M. Trozzi

Fast reactions at planar four-co-ordinate complexes. Part I. Nucleophilicity towards palladium(II) complexes

Rates of nucleophilic substitution reactions (i)[L = PPrn3, AsEt3, or piperidine, C5H11N; Y–= Cl–, N3–, Br–, I–, SCN–, or (NH2)2CS] have been measured in methanol at 25 °C. A set of nucleophilic reactivity constants, trans-[PdL2(NO2)2]+ Y–→trans-[PdL2(NO2)Y]+ NO2–(i)nPd0= log (k2[MeOH]/k1), have been calculated, k1 and k2 referring to rate constants for the solvolytic path and for direct nucleophilic attack on the complex trans-[Pd(PPrn3)2(NO2)2]. All the complexes studied obey the linear free-energy relation log k2=snPd0+ log k1, where s is a nucleophilic discrimination factor. The nPd0 sequence resembles that already found for platinum(II) complexes.

Fast reactions at planar four-co-ordinate complexes. Part II. The leaving-group effect in palladium(II) complexes of 3-azapentane-1,5-diamine

The displacement of ligand X in [Pd(3-NHpd)X]+(3-NHpd = 3-azapentane-1,5-diamine) by a variety of nucleophiles Y has been studied in methanol at 25° C and I= 0·1 M(X = Cl–, Br–, I–, N3–, or NO2–; Y = Cl–, Br–, N3–, I–, SCN–, or thiourea). The results show that, in spite of the large difference in reactivity between analogous palladium(II) and platinum(II) complexes, the lability sequence is almost the same, and, as in the case of PtII complexes, there are indications that the free-energy changes related to bond formation and weakening during the formation of the transition state are largely independent indicating that these processes are asynchronous in nature.

Steric effects in substitution reactions of cis- and trans-arylchlorobis-(triethylphosphine)platinum(II) complexes: new kinetic data for the approach to the problem of transition-state geometry

The rate of displacement of Cl– ion in the complexes trans- and cis-[Pt(PEt3)2(R)Cl](R = Ph, o-MeC6H4, and 2,4,6-Me3C6H2) with CN– has been studied in propan-2-ol containing 6% water. Analysis of the relative reactivity of the two sets of isomers gives some indication of the configuration of the transition state. It is shown that literature data concerning reactions of the same systems with pyridine (py) in ethanolic solution are incorrect. The trans-complexes do not undergo bimolecular attack and the cis-complexes spontaneously isomerize to the trans-form before or during their reaction with py.

Kinetics and mechanism of replacement of chloride ion in trans-chloro-(o-tolyl)bis(triethylphosphine)platinum(II) by substituted pyridine compounds: influence of the basicity and steric hindrance of the pyridines on their reactivity towards the reaction intermediate

Rates of reversible reactions between the complex trans-[Pt(PEt3)2(o-tolyl)Cl] and various substituted pyridine compounds has been studied in methanol at 30 °C. The strong rate dependence on the incoming group arises from competition between the incoming pyridine and leaving chloride ion for the reaction intermediate which incorporates one molecule of solvent. The efficiency of the reagent in capturing the intermediate species and the position of the equilibrium of the overall reversible reaction do not depend on the change in σ-donor ability of the pyridine compound but only on its size and steric hindrance.

Mechanism of the substitution of chloride in trans-[Pt(PEt3)2(R)Cl] by pyridine: kinetic evidence for a reaction intermediate

The rate of displacement of Cl– in trans-[Pt(PEt3)2(R)Cl](R = phenyl, o-tolyl, and mesityl) by pyridine has been studied in the presence of LiCl (0·005M) in methanol at 30 °C. The pseudo-first-order rate constants for approach to equilibrium (kobs) conform to the equation (i) where k1 and k4 represent the solvolytic rates of generation of an kobs=k1[py]+k4k2/k3[Cl–]//k2/k3[Cl–]+[py] intermediate from trans-[Pt(PEt3)2(R)Cl] and trans-[Pt(PEt3)2(R)(py)]+ respectively, and k2/k3 represents the relative efficiency of Cl– compared with pyridine in competing for the reactive intermediate, trans-[Pt(PEt3)2(R)-(MeOH)]+. k3 Has been measured directly from the kinetics of the reaction of trans-[Pt(PEt3)2(R)NO3] with pyridine, so that it is possible to calculate all the rate constants which appear in the expression for kobs. The consistency of the rate and equilibrium data has been checked by comparing the values of the equilibrium constants calculated from kinetic parameters with those obtained by means of direct spectrophotometric and conductance methods. The effect of increasing steric hindrance in the substrate upon the reactivity of simple nucleophiles is discussed.