Mechanisms of ligand substitution reactions

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

There are other reactions of transition metal complexes which are relevant to our observations on the ac electrolysis. Recently, new mechanisms of ligand substitution reactions have been reported which are characterized by electron transfer reactions as key steps although the overall reactions are not redox processes, e.g.,... 

Substitution of several metal-carbonyl complexes Cr(CO)6 and Mn(CO)5 (amine) show a small dependence on the nature and concentration of the entering hgand. Under pseudo-first-order conditions, the rate laws for these substitutions have two terms, as shown for Cr(CO)6 (as for some substitution reactions with 16e complexes, see equation 5). The second-order term was always much smaller than the first-order term. A mechanism that ascribes the second-order term to dissociative interchange (U) has been suggested for the Mo(CO)5Am system (Am = amine) and involves a solvent-encased substrate and a species occupying a favorable site for exchange. Thus, the body of evidence for the simple metal carbonyls indicates that CO dissociation and is the mechanism of ligand substitution reactions. 

The mechanism of ligand substitution reactions in the carbyne complexes /rar7.v-M(CR)X(CO)4 (M = Cr, W R = Me, Ph, NEtj X = Cl, Br, 1, SePh) was investigated by H, Fischer and co-workers (JOO). The influence of the metal center, the trans ligand, and the carbyne substituent on the M—CO dissociation step was determined. The reactions with PPhj in 1,1,2-trichloroethane [Eq. (62)] all follow first-order kinetics, with activa-... 

Kinetics and Mechanisms of Ligand Substitution Reactions of Platinum(II) Complexes... 

Figure 1 General Sf 2 mechanisms of ligand substitution reactions of square-planar metal complexes, such as platinum n) compounds, where S is solvent and Y is entering nucleophile...

As in the case of solvent exchange reactions, the rate and mechanism of ligand substitution reactions can be systematically tuned through manipulation of steric and electronic effects. The introduction of a metal-carbon bond on an inert metal... 

The remainder of this chapter presents the established mechanisms of ligand substitution reactions of complexes with 16,17, and 18 valence electrons. These are the first mechanistic data presented in this text. Keep in mind that a mechanism is a theory deduced from the available experimental data. - Logically, a mechanism can only be proven to be invalid and is, therefore, impossible to "prove" to be true the accepted mechanism explains the available data and allows additional predictions that can be tested experimentally. Modifications and refinements of the mechanisms presented in this chapter and in the rest of the text are likely to be made in the future. 

The kinetics and mechanisms of ligand-substitution reactions involving labile metal ions in aqueous solution are now well understood (1). It is therefore of interest to investigate how such reactions are affected when the reaction medium is changed either by addition of charged micelles to an aqueous solution or by dispersing the reactants in an inverse-micellar (water-in-oil microemulsion) system. 

Mechanisms of ligand substitution reactions general considerations 325... 

Campisi, A. and Tregloan, P.A. (1985) Kinetics and equilibria of Ga(III)-thiocyanate complex formation. Mechanism of ligand substitution reactions of Ga(III) in aqueous solution. Inorg. Chim. Acta, 100, 251 -259. Carriere, E. and Faure, P. (1943) Produit de solubilite I acide aluminique et hydrolyse... 

Kersey FR, Yount WC, Craig SL. Single-molecule force spectroscopy of bimolecular reactions system homology in the mechanical activation of ligand substitution reactions. J Am Chem Soc 2006 128 3886-3887. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

The possibility of ligand substitution reactions in pentacoordinate silicates Sil I31 2 and Sil FfF via hexacoordinate intermediates was studied by Fujimoto, Arita and Tamao73. Attack on each of these silicates by F or hydride produced qualitatively similar reaction pathways, leading to stable hexacoordinate intermediates, without significant breaking of the bond between silicon and the leaving group. It was concluded that a nonconcerted displacement mechanism via a hexacoordinate intermediate is likely. 

These reactions are important in that the 17-electron radicals produced may undergo ligand substitution or additional reactions and then recombine. The rates of radical recombination are generally fast, and approach diffusion-controlled hmits. This is one of the mechanisms of ligand substitution of metal metal bonded complexes that is discussed in Section 7.3.1. 

The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R", adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO)3 cationic intermediate (XVI) loss of a proton from this intermediate generates the substituted cyclobutadiene -Fe(CO)3 complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation (XVII), as well... 

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