoms and the allmetal ligands are covalent; η⁴-Sb4^2- has a stronger ability to stabilize metal-metal bonds than η⁴-Bi₄ ^2-. Nucleusindependent chemical shifts (NICS) values and molecular orbital (MO) analysis reveal that the all-metal η-Sb₄ ^2- and η⁴-Bi₄^2- ligands in M₂(η-E₄)₂ possess conflicting aromaticity (o^- antiaromaticity and π aromaticity), which differs from the all-metal multiple aromatic unit Al₄^2-. In addition, all of these M₂(η⁴-E₄)₂ species are stable according to the dissociation energies of M₂(η ⁴-E₄)₂ →2 M(η⁴-E ₄) and M₂(η⁴-E₄)₂ → 2 M+2E₄, and these stable species can be synthesized by two-step substitution reactions: CpZnZnCp + 2E₄^2- →[E₄ZnZnE₄]^2- +2Cp^- and [E ₄ZnZnE₄]^2- + 2 M₂⁺ →E₄MME₄+2Zn⁺.