Entering groups parameters

Rate Constants and LFER Parameters for Entering Groups... 

Rate laws and kinetic parameters for substitution reactions at complexes Cp2TiX2 in acetonitrile solution at 298.2 K have been reported (X = halide or alkoxide). Reactivities are discussed in terms of the nature of the leaving group, the entering group, and the non-leaving Cp ligand. A volume of activation of —15 cm3 mol-1 has been determined for the reaction with thiocyanate.1124... 

The nucleophilicity parameter, Wpj, describes the dependence of the rate of substitution in a square planar Pt(II) complex on the nucleophilicity of the entering group. 

The data show that as the size of the solvent molecule increases, the solvent is less able to solvate the ion that is the entering group and the rate of the reaction increases. The number of cases where similar observations on solvent effects are encountered is enormous. Later in this chapter, those principles will be extended to include the effects that result from using solvents that have different solubility parameters. 

The rates of reactions proceeding by an associative mechanism must depend to some degree on the nature of the entering group. The usual parameter which is referred to in substitution reactions is the nucleophilicity of ligand or nucleophile (which is a measure of the effectiveness of the nucleophile in a substitution reaction). Nucleophilicity is a kinetic term, while basicity is thermodynamic in origin. There is often a relationship... 

Trogler and co-workers studied the reactions of Fe(CO)3(PR3)2 radicals and found that CO substitution is a second-order process whose rate depends on the steric bulk of the entering group. The rate constants correlate with the pK. of the pyridine nucleophiles. For pyridine reacting with Fe(CO)3(PPh3)2 (25 C in CH2CI2), the parameters are k= 13.6 M" s", Aff = 9.8 kcal mol" and AS = -21 cal moF K these values are typical of the other systems. 

One set of empirical constants is obtained for various groupings of T, Ci, and C2 and two other sets (parameters R and N, respectively) for various leaving and entering groups. These empirical constants are then used quite successfully to calculate rate constants for several reactions (other than those from which the constants were obtained), the parameter R for reactions in which the transition state energy is determined by the bond-breaking process, and the parameter N for reactions in which the energy of bond formation is more important. 

Entering Groups. Rates of reaction of [PtCl4], [PtCl3(OH2)], and [PtCl2(OH2)2] with ethene are all rather similar. Rates and activation parameters for the reaction of a variety of uncharged platinum(ii) complexes with cyanide have been reported. The complexes in question are [PtCl(N02)(NH3)2], [Pt(N02)2(NH3>2], [Pt(CN)a-(en)], and rra s -[Pt(CN)(N02)(NH3)2] cyanide substitution follows the usual rate law [equation (1) above]. The variation of the second-order term with solvent composition for the reaction of [PtCl2(bipy)] with thiourea in aqueous dioxan and in aqueous THE has been discussed in terms of solvent effects on the initial and transition states (see Chapter 5 of this Part). ... 

Table 3 Activation parameters for the reaction o/[Pt(bipy)Cl2] with uni- and bidentate entering groups (L) in methanol...

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