Nucleophile independent dissociation reactions

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... 

It was observed that for L = NCS, NOs , Br, and C1 the rate of reaction is the same and is independent of the concentration of L. Furthermore, this rate of reaction is identical with the rate of loss of optical activity of crs-d-[Co(en)2Cl2]+ at the same experimental conditions. These results can be explained on the basis of a dissociation process yielding a symmetrical active intermediate [Co(en)2Cl] +. This means that the formation of the intermediate is the rate-determining step for these reactions, a supposition in accord with the observation that all rates are identical. However the same reaction for L = CHsO, N02, N3-, and C2H302 was found to be much faster, to differ for each of these reagents, and to depend on their concentrations. A rather obvious explanation is that, compared with the previous group of L s, these reagents are better nucleophiles and the reactions proceed by an Sn2 mechanism. 

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). 

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. 

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. 

In the dissociative model (Fig. 4.2) a phosphorus-oxygen bond is broken, yielding an alcohol or alkoxide (depending on pH) and a metaphosphate ion (PO ). The metaphosphate ion is unstable and highly susceptible to nucleophilic attack. The rate of reaction is dependent on the proton dissociation constant (pKJ of the leaving group, but independent of the basicity of the nucleophile (Wijesekera, 1992). 

Since the rates of the replacement reactions studied are independent of the nucleophilic ligands they must proceed via a rate-determining step to a reactive intermediate. This may be the five-coordinate [Rhen Xp dissociative mechanism) or the six-coordinate [Rhen XH O] (Sj 2 solvolytic mechanism). Since the reactions are reversible, they can be written as in 2 where k +Y... 

The reactions of Ni(C0)4 with phosphorus donors are first order in Ni(C0)4 and essentially independent of the nature and concentration of the attacking nucleophile. Coordinating solvents such as THF, which can interact with and stabilize the 16 electron Ni(C0)3 intermediate, accelerate the first step. The reactions are retarded by increase in pressure the volume of activation for attack of triethylphosphite is positive, +8cm mor at 0°C. All these observations point to a dissociative mechanism. 

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