Solvolysis and Anation

Palladium(n).—Solvolysis and Anation. Kinetics of solvolysis of sterically hindered (pseudo-octahedral) palladium(ii) complexes catalysed by mercury(ii) have been monitored in methanol. A relationship holds between the catalytic efficiency and the stability constant of the complex between the mercury(ii) and the leaving group. Reaction of Pdaq with ethene is thought, by analogy with earlier results on the reaction between Pd q and quinone, to be the key step in the kinetics of oxidation of ethene to acetaldehyde catalysed by palladium(ii) in the presence of phospho-molybdovanado-heteropolyanions.  

The reactions of complexes [PdX(Et4dien)]+ with nucleophiles generally obey the simple rate law shown as equation (10). Here the normal term of equation (1) is 

Kinetics of diethyl sulphide, diethyl selenide, and diethyl telluride exchange at [PdX2(ZEta)2] (X=C1 or Br, Z=S, Se, or Te) have been reported. Ligand exchange at palladium(ii) is considerably faster than at platinum(ii) in the analogous [PtXa-(ZEt2)2l series of compounds.  

---

Substitution at platinum(ii) may be involved when Pt complexes are used to catalyse substitution at platinum(iv). Some new views on the role of the platinum(ii) species in such reactions have been offered. The notion of direct relevance to this chapter is that the primary step is formation of a dinuclear intermediate directly from the simple hydrated platinum(ii) complex and the platinum(iv) complex. Aquation, Solvolysis, and Anation. The kinetics of aquation of [PtClJ over the range 50—70 °C are consistent with data published earlier for aquation at lower... 

Platinum(n).—Aquationy Solvolysis, and Anation. Kinetic parameters have been determined for the forward and reverse reactions of equation (2). Mercury(ii)... 

The relative reaction rates and the stability of the aquo complex make it possible to identify the aquo complex as an intermediate and study the individual acts separately. However, if the solvento complex were less stable and the anation rate much faster than the solvolysis, it would not be possible to observe this intermediate, and the process would be kinetically indistinguishable from a unimolecular dissociative process. Both processes would exhibit overall first-order kinetics and the usual mass-law retardation and other competitive phenomena characteristic of an extremely reactive intermediate. 

The solvolysis of hexaaquachromium(HI) in DMSO proceeds via the series of complexes [Cr(DMS0)n(H20)6 ]3+ (n = 1 to 6).945,946 The anation and solvolysis of several chromium(III) complexes in DMSO have been studied 947,948 in general the reactions proceed by /d mechanisms. A direct electrochemical synthesis of [Cr(DMSO)6][BF4]3 has been reported.949 A comprehensive review of metal ion complexation by DMSO contains an extensive section on chromium(III).950... 

Solvents. Solvent transfer parameters have been used from time to time in diagnosis of mechanisms of organic reactions. Now their first use in an inorganic system has been discussed. Comparisons between kinetic results and solvent transfer activity coefficients for solvolysis of [Cr(NCS)e] in DMSO, DMF, and dimethylacetamide have been considered in terms of a dissociative mechanism. The DMSO results can be accommodated by a dissociative model for the transition state, but those in DMF and dimethylacetamide fit less well. It is interesting to compare these conclusions with, for instance, the dissociative mechanism proposed by the same authors for anation of the [Cr(DMSO)e] + and [Cr(DMF)6] + cations, and the associative mechanism suggested on the basis of the determined activation volume for exchange of DMSO with [Cr(DMSO)e +. ... 

These results give the following values of A(A5cr-co) in cal K mol -11.5 (DMSO), -10.2 (DMF), -17.9 (NMF), and -14.0 (formamide) Equilibria occur in DMSO and DMF and the concentration of added Cl affects both the position of these equilibrium and the rates of solvolysis. The chloride anation of [Co(tren)Cl(DMSO)] was also studied and found to occur by an (5n1)ip mechanism. A number of the solvolysis products have been isolated and characterized. There is evidence that tren may sterically block nucleophilic attack. 

---