Nucleophilic attack on olefins co-ordinated to platinum. Part 4. Formation of neutral and cationic σ-adducts and of ring compounds: studies of stability

Abstract

Attack by amines (am) on alkene complexes, cis-[PtCl2(η2-C2H4)Y], can lead to neutral acyclic, cationic acyclic, and ring compounds, i.e. cis-[PtCl2(CH2CH2am)Y](Y = Me2SO or PPh3), [PtCl(am)-(CH2CH2am)Y]+Cl–(Y = NH3, pyridine, or Me2SO), and [[graphic omitted]–H)Y](Y = PPh3, Me2SO, or NHMe2). Two isomers of the second compound exist in solution. New compounds of these types are prepared and characterised and others identified in solution. Methods of distinguishing between the species are investigated. The ring compounds can be identified from the value of δ(NCHCHPt). Equilibrium studies show that in most instances the neutral acyclic compounds are readily converted to the cationic acyclic species by excess amine. This has the effect of making the starting cis-dichloro(alkene) complexes more susceptible to nucleophilic attack than their trans analogues. Ring compounds are more likely to be formed if am is bulky. Values of ΔH⊖ and ΔS⊖ have been measured for the formation of the cationic species for Y = 4-methylpyridine and am = NHMe2 for various alkenes. Some comments are made on the stability of cationic alkene complexes. A novel photochemical method is described experimentally for the preparation of some of the starting cis-dichloro(alkene) complexes from their trans isomers.