Accelerators substitutions

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

The dissociation rate constant is now composite, = k ik 2/k2- Following the first bond rupture (k 2) the competition between further bond rupture (/ , ) and reformation (Atj) which may lead to a small k j/k2 is the basic reason for the high kinetic stability of the chelate. The problem of complete dissociation is intensified when complexes of ligands of higher dentate character are examined. The situation is altered when the successively released donor atom(s) can be prevented from reattachment (see subject of accelerated substitution). 

This stabilization of what would otherwise be coordinatively unsaturated intermediates can accelerate substitution reactions. In addition, species that appear from their stoichiometry to be coordinatively unsaturated interme-... 

In the framework of the mechanism of capillary forces, the rise of a hquid cannot exceed a limiting value //max = cos djjpgr, where p is the density of the liquid and g is the gravity acceleration. Substituting the above calculated values of wetting tension q> into this formula gives //max = 2 x 10 mm (for curve 1) and 3 x 10 mm (for curve 2). However, the meniscus rises up to several centimeters and continues to move further (Fig. 20). 

This stabilization of what would otherwise be coordinatively unsaturated intermediates can accelerate substitution reactions. In addition, species that appear from their stoichiometry to be coordinatively unsaturated imeimediates may not in fact be what they seem. For example, on heating Mo(N2)2(PMc2Ph)4, N2 is lost and Mo(PMe2Ph)4 (4.19) is formed ... 

DOTG is decreasing in use because of toxicity issues. Other guanidine accelerators may substitute, but they too have toxicity issues. Other accelerator substitutes will impart different cured physical properties that must be evaluated. 

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