Dissociative substitution reactions kinetics

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

For each of these reactions kinetic data were obtained. The reactions were first order in complex concentration, and zero order in isocyanide, as expected. The complex Ni(CNBu )4, and presumably other Ni(CNR)4 complexes as well, undergo ligand dissociation in solution. In benzene solution, a molecular weight determination for this compound gives a low value (110). This is in accord with the presumed mechanism of substitution. 

Kinetic analysis of the substitution reactions indicate that they follow a dissociative mechanism. It has also been shown that two water molecules in [Cr(H20)5I]2+ undergo exchange with labeled water. It is interesting that one exchange is rapid and occurs before I- leaves. However, this is not true of the chloride compound. Therefore, it appears that the iodide ion labilizes the water trans to it, but the chloride does not. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

DR. MARGERUM We are now looking at those kinetics. There is a significant effect on the rates of formation and rates of dissociation of these complexes in the aquo replacement reactions, but perhaps not as great as for the polyamine substitution reactions. 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

DISSOCIATION KINETICS SUBSITE MAPPING SUBSTITUENT SUBSTITUTION REACTION Sn REACTIONS... 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

The above mechanistic interpretation is in contrast with the one appearing in the coordination chemistry of NO on the very labile Fe(III) porphyrins and hemoproteins, which show water substitution-controlled kinetics at the iron(III) center (22,25). The latter Fe(III) moieties are, however, high-spin systems, whilst the cyano-complexes are low-spin. There is strong experimental evidence to support the dissociative mechanism with the Fe(III)-porphyrins, because the rates are of the same order as the water-exchange reactions measured in these systems (22d). Besides, the Fe(III) centers are less oxidizing than [Fein(CN)5H20]2- (21,25). 

A useful method to probe whether the reaction mechanism involves an associative or dissociative pathway is to measure AV (the volume of activation) for the reaction. High pressure kinetics in methanol give AV 1 —12 cm3 mol-1 for an associative first step, and +7.7 cm3 mol"1 for the isomerization reaction. It is proposed that the faster reaction is a solvolytic replacement of Cl" followed by a dissociative isomerization step with [PtR(MeOH)(PEt3)2]+ (R = alkyl, aryl equation 210).580 Since isomerization and substitution reactions are mechanistically intertwined, it is useful to note here that for the rates of substitution of both cis- and frara,-PtBr(2,4,6-Me3C6H2)(PEt3)2 by I" and thiourea, the volumes of activation are negative, in support of associative processes.581 Further support for associative solvation as the first step in the isomerization of aryl platinum(II) complexes has been presented,582 and the arguments in favor summarized.583... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Kinetic studies of the substitution reaction of (CO)3M[ 3-C3(t-Bu)3] (M = Co, Rh, Ir) with P(OEt)3 provide the first examples of dissociative CO substitution for carbonyl complexes of the cobalt triad (equation 256)323. 

Operating within the framework of the Chauvin mechanism, the main consideration for the reaction mechanism is the order of events in terms of addition, loss and substitution of ligands around the ruthenium alkylidene centre. Additionally, there is a need for two pathways (see above), both being first order in diene, one with a first-order dependence on [Ru] and the other (which is inhibited by added Cy3P) with a half-order dependence on [Ru]. From the analysis of the reaction kinetics and the empirical rate equation thus derived, the sequence of elementary steps via two pathways was proposed, one non-dissociative (I) and the other dissociative (II), as shown in Scheme 12.20. The mechanism-derived rate equation is also shown in the scheme and it can thus be seen how the constants A and B relate to elementary forward rate constants and equilibria in the proposed mechanism. 

Outside Protonation. When metal-peptide complexes are placed in acidic solutions, the complexes dissociate. Metal ions which are sluggish in their substitution reactions, such as Ni(II) (43), Pd (II) (44), and Co (III) (45), add protons to the peptide oxygens prior to the metal-N (peptide) bond dissociation. The Cun(H 2GGhis)" complex is sufficiently sluggish in its reaction with acid to permit outside protonation to be observed kinetically (12). Protonation constants of 104 2 and... 

Very similar, though rather less extensive, studies have been made of decomposition reactions of MnRe(C0)jg (15), Tc2(C0)iq (17), and Re2(C0)jg (13). All the kinetics are consistent with the rather stringent predictions of rate eq 9 for the homolytic fission mechanism and are totally inconsistent with a simple C0-dissociative mechanism. Other workers have studied substitution reactions of Mn2(CO)10 (18) and MnRe(CO)10 (19) and have commented, correctly, that their kinetic results are consistent with the dissociative reaction. This seems to have led to some doubts about the correctness of our conclusion that the reactions of all the decacarbonyls proceed mainly or totally via homolytic fission. These doubts do not arise from the kinetics because the kinetics observed by these workers (18, 19) are equally consistent with the homolytic fission mechanism or, indeed, virtually any first-order activation of the complexes. The conditions under which the reactions were followed were simply not those capable of leading to a kinetic distinction between the various mechanisms proposed. The absence of any homonuclear products of the reactions of MnRe(C0)jg was offered (19) as evidence against the homolytic fission mechanism. This point had been raised before (15, 17) when it was countered by emphasizing the absence of sufficient data on... 

Kinetic studies at different phosphine concentrations indicated that the substitution reactions occur totally by a dissociative mechanism, while the ring expansion reaction is by an associative mechanism. The associative reaction could proceed by an 18-electron transition state involving either a bent NO or an rj to rj ring slippage mechanism. The bent NO mechanism seems more likely, because the ring slippage mechanism is known to result in the formation of oxocyclobutenyl product with ring expansion and because the isoelec-tronic cobalt complex does not react by a parallel associative pathway. 

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