Cyclobutadiene complexes with donor ligands

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Some of the cyclobutadiene complexes [e.g., (XIII) and (XXVIII)] appear to be inert to the action of donor ligands such as phosphines, pyridine, etc. However, a number form adducts. These have not been fully described, but triphenylphosphine adducts of tetramethylcyclobutadienenickel chloride (XIV) (28) and tetraphenylcyclobutadienenickel bromide (XXXIII) (41) and an o-phenanthroline complex (29) of the former have been reported. Other reactions with donor molecules in which the cyclobutadiene-metal bond remains intact are mentioned in Section VI, G. 

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