Platinum-olefin complexes

An attempt was also made to produce 0-iodo acyl iodides by the reaction of iodine, carbon monoxide and olefins in the presence of palladium or platinum chloride. This is, in effect, an attempt to make Dr. Tsuji s reaction catalytic rather than stoichiometric. No carbonyl insertion occurred at 1 atm. of carbon monoxide. However, it was found that iodination of the olefin was catalyzed by platinum olefin complexes and that an additional increase in catalytic activity accompanied the presence of carbon monoxide. There has been much speculation at this conference concerning the possibility of affecting catalytic activity by changing the ligands in the coordination sphere of the catalyst. This would appear to be such a case. 

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

Chatt and Duncanson extended this concept to platinum-olefin complexes such as Zeise s salt 29), In platinum-olefin complexes, the o--type bond results from a filled tt orbital of the olefin overlapped with a vacant 5d6s6p orbital of platinum, while the 7r-type bond results from a filled 5d6p orbital of platinum overlapped with the antibonding olefin tt orbital (V). 

Of particular interest from the point of view of valency theory are the binuclear platinum-olefin complexes (Chatt, 1951) ... 

Fig. 19. Proposed structures of some platinum-olefin complexes.

The intracomplex transformations on the silica surface result in the formation of product 111 of the hydros lylation, and the catalyst is displaced by another molecule of 1 olefin that is coordinated with platinum. Since the electronic spectra of the system containing the catalyst and olefin (Table 3) display the shift of the charge-transfer band from 249 to 235 nm, the formation of platinum - olefin complexes IV in the solution seems to be quite probable. 

Electronic spectra and band assignments for simple olefin transition metal complexes are relatively scarce absorption spectra (with band assignments) of Zeise s salt (K[Pt(C2H4)Cls] H2O have been reported 335> as well as the spectra of other platinum olefin complexes 132>. Details about the spectra of Ni(duroquinone) 2 and olefin-Ni-duroquinone are available 409>, and the absorption data for the tetraphenylcyclobutadiene complexes of PdCl2 and NiCl2 have been given by Fritz 184>. 

M. J. S. Dewar proposed a model (Figure 1) to describe the bonding of an olefin to silver(i) or copper(i). The model suggested that, in addition to cr-donation of olefin 7r-bonding electrons to the metal, d electrons on the metal would also interact with antibonding orbitals of 7r-symmetry on the olefin. No experimental evidence was provided to support this proposal nor, indeed, was explicit mention made of Zeise s salt or other platinum-olefin complexes. From a study of Chemical Abstracts 41 (1947)-75 (1971), it would appear that Dewar did not follow up his proposal with more detailed studies, possibly indicating that this was not a prime focus of his own interests in MO theory and its application to... 

Emeleus and Anderson discuss both the structure and bonding in platinum-olefin complexes citing explicitly Chatt and Duncanson s 1953 paper and... 

Figure 2 Calculated platinum olefin complexes 1—5 and definition of the pyramidalisation angle 6...

Some kinetic parameters for restricted rotation in rhodium-olefin complexes were reported some time ago. These values have recently been revised, and parameters for related rhodium compounds determined. Activation parameters have also been determined for some platinum-olefin complexes. Here intramolecularity of mechanism is proved by the persistence of Pt- H coupling and the lack of change in the n.m.r. spectra of the non-olefinic ligands with varying temperature. The roles of p and d orbitals both in fixing the preferred orientation of the olefin perpendicular to the co-ordination square around the platinum and in the rotational process, are discussed. Wide-line n.m.r. spectra indicate some... 

Many reactions of amines with palladium- and platinum-olefin complexes have been reported. Akermark showed that nucleophiles add to palladium-olefin complexes to generate aminoalkyl complexes, as shown by the example in Equation 11.26. In this case, reactions of a bis-olefin dichloropalladium complex with amines occurs by splitting of the chloro-bridged dimer by the first equivalent of amine to give a neutral olefin-ligated palladium-amine complex that undergoes attack of the coordinated alkene by a second equivalent of amine. The stereochemistry of the amination is cleanly trans. Akermark and Zetterberg isolated and characterized by C NMR spectroscopy the a-alkyl complexes formed by the amination of both cis- and frans-2-butene, and the stereochemistry of the product alkyl complexes results from external attack by amines, as shown in Scheme 11.5. 

Olefin-metal complexes are frequently labile, especially those of copper, silver and gold, and treatment of most olefin-metal complexes with ligands such as tertiary phosphines results in the displacement of the olefin. In the square-planar acetylacetonate complex (acac)Rh(C2H4)2 the ethylenes readily exchange with free ethylene - as is also found in the square-planar platinum-olefin complexes [17]. In these planar molecules exchange of olefins may involve olefin attack on the exposed metal atom, via a five-co-ordinated intermediate [65, 66]. 

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