Ethylene, complexes with platinum

Ethylene, complexes with platinum-(II), 6 210, 211, 214, 215 Ethylenebisbiguanide complexes with silver (III), 6 74 Ethylenebisbiguanidesilver(III) hydroxide, 6 78 Ethylenebisbiguanidesilver(III) nitrate, 6 78... 

Braunstein et al. recently reported an interesting reaction of a base-stabilized mononuclear silylene complex with platinum-ethylene complexes [Eq. (35)]. In this reaction, the Pt-bound ethylene ligand is displaced by the base-free silylene complex, but the products can be also regarded as a dinuclear complex where two metals are bridged by a silylene unit.67... 

Fig. 7.38 Effect of agostic P-H—M interactions on the phosphoms chemical shift in some platinum ethylene complexes with bridging phosphanide ligands...

In 1827 Zeise reported that ethylene reacted with platinum(II) chloride to form a salt K(C2H4)Pt CI3 2H2O, but it was not until after the elucidation of the structure of ferrocene in 1952 that attention was redirected to Zeise s salt, which was proven to be the first reported metal i-complex. 

As a final example of hydrogenation, a mixture of stannous chloride and chloroplatinic acid catalyzes the hydrogenation of ethylene almost quantitatively. It is interesting to note that the ease of hydrogenation parallels the ability of the olefin to complex with platinum. The higher olefins that do not readily complex with platinum also are found to be more difficult to hydrogenate. 

The preparation of the first phosphodiester complexes of platinum(II), in particular those containing the diphenylphosphato ligand, has been reported.139 Treatment of [PtCl2(en)] with Ag20 in DMF with two equivalents of diphenylphosphate gives [Pt(dpp)2(en)] ((26) dpp = diphenylphosphato, en = ethylene- 1,2-diamine), the first example of a bis(phosphodiester) complex of platinum(II). The complex is stable as a monomer, and it does not undergo dimerization or oligomerization reactions. 

Figure 7.35. Dinuclear palladium(II) and platinum(ll) complexes with phenylene- and ethylene-bridged bis(oxazolone) ligands.

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

Attempt to prepare Jt-complexes of triafulvenes and related methylene cyclopro-parenes285,427 428 directly by ligand exchange reaction with transition metal complexes resulted in metal insertion into the sigma bond, forming metallacyclic complexes. Thus reaction of the electron-poor triafulvene l,2-diphenyl-3-dicyanomethylenecyclopropene with (ethylene)bis(triphenylphosphine)platinum in refluxing benzene gave two crystalline products whose platinacyclobutene structure was confirmed by X-ray structure analysis (equation 364)429. 

Ethylene derivatives of organophosphines and organoarsines [75a] are used as cr, 71-chelating donors. Thus, 4-pentenediphenylphosphine reacts (3.61) with platinum halides in boiling chloroform, forming complexes of the type 623 ... 

Flynn and Hulburt, however, went further into a study of the hydrogenation of ethylene by hydrogen when a stream of these gases was passed through a solution of ethylene platinum chloride. They found that ethylene inhibits the formation of platinum metal. This inhibition seems to indicate that the first step of the reduction is a dissociation of the complex with ethylene as one of its products. The platinum formed as a result of the initial dissociation and reduction would catalyze the reduction of the complex. Ethylene, however, still inhibits the reaction markedly in the presence of platinum. 

Reactions of 250 with four electrophiles are recorded in Scheme 31. In general, the products are more stable than those from complexes with monodentate ligands. Reaction of 249 with CS2, 249 or 250 with alkynes, and 249-251 with ethylene gives products in which the C6H8 has been lost but its fate has not been determined attempts to trap free cyclohexyne failed.93 Loss of the organic ligand appears to occur more readily in nickel complexes than in those with platinum. 

In the structure of Zeise s salt, the ethylene occupies the fourth coordination site of the square planar complex with the C C axis perpendicular to the platinum-ligand plane. In this compound, the dsp2 hybridised s orbital of Pt overlaps with / -bonding... 

Di-jj-chloro-dichlorobis(ethylene)diplatinum(II) is customarily prepared by evaporating an aqueous solution of H[PtCl3(C2H4)] to dryness and then recrystallizing the dimer.1 However, this procedure has not been applied as a general synthesis of other related olefin complexes. The dimer has also been prepared, but with less success, by the reaction of Na2PtCl6 with boiling ethanol.2 The direct reaction of ethylene with platinum(IV) chloride also provides the dimer,3 but yield data are not available and presumably the method is unsatisfactory. 

---