Unsaturated organic ligands, complexed

Fluxionality. The fluxional behavior of unsaturated organic ligands in mono- and oligonuclear organometallic complexes was well documented 109a) as a result of extensive NMR spectroscopic investigations. The advent of pulse Fourier transform... 

A useful synthetic route to specific carbonyl-PF3 transition metal complexes (method C) involves displacement of relatively weakly bound ligands such as ammonia, iodide ion, or unsaturated organic ligands (e.g., dienes, arenes) from suitable metal carbonyl complexes. 

Few other reactions between higher clusters and unsaturated organic ligands have been carried out (231, 232), but there are a number of studies which involve group IB organometallic derivatives (233-236). There are also a few high-nuclearity mixed-metal cluster alkynes which are the products of reactions between two heterometal complexes (237-242). 

Since the discovery of Zeise s salt, K[Pt(ri2-C2H4)Cl3], the first organometallic complex of a transition element, and particularly since the first reported synthesis of ferrocene, [Fe(r 5-C5H5)2], the number and variety of organometallic compounds with unsaturated organic ligands has increased enormously. 

COMPLEXES OF UNSATURATED ORGANIC LIGANDS WITH TWO OR MORE METAL ATOMS MIXED METAL-CARBON CLUSTERS... 

The bonding description for a coordinated alkene can be extended to other unsaturated organic ligands. Polyalkenes may be non-conjugated or conjugated. In complexes of non-conjugated systems (e.g. cycloocta-1,5-diene (cod), 23.20, or... 

Considerable efforts are now being devoted to many other bipy-type complexes with transition metals, trying to prepare new electrocrystalline materials, solve their X-ray stmctures, and measure their conductivities as a function of temperature. It is possible that some of these materials will have similar propoerties to those of the endohedral fullerenes and the cryptatium species described above. After all, they also contain metal cation centers sorrounded by highly unsaturated organic ligands that are capable of easy reduction. As such, all of these systems, if appropriately designed and manipulated, could yield novel conducting materials widi unusual properties. 

The lanthanides and actinides also form a wide range of complexes with unsaturated organic ligands including allyls, cyclopentadienyls and cycloocta-tetraene derivatives. The bonding is strongly polar. Coordination numbers are determined largely by steric requirements around the metal centre. 

Continuing our survey of the different types of ligand, we now turn to TV complexes in which the metal interacts with the tv bonding electrons of a variety of unsaturated organic ligands. 

R SiH and CH2= CHR interact with both PtL and PtL 1. Complexing or chelating ligands such as phosphines and sulfur complexes are exceUent inhibitors, but often form such stable complexes that they act as poisons and prevent cute even at elevated temperatures. Unsaturated organic compounds are preferred, such as acetylenic alcohols, acetylene dicarboxylates, maleates, fumarates, eneynes, and azo compounds (178—189). An alternative concept has been the encapsulation of the platinum catalysts with either cyclodextrin or in thermoplastics or siUcones (190—192). 

Unsaturated organic molecules such as alkenes, alkynes, dienes, polyenes and arenes can also stabilize low oxidation states in metal complexes, being both o donors (filled bonding jt orbitals) and jt acceptors (empty antibonding jt orbitals). In these so-called Jt complexes, only jt orbitals are involved in the metal-to-ligand bonds. This latter type of complex is beyond the scope of this chapter and only a few examples will be given. 

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