Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions

By the end of the last century there were already the first indications of the possible existence of Al1 halides. However, it was only through the pioneering works of W. Klemm, who would have celebrated his 100th birthday on January 6, 1996, that detailed spectroscopic investigations became possible. Since the end of the 1970s the reactivity of AlX and GaX species in solid noble gases has been confirmed by numerous examples. In recent years formally monovalent Al and Ga species have been successfully synthesized on a preparative scale. In addition to the first halides, organometallic compounds with metal–metal bonds have been isolated and investigated with regard to their chemical properties. The fundamental importance of such species has been documented in this journal among others in the form of two highlight articles in which experimental and theoretical aspects have been examined with examples, and parallels and differences with respect to boron chemistry have been illustrated. This review is intended to give an account of the chronological development of this research area over the last few years, but an attempt is also made to categorize the experimental results achieved not only with respect to structure, thermodynamics, and reactivity, but also with the aid of quantum chemical calculations and by comparative considerations. A highlight in the rapidly developing field of AlI and GaI compounds is the synthetic work with high‐temperature molecules such as AlCl and GaCl at low temperature. Halides such as [Al4Br4(NEt3)4] and [Ga5Cl7(OEt2)5], and organometallic compounds such as [GaCp*] and [Al4Cp 4*] (shown on the right) have been prepared, and their structures have been characterized unambiguously.