Associative ligand substitution Mechanism

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

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). ... 

For a vibronically relaxed bound ES, ligand substitution mechanisms can be discussed in terms of models developed for analogous thermal reactions [36. The limiting mechanisms would be the dissociative (D) and associative (A) pathways, where the rate-limiting steps are, respectively, dissociation of the M-X bond or formation of the M-Y bond to form distinct intermediates (Eqs 6.16 and 6.17). The electronic nature of such intermediates is ambiguous, since these species may also be electronic excited states. For example, the cis to trans isomerization concomitant with the photoaquation of Cl from the Rh(lII) complex cis-Rh(NH3)4Cl2 was successfully explained by a model where Cl dissociation gave a pentacoordinate intermediate in a triplet LF excited state [37, 38]. 

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

The transmetallation of oxidative addition products of alkenyl halides and zerovalent palladium species by organotin reagents is an important step in Stille type reactions and its mechanism depends on many variables. A recent review on this topic accounts for transmetallation of alkenylpalladium(ii) species.The involvement of 14-electron T-shaped species [PdX(R)(L)] in Stille reactions involving alkenyl and other substrates has been refuted as unrealistic. Instead, associative ligand substitution at palladium is advocated, which has been corroborated by full and quantitative evaluation of kinetic data as well as MO considerations. ... 

A further interesting point comes from the comparison of the in vivo data with the other ligand substitution results in general. The in vivo half-lives listed are comparable with the rate data of the equatorial substitution and might suggest that the mechanisms responsible for the uptake/clearance of the radiopharmaceuticals, be they protein, peptide, or DNA interactions or innersphere redox reactions, might indeed be associated therewith. This is, however, just an observation and surely requires much more research to be well understood. 

Kinetic and mechanistic studies of nucleophilic substitution at metal(IV) centers are fairly rare (263). Platinum(IV) has the substitution-inert low-spin d configuration, and presumably undergoes nucleophilic substitution by an associative mechanism thanks to its high charge and large size. However there are actually very few data, probably thanks to the tendency for platinum(IV) to oxidize ligands. Substitution kinetics at metal(IV) centers may be more conveniently studied for complexes of the type ML2X2, where M — e.g., Sn, Ti, V, or... 

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... 

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

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... 

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