Cyclobutadiene complexes ligand transfer

The (tetraphenylcyclobutadiene)PdX2 dimer reacts with a variety of metal complexes via transfer of the cyclobutadiene ligand to another metal. These reactions and other ligand transfer reactions have been reviewed by Efraty15. 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

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 first cyclobutadiene complexes were obtained either from alkynes (p. 246) or by reaction of cis-dichlorocyclobutenes with metal carbonyls. Many more derivatives were then prepared by transfer of the cyclobutadiene ligand from one transition metal to another, especially from palladium in [(Ph4C4)PdCl2]2. In their complexes cyclobutadienes adopt a planar square structure. 

The cyclobutadiene complex Ru6(/r6-C)(/r-CO)3(CO)i2( -C4Ph4) 138 is formed from redox-mediated ligand transfer between [Pd(acetone)2( 7 -C4Ph4)] and [Ru6(/r6-C)(CO)i6]. The metal core consists of a bond-opened octahedron, with the 77 -organic ligand bound to a basal ruthenium atom." ... 

The reactions of the palladium-cyclobutadiene complexes with excess alkyne produced persubstituted cyclooctatetraenes in analogy to Reppe s catalytic cyclooctatetraene synthesis (Pollock and Maitlis, 1966 Reppe and Schweckendiek, 1948). Although the cyclobutadiene ligand can be transferred between metals [Eq. (76)] (Maitlis, 1971b), liberation of the ligand... 

Fi. 21. Some preparations of Tr-cyclobutadiene complexes by ligand transfer reaction. 

Since these two initial reports, several other complexes possessing tetra-substituted, cyclobutadiene ligands have been reported, notably by Maitlis and co-workers ( ), who have demonstrated that one can transfer a cyclobutadiene ligand from one metal to another to produce new cyclobutadiene-metal complexes. 

Osmocene is a photoactive compound. Photolysis of osmocene in n-hexane led to metallic osmium by reductive elimination. In the LF excited triplet state, osmocene undergoes a distortion by bending, which facilitates the transfer of a CH group between the cyclopentadienyl ligands. The resulting intermediate is an osmium complex with benzene and cyclobutadiene ligands, that decomposes to osmium metal, benzene and cyclobutadiene. ... 

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