Ligand substitution reactions associative mechanism

Poe provided solid evidence that 17-electron complexes undergo ligand substitution by associative mechanisms. He studied the reactions of Re(CO)j, which was generated by photolysis of RejfCO), in the presence of both CCl and PPh. The reaction of Re(CO)jWith CCl forms Re(CO)5Cl, and the reaction with PPhj forms Re(CO) PPh,... 

The determination of activation parameters, normally strongly associated with the nature of the transition state complex, may also be very useful in distinguishing plausible mechanisms in ligand substitution reactions. Dissociative mechanisms will be associated with a relatively high enthalpy change as well as to positive AS values. Associative processes will indeed show a not very high endothermic effect but a considerable decrease in entropy. 

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-... 

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... 

Figure 5-38. The prototypical ligand substitution reaction in octahedral complexes. In principle, the reaction could proceed by associative or dissociative mechanisms.

In addition to the type of electron transfer reaction, shown in Equations 6.142-6.145, there are examples where pure MLCT excited states induce ligand substitutions by associative or dissociative mechanisms. A well-established example of a MLCT excited state-mediated ligand labilization reaction is shown in Equation 6.149.136... 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

The neutral Ni l or Ni-R complexes undergo associative ligand substitution reactions (Scheme 5, see Associative Substitution Mechanisms of Reaction of Organometallic Complexes), and react with methylaluminoxane (MAO) to generate intermediates that polymerize ethylene to high MW poly(ethylene), and alkynes to cis, tran5 otW-poly(alkynes) (Scheme 6). ... 

E21. The two Pt(II) complexes are shown below. Both are square planar and thus the ligand substitution reactions would follow associative mechanism. The methyl-substituted complex presents a greater degree of steric... 

Numerous studies have shown that the ligand substitution reactions of 17e radicals, such as CpM(CO)3 (M - Cr, W), CpFe(CO)2 M(CO)s (M = Mn, Re), or V(CO)6 , proceed by an associative mechanism, but this study is the first evidence that a 19e species is an intermediate rather than a transition state in the ligand substitution reaction. 

The neutral Ni-Cl or Ni-R complexes undergo associative ligand substitution reactions (Scheme 5, see Associative Substitution, Mechanisms of Reaction of OrganometaUic... 

The transformation of M --C to (MC) in Eq. 2 may be further divided into individual elementary steps, MC, 1 —s-MC/, representing the successive replacements of the solvent molecules associated with the cation by the donor atoms of the ligand. As with ligand substitution reactions on transition metal ions, there are two limiting hypothetical mechanisms dissociative and associative. The first mechanism is a dissociative or pseudomonomolecular process with consecutive dissociation (Eq. 3) and recombination (Eq. 4) steps ... 

Demonstration of the nature of the intermediates is in general a very difficult experimental task, therefore most ligand substitution reactions are assumed to be interchange processes. Nonetheless, kinetic studies often permit us to establish the intimate reaction mechanism which assigns to the interchange pathways either an associative or a dissociative character. 

The 19-electron complexes also undergo very fast ligand-substitution reactions according to the same associative mechanism (second-order kinetic low), because they are in very fast pre-equilibrium (eventually intramolecular) with the 17-electron complexes. In the intramolecular case, the 17- and 19-electron forms can even sometimes be mesomer forms. 

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. 

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