In olefin complexes

The presence of two mutually perpendicular it bonds in an alkyne makes for considerably more varied and complicated ligand behavior than that shown by alkenes. There is, of course, the simple use of one it bond and the associated it orbital in exactly the same way as found in olefin complexes. This results in a lengthening of the C—C distance and a marked deviation from linearity. Typical results are shown schematically in Fig. 16-28. 

The general patterns of structures and bonding in metal poly-olefin complexes are similar to those in olefin complexes. Linear, branched and cyclic polyolefin complexes are known. The bonding is delocalized over the carbon atoms and the metal(s). These species also play important roles in organic syntheses. 

Some time later, Rob Guy, whose chemical skills were much respected by the research team and who was also envied for his ability to charm ladies, managed to synthesise a range of acetylene complexes of platinum. Their infrared spectra indicated that bonding of the hydrocarbon to the metal was very similar to that in olefin complexes. Interestingly, it was observed that a-hydroxyacetylenes are chelated to platinum through interaction of oxygen lone-pair electrons with a vacant 6p orbital of the metal. 

According to an interview, Chatt had become interested in olefin complexes long before joining ICI in 1947 and had been fascinated by the suggestion by Anna Gel man that the metal-olefin bond involved four electrons, including two d electrons on platinum as well as the 7r-electrons of the olefin. Gel man considered unsaturated molecules. ..as acceptors and donors at the same time, though in the model the metal was formulated as platinum(iv). 

The Zr—C bond distances in olefin complexes have in general been computed to be asymmetric. The Zr—C bond distances in the one observed olefin complex (ref 119) are also inequivalent. If one assumes that the closer C is bonded to Zr and the one further away not bonded, then a Zr—C nonbond distance can be estimated by averaging the longer of the two Zr—C ethylene bond distances for the eight zirconium ethylene structures of Table 1 and the six from Table 8 this yields a Zr—C nonbond distance of 2.86 A. Eor reference the longer of the two olefin bond... 

In olefin complexes bearing a positive charge, the shift to high field is attenuated, and it is often possible to observe a small net downfield shift from the position of the corresponding protons in the free olefin. These effects become rapidly attenuated as distance of proton to metal increases. Finally, the spectrum of an olefin will also show changes in proton-proton coupling constants when it has become complexed to a metal. 

The 7t-(C5)M system is also found in olefin complexes (XXVI) containing the TT-cyclohexadienyl (CgHy) radical. This series would be expected to give... 

Figure 17.1 illustrates possible variants of the interaction of the orbitals of the central atom with the substrate in olefin complexes to form o- and n-bonds. Above we present possible methods for complex formation of carbon dioxide, carbene, and carbonate. 

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