Dimer species

The inaccuracy seems not to prohibit study of the structural properties of associating fluids, at least at low values of the association energy. However, what is most important is that this difficulty results in the violation of the mass action law, see Refs. 62-64 for detailed discussion. To overcome the problem, one can apply thermodynamical correspondence between a dimerizing fluid and a mixture of free monomers of density p o = P/30 = Po/2 and dimer species [12]. The equation of state of the corresponding mixture... 

Rhodocene, [Rh()7 -C5H5)2], is also known but is unstable to oxidation and has a tendency to form dimeric species. Claims for the existence of iridocene probably refer to Ir " complexes. However, the yellow rhodicenium and iridice-nium cations are certainly known and are entirely analogous to the cobalticenium cation in their resistance to oxidation and susceptibility to nucleophilic attack. 

In addition dimeric species are formed, being in equilibrium with the monomeric RMgX. The Schlenk equilibrium is influenced by substrate structure, the nature of the solvent, concentration and temperature. 

A detailed investigation of the structure of amorphous PcRu by large-angle X-ray scattering (LAXS)269 showed that in the solid state dimeric species exist with a Ru-Ru distance in the magnitude of a double bond. Current experiments using the extended X-ray absorption fine structure (EXAFS) method confirm these results.279... 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

The postulation of the +4 oxidation state of cobalt is necessary to account for the retarding influence of Pb(II). The existence of a dimeric species of Co(II) acetate is required by the rate law and is confirmed by spectrophotometric and solubility measurements. The existence of ionic species of the reactants is inferred by the rate increase on addition of sodium acetate, an observation which cannot be attributed to a salt effect because sodium perchlorate produces a rate decrease. On this scheme an explanation of the effect of water on the stoichiometry is that the step... 

The composition of the solution depends on concentrations and pH value. In highly diluted solutions, mainly monomeric and dimeric species are present (Doesburg et ciL, 1999). 

Effect of dimer formation on deactivation. Another possible mode of deactivation is formation of inactive Co dimers or oligomers. To test for these species, we examined the ESI-mass spectram of fresh and deactivated Co-salen catalysts in dichloromethane solvent (22). The major peak in the mass spectram occurred at m/z of 603.5 for both Jacobsen s Co(II) and Co(III)-OAc salen catalysts, whereas much smaller peaks were observed in the m/z range of 1207 to 1251. The major feature at 603.5 corresponds to the parent peak of Jacobsen s Co(II) salen catalyst (formula weight = 603.76) and the minor peaks (1207 to 1251) are attributed to dimers in the solution or formed in the ESI-MS. The ESI-MS spectrum of the deactivated Co-salen catalyst, which was recovered after 12 h HKR reaction with epichlorohydrin, was similar to that of Co(II) and Co(III)-OAc salen. Evidently, only a small amount of dimer species was formed during the HKR reaction. However, the mass spectram of a fresh Co(III)-OAc salen catalyst diluted in dichloromethane for 24 h showed substantial formation of dimer. The activity and selectivity of HKR of epichlorohydrin with the dimerized catalyst recovered after 24 h exposure to dichloromethane were similar to those observed with a fresh Co-OAc salen catalyst. Therefore, we concluded that catalyst dimerization cannot account for the observed deactivation. 

The reaction occurs via the 1,2-adduct, which isomerizes to the 1,4-adduct,303 and there is an energy difference of about 5kcal/mol in favor of the 1,4-adduct. With the parent compound in THF, the isomerization reaction has been followed kinetically and appears to occur in two phases. The first part of the reaction occurs with a half-life of a few minutes, and the second with a half-life of about an hour. A possible explanation is the involvement of dimeric species, with the homodimer being more reactive than the heterodimer. 

Reaction conditions can be modified to accelerate the rate of lithiation when necessary. Addition of tertiary amines, especially TMEDA, facilitates lithiation53 by coordination at the lithium and promoting dissociation of aggregated structures. Kinetic and spectroscopic evidence indicates that in the presence of TMEDA lithiation of methoxybenzene involves the solvated dimeric species (BuLi)2(TMEDA)2.54 The reaction shows an isotope effect for the o-hydrogcn, establishing that proton abstraction is rate determining.55 It is likely that there is a precomplexation between the methoxybenzene and organometallic dimer. 

The mechanism by which the enantioselective oxidation occurs is generally similar to that for the vanadium-catalyzed oxidations. The allylic alcohol serves to coordinate the substrate to titanium. The tartrate esters are also coordinated at titanium, creating a chiral environment. The active catalyst is believed to be a dimeric species, and the mechanism involves rapid exchange of the allylic alcohol and /-butylhydroperoxide at the titanium ion. 

Fig. 15. Ball and stick representations of (a) [P4Mo6028(OH)3]9 (P4MoGOe), (b) [P4Mo6S3025(OH)3]9 (P4MogS303), (c) [P4Mo6S6022(0H)3]9- (P4Mo6S6) (d) their common polyhedral representation (e) a view of the dimeric species formed by an M2+ ion (M = Cr, Mn, Fe, Co, Ni, Zn, Cd) or a sodium cation sandwiched by two P4Mo6EG units.

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