Square-planar complexes dissociative substitution

Solvent paths and dissociate intermediates in substitution reactions of square planar complexes. R. J. Mureinik, Coord. Chem. Rev., 1978, 25,1-30 (133). 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

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). 

Organometallic dissociative substitutions, like their associative counterparts for square-planar complexes, have been investigated in detail primarily for octahedral transition metal carbonyl complexes." They involve the initial rate-limiting separation of one ligand (X) from the substrate to give an electronically unsaturated (16-electron) distorted tbp intermediate, followed by rapid binding of the nucleophile (Y) to give the substituted product (Equation (3), where kx, > k ). 

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. 

Kinetically labile and inert complexes Dissociation, association and interchange Activation parameters Substitution in square planar complexes Substitution in octahedral complexes Racemization of octahedral complexes Electron-transfer processes... 

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... 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

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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