Associative substitution mechanism square planar complexes

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

The detection of a reaction intermediate is usually not possible in coordination chemistry because lifetimes of intermediates are commonly extremely short. The simple mechanisms of reaction are commonly designated as an associative mechanism (A, with an intermediate of expanded coordination number formed) or a dissociative mechanism (D, with an intermediate of reduced coordination number formed). Intermediates of expanded coordination number are important in ligand substitution in square-planar complexes and in a few cases can actually be detected. For example, NifCNls " is known from exchange reaction of Ni(CN)4 with CN (288). Even in octahedral complexes, some evidence for associative processes exists indirectly. The [RulNHsle] " ion reacts with NO in acid to form [RuINHslsNO] and NH4 much more rapidly than can be explained by aquation of the hexaamine as the initial step, and a bimolecular mechanism with a 7-coordinate intermediate has been proposed (11, 226). 

For substitution of monodentate 77-hydrocarbon ligands (ethylene, acetylene) a priori both mechanisms are possible. In this case an ability to change the coordination number in the transition state will be decisive. It is probable that square-planar complexes react by an associative mechanism with an increase in coordination number in the transition state. For the octahedral complexes, intermediates with lower coordination number are preferable (D-type mechanism). There is as yet no evidence for a transition state involving a-bonded ethylene or acetylene. However, both molecules are capable of inserting into transition metal-carbon u-bonds 10). It is quite probable that such an insertion mechanism operates in the Ziegler-Natta ethylene polymerization 11). 

The Associative Mechanism The A mechanism can be expected to predominate in complexes that can expand their coordination number relatively easily. The obvious candidates are the square planar complexes of Pt11, Pdn, and Aum, for which many comparable five-coordinate complexes are also known. The salient feature of ligand substitution kinetics in these square planar complexes, however, is not the detectability of five-coordinate intermediates—the reaction rates are usually accurately second order overall—but rather the wide-ranging dependence of the second-order rate constants on the nature of the incoming ligand. This implies that in the scheme... 

Associative mechanism for substitution reactions in square planar complexes. Sites above or below the plane are inherently readily accessible to incoming ligands. 

Since substitution reactions of square-planar complexes in most cases tend to follow an associative mechanism, a large number of complexes were synthesized in efforts to determine whether a changeover in mechanism is induced. The increase in steric hindrance caused by introducing alkyl substituents on the three N donor atoms of diethylenetriamine (dien) in going from dien to Mesdien to Etsdien, caused a decrease in the aquation rate constant for [Pd(Rsdien)Cl]+ of six orders of magnitude, but not a changeover in mechanism. The reported activation... 

Eighteen-electron complexes react more slowly than similar complexes with either more or less electrons. The eighteen-electron rule explains why some reactions are associative and others dissociative. Complexes in which the metal has sixteen or less valence electrons tend to react by associative mechanisms, since the metal has vacant low-energy orbitals which can be used to form a bond with the entering ligand. This orbital can accept an electron pair from an entering ligand and provide a path for associative substitution. Substitution reactions in square planar complexes illustrate this point, reaction (40). 

Controversy over the interpretation of the kinetics for cis-trans isomerizations of square-planar complexes continues. The topic is important for the understanding of substitution mechanisms in general. Associative and dissociative mechanisms have been proposed, as well as intramolecular rearrangements via tetrahedral intermediates. ... 

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. 

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... 

Substitution reactions in square planar complexes take place via associative mechanisms. In the kinetic trans effect, a series can be constructed in which species are put in order of their ability to labilize ligands trans to themselves. Using this series, high-purity square planar isomers can be readily synthesized. The trans-directing ability of a ligand is also directly related to its polarizability. 

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... 

The d metal ions, such as Pt(II), Pd(II) and Ni(II), often fonai square planar complexes. The square planar complexes of Pt(II) are of particular interest in kinetic studies due to their high stability, ease of synthesis and moderate rates of reaction that enable the monitoring of the reaction. The area of discussion in these complexes is restricted only to the substitution reactions. As compared to the octahedral complexes, the crowding around the metal ion is less in square planar complexes. This is one of the important reasons that most of the substitution reactions in these complexes follow the SN (associative mechanism). 

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 ... 

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