Dissociative substitution mechanisms octahedral complexes

The mechanism may be associative (A or f) or dissociative (D or f), and it is not at all easy to distinguish between these, even though the rate laws are different. An associative mechanism involves a 7-coordinate intermediate or transition state and, sterically, an associative pathway seems less likely than a dissociative one. Nevertheless, activation volumes do sometimes indicate an associative mechanism (see Table 25.4). However, for most ligand substitutions in octahedral complexes, experimental evidence supports dissociative pathways. Two limiting cases are often observed for general reaction 25.22 ... 

Let us now look further at experimental trends that are consistent with dissociative (D or If) mechanisms for substitution in octahedral complexes is supported in very many instances. 

B.23 Assuming no experimental complications, explain, in your own words, some ways in which one might expect to differentiate between an associative (A) and dissociative (D) mechanism for the substitution of octahedral complexes. 

Explain the difference with respect to the size of the neighboring groups on substitution in an octahedral complex by associative and dissociative mechanisms. 

The addition and dissociation of pyridine and substituted pyridine molecules to a planar nickel(II) complex with a quadridentate N202 ligand have been studied by the microwave temperature-jump technique in chlorobenzene solvent (38). The data were interpreted with the assumption of mechanism C (Fig. 7), i.e., that k65 is the smallest rate constant. Subsequently, however, 14N NMR was used to measure the rate of pyridine exchange from the octahedral complex (138). The rates are the same for the two different experiments within a factor of two. This observation excludes mechanism B and is consistent with either mechanism A or C. The rate constants have consequently been presented in Table VI as k64. 

Figure 5-38. The prototypical ligand substitution reaction in octahedral complexes. In principle, the reaction could proceed by associative or dissociative mechanisms.

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). 

A variety of electronic factors influence the dissociative labilities of metal complexes. For octahedral complexes, which generally undergo ligand substitution by dissociative mechanisms, the following factors have been identified ... 

While this could be interpreted as a dissociative mechanism with an ion-pair preequilibrium, discussed above for octahedral complexes, it is notable that the similar substitution reaction of octahedral complexes that usually react by dissociative processes shows no dependence on entering group concentration. 

Electronic and steric factors also influence substitution reaction rates of octahedral complexes. The inequalities below indicate relative rates for ligand exchange via presumed dissociative mechanisms. 

Turning from the commonly studied tetrahedral compounds of tin to octahedral complexes, the kinetics of substitution at SnCl4py2 in nitrobenzene have been investigated. The activation parameters are reported to an astonishing precision the mechanism for solvolysis, as for chloride exchange, is said to be dissociative despite the very different rates reported for the two reactions. 

Many other reagents and reaction conditions accelerate ligand substitution, but are less defined than the examples described above. A variety of nucleophilic reagents, such as phosphine oxides, accelerate ligand dissociation from stable octahedral carbonyl complexes, but the mechanism that accounts for this acceleration is not well imderstood. ... 

Substitution for polyhapto ligands occur in many cases by mechanisms that are similar to those for replacement of monodentate ligands. Thus, substitutions for polyhapto ligands in 18-electron octahedral complexes, like substitutions for CO in the 18-electron octahedral complexes presented in Section 5.4.2.1, often occur by competing dissociative and associative pathways with a two-term rate expression, = k + k ... 

The dissociative (D) mechanism for the substitution of one ligand for another in an octahedral complex, ML5X (L = inert ligands, X = labile ligand, and Y = incoming ligand). 

Now that we have fairly well established that the dissociative mechanism generally applies for the substitution reactions of octahedral complexes, we are in a good position to begin to answer some of our earlier (p. 100) critical questions about inert versus labile complexes. As defined earlier,and inert-xro, kinetic terms describing the rates of reactions of coordination compounds. As you should recall from earlier courses, rates depend on the magnitude of the energy of activation, of the ratedetermining step. 

Although the overwhelming bulk of evidence for substitution reactions of octahedral complexes indicates that they most often proceed via a dissociative mechanism. 

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... 

Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

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