Discrimination factor, nucleophilic

The constant s, characteristic of the substrate complex, reflects its sensitivity to variation in nucleophilicity as assessed by the Ptpy2Cl2 reaction. It is called the nucleophilic discrimination factor (ndf). The intercept log k turns out to be related to the value of the k term in the rate law for the solvent in question. Some representative ligands involved in attack on Pt(II) complexes may be listed in order of decreasing as follows ... 

On the basis of this equation, an index of nucleophilicity pt can be assigned to each nucleophile Y (see Table 4.13). It is found, moreover, that a plot against pt of logfcy, for reaction of Y with another Pt(II) neutral substrate, is also often linear. Thus, Eq. (2.168) applies, and 5 is termed the nucleophilic discrimination factor (Sec. 4.7.1). Some of the departures from linearity of plots of Ary vs p, which have been observed, disappear if the Pt reference substrate chosen is of the same charge as the Pt reactants. The value of p, for a bulky nucleophile has also to be modified to allow for steric hindrance features. 

The term s is the nucleophile discrimination factor. The values of pt, the nucleophilic reactivity constant, is useful for correlating kinetic data for other Pt(II) complexes. 

In the acid hydrolysis of the pentamminecobalt complexes where you have a leaving group, such as nitrato or bromo, H. Taube and A. Haim came out with some very interesting work, with which I am sure you are all familiar. They suggested that the five-coordinated pentamminecobalt species was formed which then discriminated between various nucleophilic reagents, sometimes reacting with water, sometimes with- thiocyanate ion. In fact, they were able to measure these nucleophilic discrimination factors in a number of cases, and they were able to correlate different types of reactions in which the pentamminecobalt would be generated in different ways. 

This relevant finding is attributed to the greater nucleophilic discrimination factor of DMSO in its complex, whereas the trans effect is attributed to transition-state stabilisation arising from the x-acceptor properties of the ligand, S-bound DMSO being considered a better o-donor and x-acceptor than ethylene (22). 

The intercepts of plots such as Figure 3 are close to the values of k for each substrate, and the constant D is dependent on the nature of the substrate. It is a nucleophile discrimination factor, and a large value of D means the complex is very sensitive to changes in the nature of the nucleophilic reagent. Finally the most significant point that can be made from values of n p (Table 2) is that platinum(ii) is a class (b) or soft metal. The strongest protonic base CH30 in... 

The nucleophilicity discrimination factor, s, is a characteristic of a given square planar Pt(II) complex and describes how sensitive the complex is to variation in the nucleophilicity of the entering ligand. 

Table 25.3 Nucleophilic discrimination factors, s, for selected square planar Pt(II) complexes.

When the platinum nucleophilicity scale was first proposed it was implied that one np, scale was applicable to all substrates and that plots of logk2 against npt°(Y) were linear, taking the form log 10 2 = S np, (Y) + C, where S is termed the nucleophilic discrimination factor of the substrate and C its intrinsic reactivity. Discussions of mechanism based on a comparison of nucleophilic discrimination factors are frequently encountered. Nucleophiles that do not retain their positions in the nucleophilicity scale, e.g. NO2", SeCN and SC(NH2)2, were termed biphilic by Cattahni since their behaviour could be explained by their n-acceptor properties. When the Pt reaction centre had a greater n-basicity than the standard complex (for example a smaller effective nuclear charge) the substrate was more reactive than predicted and vice versa. This concept had been deduced some years earlier by Bosnich from his work with octahedral Ru complexes... 

The nucleophilic discrimination factor s is defined as 1.00 for tran -[Pt(py)2Cl2] this nucleophilicity scale has as a reference substitution reactions such as those described in the first column in Table 12.12, carried out in methanol solvent at 30 °C. Values of rjpt are subsequently found with kinetic data from these reactions via... 

Fig. 26.6 (a) The nucleophilicity discrimination factor, s, for a particular square planar Pt(II) complex can be found from a plot of log 2 (the second order rate constant, see equation 26.12) against npt (the nucleophilicity parameter, see equation 26.22). Experimental results are plotted in this way in graph (b) which shows data for the reaction of tranx-[PtCl2(py)2] with different nucleophiles in MeOH at 298 or 303 K. [Data from R.G. Pearson et al. (1968) J. Am. Chem. Soc., vol. 90, p. 319.] (c) Plots of log 2 against npt for three square planar Pt(II) complexes each plot is of the same type as in graph (b). The gradient of each line gives s, the nucleophilicity discrimination factor, for that particular complex. [Data from U. Belluco et al. (1965) J. Am. Chem. Soc., vol. 87, p. 241.]... 

If we now consider substitution reactions of nucleophiles with other Pt(II) complexes, linear relationships are found between values of log 2 tid pt as illustrated in Figure 26.6. For the general reaction 26.11 (in which the ligands L do not have to be identical), equation 26.23 is defined where s is the nucleophilicity discrimination factor and k2 is the rate constant when the nucleophile is MeOH. 

The parameters S ("nucleophilic discrimination factor") and log kj ("intrinsic reactivity") are dependent upon the nature of the Pt(II) complex and not on the nature of Y. 

The rates of several substitution reactions at palladium(n) have been measured in methanol. For rra 5-[PdL2(N02)2] + Y ->-fra//j-[PdL2(N02)Y] + NO2-, where L = PPr"s, AsEtg, or piperidine and Y = Cl, N3-, Br, I, SCN, or (NH2)2CS, a set of nucleophilic reactivity constants npd = log ( alMeOHj/Ari) has been calculated, where and refer to rate constants for the solvolytic path and for direct nucleophilic attack on the complex rra/ij-[Pd(PPr 3)2(N02)2l. All the complexes studied obey the linear free-energy relation log kz = jnpd + log kx, where j is a nucleophilic discrimination factor. The npd sequence resembles that already found for platinum(n) complexes. 

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