The tetrameric, η5‐coordinated aluminum(I) compound [{Al(C5Me5)}4] (1) is stable at room temperature. It was prepared from AlCl and [Mg(C5Me5)2]. In this cluster, four Al atoms form a regular tetrahedron and the planes of the C5Me5 rings lie nearly parallel to the respective faces of the tetrahedron. (see also Highlight on p. 544).

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.

Bereits Ende des letzten Jahrhunderts gab es die ersten Hinweise auf die Existenz von Al1‐Halogeniden. Aber erst durch die grundlegenden Arbeiten von W. Klemm, dessen Geburtstag sich am 6. Januar 1996 zum 100. Mal jährte, wurden detaillierte spektroskopische und anwendungstechnische Untersuchungen möglich. Nachdem seit Ende der siebziger Jahre auch die Reaktivitäten von AlX‐ und GaX‐Spezies in festen Edelgasen durch zahlreiche Beispiele nachgewiesen wurden, gelang es in den letzten Jahren, Spezies mit formal einwertigem Al und Ga im präparativen Maßstab darzustellen. Außer den ersten Halogeniden konnten dabei auch metallorganische Verbindungen mit Metall‐Metall‐Bindungen isoliert und bezüglich ihrer chemischen Eigenschaften untersucht werden. Die grundlegende Bedeutung solcher Spezies ist in dieser Zeitschrift unter anderem in zwei Highlights beschrieben worden, in denen exemplarisch experimentelle und theoretische Aspekte beleuchtet sowie Parallelen und Unterschiede zur Borchemie aufgezeigt wurden. In dem vorliegenden Übersichtsartikel wird zum einen die chronologische Entwicklung dieses Forschungsgebiets während der letzten Jahre dargelegt, und zum anderen wird versucht, die experimentellen Ergebnisse hinsichtlich Struktur, Thermodynamik und Reaktivität sowohl mit quantenchemischen Rechnungen als auch durch vergleichende Betrachtungen einzuordnen.

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The first sandwich complex of an element from the third main group, the cation in 1, has been synthesized and characterized by spectroscopy and X‐ray crystallography (see picture). This cation is stabilized by π interactions, and the Cp* ligands are staggered.

Two face‐sharing Al4P4 cubes, which lack two P atoms at the corners—this is the (idealized) framework of the title compound in the crystal. According to calculations, the isomer with adamantane structure is less stable by 30 to 80 kJ mol−1. As a result of quantum mechanical calculations on the model [P4(CpAl)6], [P4(Cp*Al)6] can be characterized as an electron‐deficient compound that is stabilized by considerable ionic bonding contributions.

Eine bei Raumtemperatur stabile Aluminium(I)‐Verbindung ist das tetramere, η5‐koordinierte [{Al(C5Me5)}4] 1, das aus AlCl und [Mg(C5Me5)2] herstellbar ist. In diesem Cluster bilden vier Al‐Atome ein reguläres Tetraeder, und die Ebenen der C5Me5‐Ringe liegen nahezu parallel zur jeweils gegenüberliegenden Basisfläche des Tetraeders. Siehe auch Highlight auf S. 559.

A metastable AlCl solution obtained by co-condensation of the high-temperature molecule AlCl and a mixture of toluene/diethyl ether reacted with SiCp* 2 or SiCl 4 /AlCp*, giving a unique SiAl 14 cluster species that bears six Cp* ligands protecting the compound from disproportionation and formation of the bulk material (elemental Al or an Si/Al alloy). The structure of the SiAl 14 core represents a section of the bodycentered packing where a Si atom resides in the center of an Al cube. Each of the six faces of the cube is capped by an additional Al(η 5 Cp*) moiety. This cluster compound was investigated by mass spectrometry, X-ray diffraction, 27 Al NMR spectroscopy, and ab initio theory. The solid-state structure contains minor amounts of molecules with additional Cl atoms bonded to the Al atoms at the corners of the cube, and the presence of Cl-containing molecules in the crystal is rationalized on basis of the suggested reaction path.