CATIONIC COMPLEXES OF PLATINUM(II) WITH ETA(2)-OLEFINS - SYNTHESIS AND REACTIVITY OF THE ALKENE

New cationic olefin complexes of platinum(II), [Pt(eta2-olefin)X(tmen) ](ClO4) [olefin = ethene, 1; propene, 2; styrene, 3; (Z)-2-butene, 4; (E)-2-butene, 5. X = Cl, a; NO2, b. Tmen = N,N,N',N'-tetramethylethyle nediamine], have been prepared by olefin metathesis from 1. Substituti on of propene and styrene for ethene takes place with high yield of pr oduct, independent of the nature of X; in contrast, substitution of (E )- and (Z)-2-butene for ethene occurs only in the case of X = NO2. Com plexes 2b and 5b gave evidence of a metallacycle formation promoted by nucleophilic attack of one oxygen of the nitro ligand onto the cis ol efin, which shifts from eta2 to eta1 coordination. The olefin, in the cationic species, easily undergoes nucleophilic attack by OH-, MeO-, a nd acetylacetonate anions. In the case of unsymmetrically substituted olefins the addition reaction takes place with a high degree of regios electivity according to Markovnikov's rule. H-1 NMR data and, in parti cular, the J(Pt,H) values have proved to be very valuable tools for de termining the coordination mode of the carbon ligand and the configura tion of the complexes. Acid cleavage of the products of carbon nucleop hile addition was performed under different experimental conditions (g aseous HCl in chlorinated solvents or aqueous perchloric acid). The yi eld of Pt-C bond cleavage (and consequent alkylation of the carbon nuc leophile) was greater in the chloride than in the nitro species and in aprotic than in protic media.