Ruthenium complexes proposed dissociative

Kinetics of aquation of [Ru(LLLL)X2], with LLLL = cyclam, 2,3,2-tet, en2, or (NH3)4, and X = Cl or Br, have been followed by cyclic voltammetry. Rate constants, and activation parameters (A// and A5 ) have been evaluated, and compared with kinetic parameters for reactions of analogous compounds of ruthenium(III) and cobalt(III). Similar trends obtain for all three sets of complexes. There is retention of stereochemistry, rates decrease as the extent of chelation in LLLL increases, and trans complexes are less labile than cis analogs. Reactivities are determined by solvation of the initial and transition states, by nephelauxetic effects, and by a-trans effects. A limiting dissociative (D) mechanism is proposed for the ruthenium complexes, with square-pyramidal geometry for the transient intermediate [cf. rhodium(III) photochemistry below. Section 5.8.10]. Differences in isomer lability have also been described for... 

This value is much larger than that observed in the previously reported deracemizations of [Cr(ox)3]3 and Cr(acac)3. In this reaction, the basic condition is necessary, and the addition of Hacac increases the enantiomer excess, for which the reason will be discussed below. The reaction mechanism shown in Scheme 17 was proposed. In the mechanism, the 3MLCT excited A- [Ru(( — )-men-bpy)3]2+ is oxidatively quenched by Co(acac)3 to form an exciplex with Co(acac)3 followed by electron transfer to Co(acac)3 from A- [Ru(( — )-menbpy)3]2 +, which leads to the formation of a successor complex, [A-Ruin(( — )-menbpy)33 + Con(acac)3 ]. This successor complex dissociates to A-[Rum(( — )-menbpy)3]3+, Co(acac)2, and acac. If the reducing reagent is absent or the reducing reagent does not effectively reduce the ruthenium(III) complex, Co(acac)2 reduces A-[Rum(( — )-menbpy)3]3+ to A-[Run(( — )-menbpy)3]2+ concomitantly with the formation of Co(acac)3. As discussed in Sec. II.A., the photoreduction of Co-(acac)3 occurs stereoselectively. In addition, the oxidation of Co(acac)2 to Co-(acac)3 occurs stereoselectively, because Co(acac)2 reacts with the chiral ruthen-... 

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). 

Group V Donors. The force constant of the Rh—N bond in [Rh(NH3)g] has been determined as 210 Nm" and is thus higher than for the ruthenium analogue. Electronic spectra of rhodium pyrazine complexes, e.g. [Rh(NH3)5(pyr)], have been recorded N-protonation causes a red shift in the n- n pyrazine transition. The use of n.m.r. to differentiate cis- and trans-isomers of [Rh(en)2X2]Y (X = Cl or Br) has been discussed. At room temperature, spectra are, to a greater or less extent, exchange-simplified and thus unreliable. Equilibrium (7) and isomerization of both species was monitored. A dissociative mechanism was proposed for the isomerizations. 

Mass spectrometry (MS) studies have played a key role in the study of metathesis reactions, particularly in the hands of Chen and coworkers, who have identified intermediates in the catalytic cycle,and probed the energetics of their reactions, using electrospray MS techniques. Species such as 14e ruthenium carbene complexes can be detected by MS in the presence of different alkene substrates, the different carbene products (from CM or ROMP, for example) can be detected. Further, the fragments into which any proposed species can be broken by successively higher lens potentials can be used to check the species structure. In successive and more advanced studies, interpretation of data from the energy-resolved, coUision-induced dissociation cross-section measurements allowed the construction of potential energy surfaces for some steps of the metathesis reaction.Metathesis precatalysts were typically custom-made species, modified with ionic tags, to facilitate detection by MS. 

The complexes frans-[Ru(NH3)4 P(OR)3 2] and trans-[Ru(NH3)4 P(OEt)3 P(OR)3 ], with R = methyl, isopropyl, or butyl, aquate to give a monophosphite product with rate constants which vary little with the nature of the complex or solvent composition (up to 80% ethanol). The results support the dissociative mechanism previously proposed for the triethyl phosphite complex. Trans effects are here dominated by 7T acceptor properties of these phosphite ligands. Trans effects in substitution in bis-ligand ruthenium (and iron) phthalocyanine complexes follow the order... 

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