Nucleophile independent dissociation

Also, the nucleophilicity of cyanide overpowers the nucleophile independent dissociation and intramolecular racemization reactions, which had been well studied many years ago (11-16). 

The replacement of X by pyridine in the complex PtX(dien)+ to give Pt(dien)py2+ (equation 549) has been studied under controlled conditions with a range of leaving groups X. These data are shown in Table is.1 87-1988 From these data the leaving group order is NOj > H20 > Cl- > Br > I- > N3 > SCN- > NO2 > CN-. Reactions such as these must be carried out under thermal conditions for accurate comparison since photoaquation can occur, albeit with a rather low quantum yield.1989 The volumes of activation of these reactions (equation 550) are all negative. An associative mechanism is proposed for the nucleophilic dependent path, but for the nucleophile independent pathway both associative and dissociative mechanisms need to be considered.1990... 

Reaction (31) follows a two-term rate law, equation (32), with the ki term nucleophile independent and presumably due to CO dissociation. The dependence of k2 on the nucleophile is large, the relative reactivities... 

In a classic study in 1940, Crossley and coworkers demonstrated that the rates of nucleophilic substitution of the diazonio group of the arenediazonium ion in acidic aqueous solution were independent of the nucleophile concentration, and that these rates were identical with the rate of hydrolysis. Since that time it has therefore been accepted without question that these reactions proceed by a DN + AN mechanism, i.e., that they consist of a rate-determining irreversible dissociation of the diazonium ion into an aryl cation and nitrogen followed by rapid reactions of the cation with water or other nucleophiles present in solution (Scheme 8-6). 

This CO insertion occurs under the influence of nucleophiles other than carbon monoxide, e.g., triphenylphosphine. The independence of the rate on the concentration of ligand suggests (52) a rate-controlling dissociation of the octahedral complex assisted by a nucleophilic (ether) solvent. 

The catalysis of CO2 hydration by carbonic anhydrase II occurs via the two chemically independent steps outlined in Scheme 2 a general mechanistic profile is found in Fig. 23. The first step involves the association of substrate with enzyme and the chemical conversion of substrate into product. The second step is product dissociation and the regeneration of the catalytically active nucleophile zinc hydroxide (Coleman, 1967). Below, we address the structural aspects of zinc coordination in each of these steps. 

At first glance the k, term, first ol der with respect to complex and independent of Y, would suggest a dissociative pathway. Strong evidence, however, supports the view that this pathway also is associative. It must be recognized tbat, in general, solvent (S) molecules will be nucleophiles and will therefore compete with Y for ML2TX to form ML2TS (sometimes called the solvento complex). Thus the two-term rate law could be written as ... 

While some specific role of the K+ cation may account for the increased reactivity of benzothiophene with increasing amounts of KOH in the hydroxide mixture, the possible role of the total base, KOH, cannot be neglected. Potassium hydroxide is a stronger base and nucleophile in this system than NaOH is (10), and the increased basicity and nucleophilicity could account for increased reactivity toward benzothiophene decomposition. The chemical nature of ionic melts is not fully understood. While the hydroxide melts are believed to be fully dissociated (H), explanations have also been given for the formation of "quasicrystalline states (12), where order within the melt exists and dissociation is not complete. It is difficult to deduce how much independent freedom K+ and 0H have and if either the K+ cation or KOH or both are the important species. 

One useful approach to examining the dynamics of reactive bimolecular collisions involves analysis of the unimolecular dissociation of species that correspond to a reaction intermediate. This criterion was applied to the S 2 reaction in three independent investigations which made use of different experimental techniques and conditions to study the decomposition to products of specific ion-dipole complexes, the presumed intermediates of these nucleophilic displacement reactions239-241. 

AsPh3) occurs by a primarily hgand-independent mechanism, probably CO dissociation. For more nucleophilic entering ligands (PBu3, CN-t-Bu), the hgand-dependent path still predominates. The CO dissociation rates from Ir4(CO)uL show the same trends with changes in L as mononuclear complexes and metal-carbonyl dimers. 

In this way it was possible to determine the pH-dependence of individual rate constants over a range of three pH-units. It was found that k[ is proportional to hydroxide ion concentration (Aii = A i[OH ]), that k i is practically pH-independent (ilj = and that the constant 2 depends on pH in the shape of a part of a dissociation curve, indicating that a rapidly established acid-base equilibrium precedes the rate-determining step. These differences in pH-dependence explain why at [0H ] < 0 3m, k[ (the value of which decreases Avith decreasing pH) can be neglected compared with (pH-independent) k i and why at [OH ] > 0 3m, is small compared with the rapidly increasing k. The rate of nucleophilic attack (with constant k[) decreases with increasing ethanol concentration. 

For all investigated reactions, the photosubstitution quantum yield decreased significantly with increasing pressure. Under the assumption that nonradiative deactivation is relatively independent of pressure, the pressure dependence of /(l — < ) represents that of the photochemical reaction [Eq. (27)]. The positive volumes of activation fit well into the picture of a dissociative mechanism, that is, release of CO. This model cannot account for the observed trends in AF ( /(1 — < )) especially as a function of solvent. For this reason, a second way to account for the observed data was presented [100] according to which CO dissociation leads to a trigonal bipyramidal M(CO)5 fragment with dissociated CO within the solvent cage. The latter species can either recombine with CO, be trapped by solvent, or bind to the nucleophile L, which results in a competition between these reaction paths. The difference in the pressure dependence for the recombination with CO or combination with L can be used to account for the observed activation volumes. 

---