Fünf ist Trumpf: Fünffachbindungen von Gruppe‐VI‐Metallen können Friedel‐Crafts‐Halogenacylierungen eingehen. Entsprechende Dimolybdänkomplexe reagieren mit einem Äquivalent RCOX (R=Me, C6H5, 2‐MeC6H4; X=Cl, Br) zu Acylkomplexen mit Metall‐Metall‐Vierfachbindung. Der Zusatz eines zweiten Äquivalents an Acylhalogenid führt zu einer Disproportionierung unter Bildung von Dimolybdän‐Carbin‐Carboxylat‐Komplexen.

The addition of acyl chloride to alkynes is not only of fundamental interest, but also important in chemical and pharmaceutical industries, because both carbonyl and chloro functionalities are simultaneously introduced to yield bchloro a,b-unsaturated ketones. [1] Many N-and O-containing heterocycles of industrial importance have been subsequently prepared from these ketones. [1,2] b-Chlorovinyl ketones are readily synthesized from alkynes and under acyl chlorides under the traditional Friedel-Crafts conditions. [1,3] Like the CÀC p bond, the metal-metal d bond is a twoelectron two-center bonds formed by the highest occupied molecular orbital, [4] thus the reactivity of quadruple-bonded dinuclear complexes has been highly anticipated. Although a large number of quadruple-bonded dinuclear species have been characterized in the past five decades, [5] their reactivity is far less explored. After a long hiatus, interest in metalmetal multiple bonding was reinvigorated by Power and coworkers who reported the first Cr À Cr quintuple bonded compounds. [6] This field was propelled by the identification of few nitrogen-donor-stabilized quintuple bonded Group VI compounds. [7] These low-coordinate quintuple-bonded species not only have interesting metal-metal bonding, but also show remarkable reaction chemistry. [8][9][10][11][12][13][14][15][16] Recent studies have indicated that there are strong analogies between the d component of the metal-metal quintuple bonds and the p component of the carbon-carbon multiple bonds. For example, the quintuple bonded molybdenum amidinate dimers [Mo 2 {m-k 2 -RC(N-2,6-iPr 2 C 6 H 3 ) 2 } 2 ] (R = H (1), Ph) [17] react with two equivalents of terminal alkynes to give the first aromatics containing two multiple-bonded dimetal units by a [2+2+2] cycloaddition process, [14] while its chromium analogue [Cr 2 {m-k 2 -HC(N-2,6-iPr 2 C 6 H 3 ) 2 } 2 ] (2) [18] reacts with alkynes to afford [2+2] adducts. Kempe and co-workers have

A new family of the quintuply bonded dichromium complexes [Cr2{μ‐κ2‐HC(N‐2,6‐R2C6H3)2}2(μ‐κ2‐HC[NAr]2)] (R = iPr, Ar = 4‐MeC6H4 (5), Ar = 3,5‐Me2C6H3 (6), and Ar = 2,6‐Me2C6H3 (7); R = Et, Ar = 4‐MeC6H4 (8), Ar = 3,5‐Me2C6H3 [9], and Ar = 2,6‐Et2C6H3 (10)) with a heteroleptic lantern configuration was obtained upon the addition of one equivalent of amidinate to the quintuply bonded dichromium amidinates [Cr{μ‐κ2‐HC(N‐2,6‐R2C6H3)2}]2 (R = iPr, Et). Additionally, the same approach was applied to the preparation of the acetate derivative [Cr2{μ‐κ2‐HC(N‐2,6‐ iPr2C6H3)2}2(μ‐κ2‐CH3CO2)] (11), which represents the first example that the quintuply bonded dinuclear complex contains an oxygen‐containing ligand. Of particular interest is that the Cr‐Cr bond lengths in these new trigonal paddlewheel quintuple Cr‐Cr bond species are comparable with those in their precursor compounds. They show ultrashort Cr‐Cr bond lengths in a narrow range of 1.740–1.755 å on the basis of single‐crystal X‐ray crystallography. The small Mayer bond orders of the long Cr‐N bonds as well as divergent, C2v and D3h, structural conformations in 5–11 suggest that the metal–ligand interactions possess minor covalent character and the electrostatic interactions play a dominant role. As a result, these extremely short Cr‐Cr quintuple bonds are caused by the overlap between five pairs of d orbitals that do not involve much in metal–ligand bonding. Additionally, anionic lantern dichromium trisamidinates 5–10 can be chemically oxidized by one electron, supported by electrochemistry, and their ease to undergo oxidation is presumably associated with their neutral lantern dichromium trisamindinate products, whose structures inherently display a Jahn‐Teller distortion, exemplified by the structure of the homoleptic dichromium complex [Cr2{μ‐κ2‐HC(N‐2,6‐Et2C6H3)2}3] [12] determined by X‐ray crystallography. These results unambiguously support the Cr‐Cr quintuple bonding in these novel anionic lantern dichromium complexes.