Arene Complexes of Univalent Gallium, Indium, and Thallium

Arene complexes of main‐group metals were, until recently, rare species—in contrast to the now classical, analogous complexes of transition metals. In systematic investigations, it has been possible to prepare and structurally characterize arene complexes of the univalent elements gallium, indium, and thallium, which directly follow the d‐block elements in the periodic table. This new type of compound is characterized by centric (η⁶) coordination of the metal to the arene; both mono‐ and bis(arene) complexes are known. The interaction can be explained by the perfect agreement between the HOMO/LUMO symmetry of the arene and of the low‐valent metal. The electronic states of the nd¹⁰(n + 1)s² configuration, which are partially modified by relativistic effects, play a particularly important role. The relationship to the few known complexes of the neighboring elements (Sn^II, Pb^II) becomes plausible via the isoelectronic principle. The arene/Ga^I, In^I, Tl^I systems are of potential significance as homogeneous reducing agents and as agents for the activation of aromatic compounds, the purification of metals, and the separation of metals from nonaqueous media.