The elements of the group 13 share a ns²np¹ electron configuration of the valence shell in the ground state, so that the oxidation states of +I and +III are most common. However, the +III oxidation state is more stable for the group 13 metals, except for thallium. Thus, metastable univalent gallium and indium species – the focus of our group – are accessible most likely as a result of the “inert-pair” effect of the ns² (n = 4, 5) lone pair orbital and d-block contraction. Nevertheless, both tend to disproportionate to M^III compounds and M⁰ metal. The introduction of positive charge yielding a M⁺ cation already very early on proved as a viable way to univalent M^I compounds, for example with the preparation of the mixed valent M^I[M^IIIX₄] (X = Cl, Br, I) salts. Yet, even though M[MX₄] does contain a M⁺ ion that can be solvated in aromatic solvents, the coordinating [MX₄]^– anion often undergoes unwanted side-reactions and dis- or comproportionations take place in many cases. This prevented the development of a coordination chemistry especially of Ga⁺ for a long time. Only with the synthesis of perfluoroalkoxy-aluminate salts of type [M(arene)_n]⁺[pf]^– (n = 2,3; [pf]^– = [Al(OR^F)₄]^–, R^F = C(CF₃)₃) that were first synthesized some years ago by us for Ga, and only a few months later in the group of Scheer for In, the field was advanced.

Note that the gallium p-orbitals play the main role in electron acceptance from the π-basic arenes. In contrast, the lone-pair orbital located at the gallium center remains largely unaffected by the arene-bonding. This results in a significant contraction and stabilization of the 4s²-electrons at the metal induced by the positive charge. Hence, the susceptibility of Ga^I towards dispropotionation is effectively inhibited. The cationic gallium arene complexes are characterized by comparatively high HOMO-LUMO-gaps > 6 eV. However, especially for the PhF-complex, the LUMO with high p-character at the gallium cation remains at low-energy, allowing for the complex to react as π-acid.