Platinum 2-olefin

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 infrared spectra of several of these complexes confirm that the olefin exists, with its double bond, in the complex (35, 177, 178), but that the double bond is weakened by complex formation, as shown by the decrease of some 140 cm-1 in the C C stretching frequency (35). The total platinum-olefin bond order is approximately 4/3 (35), and the C C bond order approximately 5/3 (126). 

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. 

Platinum olefin compounds have long been known, the most familiar being Zeise s salt, K(PtCl2C2H4)Cl. The fundamental compound of such... 

Another unusual behavior of NHC TM complex was recently reported by Nolan et al. and involves the insertion of an NHC into a platinum-olefin bond. In the course of preparation of new NHC-containing platinum complexes by reaction of equimolar amounts of [(l,5-hexadiene)PtCl2] and free NHC, the substitution product (286) in which one coordinated double bond was substituted by an NHC was isolated in good yield (Scheme 48). A by-product was also... 

In both Zeise s salt and the dimer, the frequency of the platinum-olefin stretching vibration is about 407 cm (232, 259, 261). This is lowered somewhat by deuteration of the olefin, by substitution of another olefin for ethylene, or by substitution of bromine for chlorine (Table II) (259, 261). The stretching force constant for this vibration... 

Some investigators have assigned additional infrared bands at 480-500 (232, 485) and 360-370 cm" (485) to platinum-olefin vibrations as well. Grogan and Nakamoto suggest (259, 261), however, that these are probably due either to overtones or combination bands or to metal-chlorine vibrations. 

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>. 

Stable, isolabk complexes of olefins with platinum, palladium, and some other Group YIII metals are known, and it is likely that such complexes are formed in the polymerization catalysts, at least transiently. In the platinum olefin case, the n-bonding has been treated theoretically (27) and has been assumed to consist of two factors 1. donation of the 7t-electrons from the olefin to an empty d-orbital (or hybrid orbital) of the metal, and 2. overlap of a filled d-orbital of the metal with the anti-bonding rc-orbitals of the olefin (Fig. 7). The fact that this bonding... 

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...

An MO treatment of the platinum-olefin p/ -bond was applied by Chatt and Dun-canson in 1953 131>. It is similar to Dewar s explanation of bonding in silver complexes but o-bond overlap is between a filled ligand 7r-orbital and an empty 5d6s6p2 hybrid orbital of the metal. Back co-ordination from the filled metal 5d6p hybrid orbital to the empty olefin jt orbital assists in restoring a more favourable charge distribution as demonstrated in E.II. A result of the mixing in of a 6p with the Sd orbital of the metal is that the C=C bond will be perpendicular to the co-ordination plane of the metal atom if overlaps are to be maximised. The orientation of ethylene in Zeise s salt and related compounds... 

The authors concluded that their NMR parameters suggested a con-tinum of bonding in platinum-olefin/acetylene complexes. The NMR parameters for this series of platinum-olefin/acetylene complexes are given in Table XLI. 

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