Reactivity associative substitution mechanism

At the other extreme is the associatively (a) activated associative (A) mechanism, in which the rate-determining step for substitution by 1/ proceeds through a reactive intermediate of increased coordination number, [M(H20) L](m x,+, which has normal vibrational modes and survives several molecular collisions before losing H20 to form [M(H20) 1L](m t,+, as shown in Eq. (8). Equation (9) indicates the linear variation with excess [I/-] anticipated for obs, which is similar in form to that of Eq. (5) when if0[I/ ] 1 and kohs + k. ... 

Return now to the questions surrounding the actual sequence of events leading to substitution following population of the reactive state. As in thermal substitution mechanisms it is appropriate to determine whether a dissociative or an associative mechanism obtains. Certainly, this point is the one most often clarified, but other aspects also deserve some scrutiny. These include the possibility of acid-base equilibria in the excited state, isomerization of potentially ambidentate ligands, the extent to which the extruded ligand is electronically or vibrationally excited, the degree of molecular distortion upon population of the reactive state and the possibility of competing chemical processes which may be influenced by the environment or by structural modifications of the molecule. 

In other complexes the introduction of a C atom as a strong u-donor ligand, increases the reactivity of Pt(II) by six orders of magnitude compared to [Pt(H20)4] , but does not induce a changeover in mechanism. Surprisingly, there is only one example in the literature where the authors report activation volumes in support of a changeover in the substitution mechanism, i.e. from the usual associative to the unusual dissociative mechanism. The authors point out that at least two strong... 

The use of 77-acceptors can also achieve a higher rate of substitution. In our group we were able to compare the substitution behavior of the Pt(II) aqua complexes of ethylenediamine and 1,10-phenanthro-line, and although the reactivity of the phenanthroline complex is —102 higher than the ethylenediamine complex, the activation parameters strongly indicate that an associative mechanism is operative (65). 

Only limited data are available for substitution in this state. An associative mechanism is favored and Au(I) appears to be less reactive than Ag(I) or Hg(II). ... 

Solvolysis studies of meta- and para-substituted phenyl phosphates (240) in anhydrous Bu OH and in Am OH have revealed that generally reactions of dianions are much faster in alcohols than in water. For example, the dianion of p-nitrophenyl phosphate (240 X = 4-NO2) reacts 7500- and 8750-fold faster in Bu OH and Am OH, respectively, than in water." The results of a theoretical study of the reactivity of phosphate monoester anions in aqueous solution do not support the generally accepted view that Brpnsted coefficients fhg = —1.23 and jSnuc = 0.13 determined more than 30 years ago for the uncatalysed reaction of water and a monophosphate dianion (241) represent conclusive evidence for the dissociative mechanism. It is suggested that, instead, the observed LFERs could correspond to a late transition state in the associative mechanism." ... 

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