Associative mechanism for ligand

Associative mechanisms for ligand exchange in tetrahedral complexes of transition metals have been reported in several cases. Studies of tetrahedrally distorted copper and nickel complexes have already been mentioned above (see Chapter 2, refs. 131 and 133). Activation parameters for the solvent exchange in the tetrahedral nickel(ii) solvento complex in hexamethylphosphoramide, determined using n.m.r. spectroscopy, are compatible with an associative... 

Heteroatom ligands of A3-iodanes are readily displaced with external nucleophiles. Detailed mechanism for ligand exchange on iodine(III) is not known, but two mechanistic pathways, associative and dissociative, are considered for the process [9]. There are many evidences supporting the associative mecha-... 

VanQuickenbome and Ceulemans have advanced a model, based on the angular overlap model variant of ligand field theory, to rationalize the observations of the preferentially labilized ligand [68], This model was focussed mainly on T2 as the reactive state. Although not a necessary feature of their treatment, they concluded that a dissociative mechanism was operative. Kirk suggested an edge displacement mechanism, which is associative [5], but he also considered a dissociative mechanism [12], An associative mechanism for the prompt T2 reaction has been inferred on the basis of symmetry arguments [6]. 

Two theoretical models for allosteric effects have been proposed to explain the mechanism for ligand-protein cooperative interactions the concerted (or symmetry) model of Monod, Wyman, and Changeux and the sequentially induced-fit model of Koshland. The nomenclature associated with allosterism and cooperativity originated from the concerted model. Both models assume that... 

Associative mechanism for substitution reactions in square planar complexes. Sites above or below the plane are inherently readily accessible to incoming ligands. 

The tra j -labilising effect of sulphite ligands is illustrated by the rates and activation parameters for aquation of the complexes [CoX(S03)(DMG)] and [CoX(S03H)(DMG)]. Variation of rates with composition of mixed solvents indicates an associative mechanism for these aquations. ... 

Figure 7.2 Dissociative and associative mechanisms for phosphine-olefin ligand exchange.

Reactions in Water.—The ultrasonic absorption technique has been applied to silver(i) thiosulphate in solutions of which the predominant species is the ion [Ag(S203)2] . Both of the relaxation effects observed over the 15—230 MHz frequency range were attributed to the stepwise mutual approach of [AgS203] and [SaOal - with the elimination of co-ordinated water. At 25 °C the rate constant for this association reaction is 1.0 x 10 1 mol s, which is close both to the value calculated on the basis of the process being essentially diffusion controlled and to the literature value for the reaction of a second molecule of phen with the mono-complex [Ag(phen)]+. The authors favour an associative mechanism for these reactions of Ag+, with the bound ligand having essentially no effect on kr. 

The ratios k k are observed to be 2.2 0.39 1. Thus, if At2 is taken to be normal , the rate constant for the reaction of ser with [Cu(en)] + is also consistent with normal substitution whereas the presence of the bulky histamine molecule leads to a relatively low rate constant. Furthermore, k- the rate constant for the dissociation of ser from [Cu(hm)(ser)]+ is also substantially lower than the rate constant for the dissociation of this ion from the other serinato-complexes (Ar i, A 2, At-ij), which all have the same value to within a factor of two. This rate reduction may indicate steric hindrance between the co-ordinated histamine molecule and an entering water molecule. In a dissociative mechanism for ligand substitution bulky non-leaving ligands appear to enhance the rate. The opposite effect shown here by histamine suggests that an associative mechanism holds for Cu substitution. Further evidence for this conclusion has been obtained from the copper(n)-ethylenediamine-histamine system. 

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