Tetramethylcyclobutadiene nickel chloride

On treatment with aqueous silver nitrate it forms a stable crystalline complex (78), from which 75 can be recovered almost quantitatively by addition of ammonia ". Anhydrous nickel(ii) bromide converts cyclooctyne into the trimer (79) with a quantitative yield . However, when the reaction was carried out in the presence of a trace of water, the dimeric cyclobutadiene-nickel bromide complex (80) was obtained in 9-4% yield, together with 79 (85%) . The spectroscopic properties of 80 showed close similarity with those of the tetramethylcyclobutadiene-nickel chloride complex . 

Criegee and his co-workers 28,29,32) were the first to study the thermal decomposition of a cyclobutadiene complex, tetramethylcyclobutadiene-nickel chloride (XIV). The most important reactions and products obtained are summarized in Fig. 4. 

While sodium cyclopentadienide attacks tetramethylcyclobutadiene-nickel chloride both at carbon and nickel (Section VI, F), the discovery of a novel cyclopentadienylation reaction which is in effect a ligand-transfer reaction involving attack at the metal only has allowed other types of cyclobutadiene complexes to be prepared. Thus on reaction of tetra-phenylcyclobutadienenickel and -palladium bromides (LXXXVI) with cyclopentadienyliron dicarbonyl bromide, the paramagnetic (cyclopentadi-enyl)(tetraphenylcyclobutadiene)nickel and palladium tetrabromoferrates (LXXXVII M=Ni, Pd) are obtained 64, <55). 

The prediction, in 1956 by LONGUET-HIGGINS and ORGEL (1), that while cyclobutadienes are likely to be unstable, their complexes with transition metals would be stable, has been amply confirmed. In 1959 CRIEGEE and SCHRODER prepared tetramethylcyclobutadiene nickel chloride (2) and this was quickly followed by the preparation of tetraphenylcyclobutadiene [Ph C ] complexes of iron, [II] (3), nickel, [III] (4), and cobalt, [VI] (5). 

The first example of reaction of a half sandwich metallocene with a protein was reported a few years later. Tetramethylcyclobutadiene nickel(II) dichloride 2 was shown to react with the enzyme horse hver alcohol dehydrogenase to afford a modified enzyme devoid of zinc and enzymatic activity and containing 8 mol nickel per mol protein. The mode of binding of this organometallic complex had not been fully elucidated at that time but was thought to occur by substitution of the chloride ligands by nucleophilic residues [3],... 

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