Square-planar substitution reactions dissociative activation

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

The reaction rate is primarily determined by the enthalpy of activation (A// ), which is usually the case in square planar nucleophilic substitution reactions. Of greater importance, so far as a dissociative versus an associative mechanism is concerned, are the entropies and volumes of activation, AS and AY, respectively. Note that the values are negative for both the fct and the steps. The observed decrease in entropy is what we would expect for a mechanism in which two particles come together to give an activated complex. The volume of activation is determined by doing the reaction under high pressure ... 

Craig et al. demonstrated that the mechanical activation of a leaving group accelerates the nucleophilic substitution of dimethyl sulfoxide for substituted pyridines at square-planar pincer Pd(ll) metal centers (Figure 14). For the first time, they compared the dynamic SMFS behavior and stress-free kinetic data for bimolecular reactions. The stress-free dissociation rate constants of 1 x (2a)2 and 1 x (2b)2 extrapolated from the corresponding DFS are 0.7 0.4 and 20 3 s , respectively, which are in excellent agreement with the values determined by dynamic NMR (1 and 17 s , respectively). ... 

PIatinum(ii).—General. Activation volumes are often useful guides to the diagnosis of mechanism, frequently permitting distinction between associative and dissociative mechanisms and sometimes permitting more subtle distinctions, as between /a and/) or 7a and A alternatives. Reactions of a series of complexes [Pt(dien)X]+ (X = Cl, Br, I, or N3) with a range of incoming nucleophiles Nu (Nu = OH, I", Ng-, NO2 , SCN, or py) follow the usual rate law (1) characteristic of substitution at square-planar species. Activation volumes have been determined, in aqueous solution, both... 

This two-term form, normal for square-planar complexes, is extremely unusual for substitution at an octahedral complex. The tantalum(v) appears to be present in the reaction system solely as [TaF ], but of course [TaF ] is a stable anion so that parallel associative and dissociative paths for fluoride exchange represent a reasonable mechanism. Rate constants and activation parameters are listed in Table 9. The activation entropy for the ki term is entirely consistent with associative fluoride exchange via a... 

---