13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.

Methylene polymerisation, which occurs during the heterogeneously catalysed hydrogenation of CO (Fischer-Tropsch synthesis) is analysed as an example of a transition metal-promoted C-C coupling reaction. A new catalytic cycle has been proposed, the alkenyl mechanism, which is based on simple stoichiometric organometallic reactions. Results from experiments using isotopically 13C2-labelled probe molecules indicate that surface vinyllalkenyl species are directly involved in the Fischer-Tropsch polymerisation over rhodium and ruthenium surfaces. The data indicate that C-C couplings are favoured if one of the participants has a metal to sp2 carbon bond, and that vinyl (or alkenyl) is a better combining ligand than phenyl, which in turn is better than methyl (alkyl). catalysis are well modelled by soluble organometallic metal complexes, where the C-C coupling reactions take place at one or two metal atoms. Model studies further suggest that no step of the Fischer-Tropsch process requires the participation of cluster arrays. occur on metal surfaces under heterogeneous catalysis conditions, those that take place on isolated and characterised metal complexes, and those that occur under ultra-clean conditions on single planes of metal crystals.

Treatment of the neutral trimethyl compounds Cp*MMe 3 (M ) Ti, Zr, Hf) with the highly electrophilic borane B(C 6 F 5 ) 3 in methylene chloride in the presence of various arenes results in methyl carbanion abstraction and coordination of the arene to form complexes of the type [Cp*MMe 2 (η 6 -arene)][MeB(C 6 F 5 ) 3 ] (M ) Ti, Zr, Hf; arene ) benzene, toluene, m-and p-xylene, anisole, styrene, mesitylene). Indications of relative metal-arene affinities have been gleaned from a variety of low-temperature 1 H NMR experiments, and it seems that both steric and electronic factors play significant roles in determining stabilities. The crystal and molecular structures of [Cp*HfMe 2 (η 6 -toluene)][MeB(C 6 F 5 ) 3 ] have been determined; as anticipated, there are no significant contacts between anion and cation, and the latter assumes a bent-metallocene type of structure.

A variety of monocyclopentadienyl and mono(pentamethylcyclopentadienyl) complexes of titanium, zirconium, and hafnium are assessed for abilities to initiate polymerization of ethylene, styrene, and, in part, α-methylstyrene. In general, little or no activity was found for either neutral species of the types CpMMe3 and CpMMe2OR or for cationic 12- and 14-electron species of the types [CpMR2L]+ and [CpMR2L2]+, respectively (Cp = η5-cyclopentadienyl; R = alkyl; L = amine, phosphine ligands). In contrast, much better olefin polymerization initiators result from abstraction of a methyl carbanion from Cp*MMe3 (Cp* = η5-pentamethylcyclopentadienyl) by B(C6F5)3, a reaction which gives cationic, 10-electron species of the type “[Cp*MMe2][BMe(C6F5)3]”. Of these, the complex [Cp*TiMe2][BMe(C6F5)3] (A) is an excellent initiator or initiator precursor for the polymerization of ethylene and styrene, resulting in high yields respectively of high molecular weight polyethylene and atactic (a-PS) and/or syndiotactic polystyrene (s-PS), depending on conditions; the tacticity of purified s-PS, as judged by 13C{1H} NMR spectroscopy, approaches 100%. While the polymerization of ethylene probably involves a classical Ziegler−Natta process, polymerization of styrene to s-PS and a-PS apparently involves respectively a Ziegler−Natta process and carbocationic initiation. High yields of essentially syndiotactic poly(α-methylstyrene) are obtained by utilizing the same initiator system, also apparently via a carbocationic process.

The utilization of metallocene compounds of titanium, zirconium, and hafnium as homogeneous Ziegler-Natta catalysts for olefin polymerization is well established,1 and it is recognized that useful catalytic properties generally require the presence of an alkyl ligand, a vacant site, and a positive charge. Thus 16electron complexes of the type [Cp,2M(Me)L]"1• (M = Ti, Zr, Hf;Cp' = substituted T?5-cyclopentadieny 1; Me = methyl; L = labile ligand) form the most extensively studied class of homogeneous catalysts for the polymerization of olefins. In contrast, although