Dissociative ligand Stereochemistry

The anion dissociates, and the coordinatively unsaturated metal center then picks up a monomer molecule for subsequent enchainment. This dissociative model has been favored in the past [16, 21-23, 27-28] since it allows a convenient explanation of the observed polymer stereochemistry by considering only the roles of the ligand and the alkyl chain in the cationic metallocene complex. However, anion dissociation opposes the electrostatic attraction between cation and anion and is therefore energetically expensive. So does it operate at all ... 

Only octahedral complexes have been discussed, mainly because much less is known about conformational effects in other configurations. In some tetrahedral complexes, for example, [Be(OCH2CH20)2]2 or [B(OCH2 CH20)2]-, the ligands may be sufficiently inert to dissociation to be examined with respect to their stereochemistry. Similar remarks apply to planar and other structures. 

The variable-temperature NMR spectra help to explain the catalytic properties of the dppp complex system which were outlined previously in Table I. The reduced catalytic activity compared with the tris(triphenylphosphine) complex system is apparently due to the reduced dissociation of the cyclic complexes. For example, the 90°C spectra of Figures 3 and 13, clearly show that the ligand-exchange rate is much slower in the case of dppp. However, temperature-dependent ligand exchange of the monocyclic complex occurs and leads to cis-bisphosphine species that catalyze the hydroformylation of olefins at minimal partial pressures of CO. The hydroformylation rate of the dppp system is faster at 1 atm CO pressure than that of the dppe system. Of course, such hydroformylations are nonselective due to the cis-stereochemistry. 

Of at least as great importance to the chemistry of PX3 compounds as the electronic factors are steric factors.6 Indeed these may be more important or even dominant in determining the stereochemistries and structures of compounds Steric factors also affect rates and equilibria of dissociation reactions and the stereochemistry of phosphine ligands is the prime factor in many highly selective catalytic reactions of phosphine complexes, such as hydroformylation and asymmetric hydrogenation. 

The transmetallation step (iii) is certainly the most enigmatic part of the catalytic cycle. Generally, it is assumed to be rate limiting, and several mechanisms are proposed depending on the solvent. An open transition state with inversion of the stereochemistry would arise with polar solvents which are able to stabilize the transient partial charges , whereas a cyclic transition state with retention of the stereochemistry would arise in less polar solvents. It should be noted that the nature of the ligands on the palladium may influence dramatically the kinetics of the transmetallation step. A 1000-fold rate enhancement was observed when replacing triphenylphosphine by tri(2-furyl)phosphine . However, the dissociative or associative nature of the substitution on the palladium is stiU under discussion . ... 

For example, the cis stereochemistry in reaction (i) is maintained in the product enedione. The reactions may involve concerted addition to the n system of the organic ligand. Further evidence for oxygen insertion by the homolytic dissociation of a... 

Additional studies demonstrated that replacement ofthe ligand ethylene by CO or PPh j did not influence the enantioselectivity of the reaction, suggesting that the ancillary ligand dissociates before the C H insertion. A bisimido ruthenium(VI) complex SO operating through an H abstraction/radical rebound mechanism, established in ruthenium porphyrin systems by Che, is proposed to account for the observed stereochemistry (Figure 12.2). 

There are a number of LF photosubstitution reactions that result in geometrical isomerization, for instance, cis-trans isomerization of Rh1" ammine complexes, that have been studied in great detail [120-122], These reactions involve ligand dissociation from the hexacoordinate ES prior to stereorearrangement. The product stereochemistries are principally interpreted in terms of the comparative energies of the square pyramidal,... 

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