Stoichiometric mechanism

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

Note that the three transition states, a, b and c, in Fig. 3 all contain both the 

Here MX, Y designates an outer sphere or second sphere complex. There is every reason to suppose that formation and dissociation of MX, Y occurs at rates approaching the diffusional-control limit so that the slow conversion to MY is a negligible perturbation on the equilibrium of the first step. There is a similarity here the Langmuir, the Michaelis-Menten and the Lindemann-Hinshelwood schemes. 

Two common limiting forms of the rate law for mechanism (1) are encountered experimentally. In the event that the equilibrium constant, K, for outer sphere complexation is small in relation to the concentration of MX and Y, the rate law 

Recall that three transition states m-ght be cons dered as falling within the pattern (1). Transition state a of Fig. 3 involves strong binding of both X and Y. In this case, it is quite possible that an intermediate of increased coordination number is formed during the reaction. Since the initial attack of Y determines the stereochemical course of any reaction obeying the rate law (4) there is no 

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The hypothesis of dissociative activation in Co(III) reactions stands the available tests well. It is therefore profitable to attempt to distinguish the D from the /j pathways. Fig. 7 summarizes the two pathways consistent with d activation, and the general methods for establishing the stoichiometric mechanism 1 are illustrated by the example of Co(NH3)50H2 ". ... 

Scheme 3. Stoichiometric mechanism of demetalation of robust FeIII-TAMLs such as lm by picolinic acid (L) in the presence of other pyridine bases (P). Axial aqua ligands are omitted for clarity.

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... 

In discussing the molecularity of substitution reactions the Langford—Gray nomenclature will be used throughout.6 This defines a stoichiometric mechanism, which distinguishes processes... 

The need for the presence of at least one amine proton in the substrate and a wealth of experimental evidence support a mechanism in which the lyate ion (or even the solvent itself) acts as a base to remove a proton from a suitably placed amine group, thereby generating a substitutionally labile amido species. There is strong evidence to support the idea that the mode of activation of this species (at least in the case of the Co111 species) is dissociative, but there is still disagreement as to whether the stoichiometric mechanism is dissociative (Z)) or interchange (7d). The distinction between these mechanisms rests upon the presence or absence of an identifiable five-coordinate intermediate that has lost all memory of its origins. 

The above parameters define the stoichiometric mechanism of the reaction but have little bearing on its intimate character (137) thus, the participation of the weak acid, AH, rather than its conjugate base. A", in the rate-limiting step of the reaction does not necessarily imply that AH is ligated in the final product. 

Hence, barring the formation of EO(RSH) complex in a rate-determining step, the nucleophilic attack by sulfur at EO oxygen, or transfer of hydrogen atom from S—H, does not appear to be the preferred reaction pathway, Further, the reduction of Compound I by azide does not conform to the stoichiometric mechanism [Eq. (27)] (143,175). 

Disjunctive metal exchange involves dissociation of the initial complex and reaction of the incoming metal with the free (or protonated) ligand intermediate as compared with the direct attack of the incoming metal on the initial complex in the adjunctive pathway. We have introduced this terminology to denote these as stoichiometric mechanisms which may be distinguished by the dependence of... 

C. Zerner, Michael J. Scott and C. Russell Bowers. He then joined Michael B. Hall s research group at Texas A M University and is currently an Assistant Research Scientist. His research focuses on using theoretical and computational chemistry to research and answer questions in a variety of areas, including biological enzyme catalysis, catalytic and stoichiometric mechanisms of bond activation and functionalization of organic molecules by organometallic transition metal complexes, and the elucidation of structure and bonding of various compounds of interest. 

Secondary antioxidants work by preventing the formation of free radicals. Some of them will decompose hydroperoxides by a safe reaction before they get the chance to generate fiee radicals. Hydroperoxide decomposers fall into two categories some act by a catalytic mechanism. These include the sulfur-containing acids that are formed by the oxidation of thiodipropionate esters or metal dialkyldithiocarbamates. The last-mentioned, if they contain a transition metal, are also ultraviolet light absorbers. An alternative type of hydroperoxide decomposer acts by a stoichiometric mechanism, namely the phosphite esters. 

The mechanism of formation of radicals from saccharin/amine complexes by higher oxidation state metals is unclear, but perhaps is related to oxidation of tertiary amines by benzoyl peroxide. Scheme 2 describes a catalytic effect of the transition metal on free radical production when both saccharine/amine and hydroperoxide initiators are present, instead of an essentially stoichiometric mechanism if only one initiator were present. 

The stoichiometric mechanism can be determined from the kinetic behavior of one system. The classifications are as follows ... 

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