Platinum complexes electrophilic cleavage

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

The kinetics of solvolysis of complexes cis- and // a 5-[Pt(N02)2L2] (L=ammonia, pyridine, or triethylphosphine) in acetic acid-sulphuric acid mixtures have been compared with those in aqueous sulphuric acid. The mechanism is loss of a nitro-sonium ion from the protonated complex. Whether this protonation takes place at the nitrito-ligand or at the metal is not known, but definitive evidence favouring the latter could not be obtained. Platinum-carbon bond cleavage in cw-[Pt(C6H4Y)2-(PEt3)2], studied in methanol and methanol-water mixtures, takes place by electrophilic attack by a proton,... 

The further reaction of these platinum complexes with an excess of acid to afford alkenes has received much less attention. If the alkyne bears the strongly electron-withdrawing trifluoromethyl group, the intermediate vinyl species is resistant to further electrophilic cleavage (Kemmitt et al., 1973). The diphenylacetylene complex, on the other hand, reacted with excess HCl to product trans-stilbene in almost quantitative yield (Tripathy and Round-hill, 1970). The corresponding 2-butyne complex gave a 1 4 mixture (the thermodynamic ratio) of cis and trans butenes under similar conditions. It is... 

Platinum-lead complexes undergo a cleavage of Pt—Pb bond with halogens and halogen acids506,507 510. These reactions are believed to occur through an electrophilic attack on Pt(II) leading to oxidative addition with the formation of a hexa-coordinated Pt(IV) complex. Reductive elimination of a plumbane results in the observed products (equation 193). 

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