Platinum alkenes

Figure 3.63 Platinum-alkene bonding in Zeise s salt. (Reproduced with permission from S.A. Cotton and F.A. Hart, The Heavy Transition Elements, Macmillan Press Ltd, 1975, p. 126.)...

Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

It has also been proposed (96) that multiple exchange occurs via formation of platinum-alkene complexes from the alkyl intermediate (XXXI) ... 

Even multiple exchange in methane could be explained this way. For methane, a carbene complex would be postulated, and such complexes are now known (87). One objection to the involvement of platinum-alkene complexes is that they are very stable and once formed might not be expected to react further, as was found when exchange in chloropro-pane was attempted (Section III,C) (55). Also the compound formed is a platinum(O) complex. A mathematical analysis of multiple exchange similar to that developed by Anderson and Kemball (/) has been carried out (82). 

Platinum alkene complexes have been known since 1830 when Zeise s salt was discovered. Alkene complexes of platinum(II) are kinetically more stable than their palladium counterparts, but this feature makes them less attractive for applications in homogeneous catalysis. A number of reviews have been written on alkene complexes.620-623... 

Under certain conditions, platinum alkene complexes can be converted to jr-allyl complexes by hydrogen loss (equation 281).860 Allyl alcohol insertion into a platinum(II) carbonyl bond gives an allyloxycarbonylplatinum(II) intermediate which can be decarboxylated to form the jj3-allyl complex (equation 282).861,863... 

The field of organometallic chemistry, concerned with compounds involving metal-carbon bonds, has also expanded enormously since the 1940s. Mention has already been made of Grignard reagents (Chapter 10) and the zinc alkyls discovered by Frankland (Chapter 8). Predating Frankland s work was the discovery in 1827 by William Christopher Zeise (1789-1847) of the salt which was for many years named after him and is now known as potassium trichloro-(ethylene)platinate(ll) monohydrate (Figure 12.13). Many other platinum-alkene... 

The final paragraph of a paper on the preparative and structural chemistry of amino-acid-alkene mixed complexes of platinum(n) reports some qualitative observations on racemization rates of complexes of formula [Pt(aIkene)(L-prolinate)a]. These rates correlate with platinum-olefin bond strengths. MO calculations of barriers to internal rotation in platinum-alkene and -alkyne complexes have been made. ... 

Five coordinate platinum-alkene-hydrido complexes of the type [Pt(2,9-dimethyl-l,10-phenanthroline)(X)(H)(cjs-Me02CCH=CHC02Me)] (X = Cl, Br, I) can, in some cases, be isolated from reaction of the protic acid with [Pt(2,9-dimethyl-l,10-phenanthroline)(cw-Me02CCH=CHC02Me)]. In other cases insertion of the alkene into the Pt-H bond leads to the formation of four coordinate hydrocarbyl derivatives 5. 

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