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Dedicated on the occasion of his 75th birthday to Professor Günther Wilke, an outstanding pioneer in transition metal catalyzed organic synthesisA comparative mechanistic study of the nickel(0)-catalyzed homo-oligomerization and co-oligomerization of alkynes and of nitriles has been undertaken, with diphenylacetylene and benzonitrile, towards an array of nickel(0) reagents, such as finely divided nickel, (COD) 2 Ni, (Bpy)(COD)Ni, (Et 3 P) 4 Ni, (Bpy)(PhCϵCPh)Ni and (COD) 2 Ni−MeAlCl 2 combinations in donor (THF) and nondonor (PhMe or neat substrate) solvents. Special attention has been given to the detailed molecular structures of the initial 1:1 adducts, (Bpy)(PhCϵCPh)Ni, (Ph 3 P) 3 (PhCϵCPh)Ni and [(Ph 3 P)(PhCϵN)Ni] 4 by a consideration of XRD and IR data. Data from the single crystal XRD analysis of (Bpy)(PhCϵCPh)Ni, reported here for the first time, are shown to be in excellent accord with the presence of a 2,3-diphenylnickelacyclopropene ring for the (PhCϵCPh)Ni moiety with almost coplanar chelating coordination of the bipyridyl ligand, rather than with the presence of simple ''side-on'' coordination of the alkyne with the metal center. A parallel analysis of XRD and IR data for the two benzonitrile-nickel(0) complexes, which were drawn from previous publications, has concluded that the nickel in (Ph 3 P) 3 (PhCϵN)Ni is coordinated in an ''end-on'' fashion and the nickel centers in [(Ph 3 P)(PhCϵN)Ni] 4 are coordinated as bridges between nitrile units in both an ''end-on'' and ''sideon'' manner. The stereochemistry of the acid hydrolysis of the

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Two equivalents of the complex (2,2/-bipyridyl)(l,5-cyclooctadiene)nickel(0) cleave dibenzothiophene in homogeneous media to form nickel-substituted biphenyl derivatives and nickel sulfide. The cleavage proceeds more readily in THF than in benzene solution, and excess amine (either bipyridyl or pyridine) retards the reaction. Neither biphenylene nor 2-phenylbenzenethiol was a detectable transient in the desulfurization process. The possible fleeting formation of biphenylene was ruled out by a study of the desulfurized products obtained from the 2,8-dimethyl and the 3,7-dimethyl derivatives of dibenzothiophene. A possible role of nickel hydride in some of the C-S bond cleavage is suspected from the observed effects of either acid or LiAlH4 used in the workup of the reaction mixture. The reaction is not catalytic in nickel when an excess of LiAlH4 is used; this, indicates that Ni(0) cannot be regenerated from NiS under these conditions. The observed cleavage of ~50% of the dibenzothiophene by 2 equiv of (C10H8N2)Ni(C8H12) is ascribed to the gradual formation of (C10H8N2)2Ni, which is ineffectual in cleaving dibenzothiophene. Finally, the relative reactivity of the dibenzothiophenes (dibenzothiophene > 2,8-dimethyldibenzothiophene » 3,7-dimethyldibenzothiophene), the relative effectiveness of amine donors or solvents, and the observed hydrogen abstraction from the solvent are interpreted in terms of an electron-transfer mode of action by the nickel(O) desulfurizing agent.

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