-arene}] (3; R 1,2,4-tri-tert-butyl) is described. A unique feature of the compound is its h 4 :h 4 antifacial-coordinated arene ligand, which displays complete planarity, as determined by X-ray crystallography, and represents the first example of its kind. In 3 two opposite carbon atoms of the arene moiety are bonded to both cobalt centers. To probe possible reasons for arene lability in this complex we compared arene exchange with benzene and azulene in 3 and in the related complex [{(h 5 -Cp -arene}] 4 were performed with the semiempirical ZINDO approximation. From a crossover exchange experiment between 1, 3, benzene, and azulene it was deduced that arene exchange in these tripledeckers proceeds by a dissociative pathway involving mononuclear {(h 5 -Cp R )Co} intermediates (R Me 5 , 1,2,4-tri-tert-butyl). The formation of the mixed triple-decker 13 as main product in this experiment is highly indicative for this type of ligand substitution. Such a mechanism is described herein, for the first time, for ligand exchange in tripledecker complexes.

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Mechanistische Untersuchungen zum reversiblen Gleichgewicht von metallorganischen Tripeldecker-und Sandwichkomplexen des Typs [Bis{(r75-CpR)Co}-/z-{774:774-aren}] und [(r75-CpR)Co(r76-aren)]Mechanistic Studies towards the Reversible Equilibrium between Metal Organic Triple Decker and Sandwich Complexes [Bis{(/75-CpR)Co}-^-{/74 :774-arene}] and [{(775-CpR)Co(776-arene)] The arene ligand exchange mechanism o f slipped arene triple deckers [Bis{(?;5-CpR)C o}-^-{r/4:?74-arene}] (R = M e5, 1,2,4 tri-tert butyl, arene = benzene, toluene) 1 was studied by 1 H-NMR spectroscopy for different concentrations and solvents. It has been found that triple deckers of type 1 decom pose slow ly in solution. A unique equilibrium, between these triple deckers and the mixed sandwich com plexes [(?/6-arene)Co(?/5-CpR)] and 14 e [(^5-Cp)Co]soW fragments generated in situ by decom position o f 1 exists. In addition to this equilibrium arene lability of the thus formed mixed sandwich com plex type has been detected by NMR making slipped triple deckers 1 ideal single source compounds for the generation o f two [(7^-Cp)Co] fragments in one reaction step. Such fragments are valuable metal ligand components with high synthetic utility in organometallic chemistry. * Sonderdruckanforderungen an Priv.-Doz. Dr. J. J. Schneider. Fax: +49 201 183 2402,

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