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The soluble catalyst system methylaluminoxane (MA0)-Ni(acac), (acac: acetylacetonate) gives polystyrene consisting of an amorphous (aPS) and a crystalline isotactic (ips) fraction. Al(CH,),, which is always present in commercial samples of MAO, decreases both polymer yield and stereospecificity. The polymer yield increases with increasing the MAO/Ni ratio but, at the same time, the iPS/aPS ratio decreases. Addition of N(C2H5), (mole ratio N/Ni = 1) increases the proportion of the isotactic fraction, while it decreases the polymer yield. A tentative interpretation of the stereospecificity is reported. Gel permeation chromatography: Mw = 33 -lo3; M, = 18 -1W3. dl Mw = 33 * lo3; M,, = 18lo'. Mw = 110 * lo3; M,, = 53 * 10'. e, M, = 42 -lo3; M,, = 24 -lo3.4

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Regioselective addition of organolithium compounds at the ortho position in oxazoline, imine, or hydrazone (h 6 -arene)tricarbonyl chromium complexes afforded anionic [(h 5 -cyclohexadienyl)Cr(CO) 3 ] intermediates. A single-crystal Xray structure determination of the product of the addition reaction of naphthyllithium with a phenyl oxazoline complex showed structural characteristics of an aza-enolate superimposed on the predominant cyclohexadienyl structure. Reaction of the anionic cyclohexadienyl complex with a trityl salt yielded 1,2-disubstituted [(h 6 -arene)Cr-(CO) 3 ] complexes. Labeling studies, carried out to provide mechanistic details of the aromatization process, ruled out an isomerization/exo-hydride abstraction sequence but were in accord with a process involving endo-hydride abstraction.