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Whereas the reaction of ethylene and triethylaluminum under pressure at 100°C yields trialkylaluminum compounds having long alkyl chains, the presence of small amounts of nickel salts induces the formation of butene. The discovery of this "nickel effect" represents the starting point for the development of the Ziegler catalysts. Comparatively little was formerly known about the nature and the mode of action of the catalytically active nickel species. A basis for the elucidation of the effect was provided by studies on the reduction of nickel compounds by organoaluminum compounds, on the occurrence and existence of nickel hydrides, and on interactions between nickel(0) and Lewis acids as well as organic compounds of main group metals. The most significant result of these studies is the recognition that multicenter bonding systems involving trialkylaluminum compounds and nickel atoms are involved. These react further with complex bonded ethylene in what is probably a concerted manner.

In the compounds 8a, 8b, and 8 d the vanadium atom has a distorted pseudo-trigonal geometry. The magnitudes of the angles DI-V-D2 and Dl-V-D3 differ only slightly (148-152"), as of necessity do those of D2-V-D3 (59-61"). The V-C bond lengths within the vanadium-Cp and vanadium-Cp* moieties lie between 2.20 and 2.26 A and in all cases V-D1 is 1.89 A. The most prominent structural feature of the new vanadium complexes 8 is the strong folding of the pentalene or methylpentalene ligand towards the metal atom along the bond between the bridging carbon atoms. In all three vanadium complexes the angle between the plane through the atoms C7-C9, C13, and C14 @a, 8d) and C7-C9, C8*, and C9* (Sb), respectively, and that through the plane of the atoms C9-Cl3 and C9-Cl1, C9*, and C10*, respectively, is 137". The distances between vanadium and the C atoms of the pentalene ligands can be divided into three groups: V-C9zV-C13 (2.04-2.07 A)

In this progress report a new principle is described for the synthesis of multimetal n-complexes with hitherto unknown structural features. The nickel(0)-olefin complexes 1,5,9-cyclododecatrienenickel(0) and bis(l,5-cyclooctadiene)nickel(o) react unexpectedly with main-group metals, in particular alkali metals, their hydrides, and organometallic compounds, to give nickelligand species with a surplus charge. These species endeavor to transfer the excess charge onto m-ligands such as olefins or dinitrogen. Multimetal complexes with electron rich transition metal n-ligand units can, in addition, be prepared from metallocenes, alkali metals, and unsaturated compounds. The syntheses, structures, and reactions of this new class of substances will be summarized. 0 Verlag Chemre, GmbH, 6940 Wernherm, 1980 0570-0833/80/0707-0520 S 02.50/0 Angew Chem I n / Ed Engl 19. 520-5.17 (1980)

The new room temperature stable halfsandw ich complex Cp*Fe(tmeda)Cl (2) has been synthesized by the 1:1 reaction of FeCl2(thf)1.5 with LiCp* in a mixture of THF and TMEDA at - 30 °C. 2 is an ideal starting material for the synthesis of a wide range of new iron complexes. Treatment of 2 with lithium sand in THF in the presence of COD or ethene followed by the addition of TMEDA yields the ferrates [Li(tmeda)][Cp*Fe(cod)] (3) or [Li(tmeda)][Cp*Fe(C2H4)2] (4). By delithiation with dichloroethane, 3 and 4 can be transformed into the novel 17 e iron complexes Cp*Fe(cod) (5) and Cp*Fe(C2H4)2 (6). 6 is extremely labile. Since the ethene ligands can be easily displaced, the title compound is a synthetically valuable source of the Cp*Fe fragment. Whereas the photochemically generated 17e dicarbonyl species CpFe(CO)2 rapidly recombines to give the dimer [CpFe(CO)2]2, the isoelectronic 6 is stable in ethene saturated pentane for several days at 0 °C. Without the stabilizing effect of ethene (in pure pentane or under vacuum ), 6 loses ethene to give the dinuclear complex (Cp*Fe)2(C2H4)2 (8) irreversibly. The structure of 8 has been characterized by X -ray analysis.