Octahedral substitution associative mechanisms

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). 

Associative reactions are also possible in octahedral substitution, but are much less common. Table 12-5 gives data for both dissociative and associative interchanges for similar reactants. In the case of water substitution by several different anions in [Cr(NH3)5(H20)] ", the rate constants are quite similar (within a factor of 6), indicative of an Id mechanism. On the other hand, the same ligands reacting with [Cr(H20)6] show a large variation in rates (more than a 2000-fold difference), indicative of an mechanism. Data for similar Co(lll) complexes are not conclusive, but their reactions generally seem to have 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). 

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 ... 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Substitution react ions are generally classified as having either dissociative or associative mechanisms. We will use the general octahedral substitution reaction (23)... 

In general, the LFAEs are more positive for the associative mechanism than for the dissociative mechanism, adding further support to the theory that most octahedral substitution reactions are dissociative. The largest LFAEs for the associative mechanism largely mirror those for the dissociative mechanism d, LS d, and d , with the main exception being that LS d is also now included as Inert. 

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). 

Bidentate coordination of the imine was also demonstrated in crystal structures of Ir(i) bis(ethylene) and 1,5-cyclooctadiene complexes. An analogous chelate iridium-amine adduct was also synthesized and structurally characterized. The kinetics of substitution of amine by imine (Scheme 14) were investigated to probe the mechanism of the product/substrate exchange reaction in the proposed catalytic cycle. The data were interpreted in terms of an associative mechanism in which the imine binds reversibly to Ir, prior to the release of amine. However, since the catalytic mechanism may well involve octahedral Ir(m) rather than square-planar Ir(i) complexes, a different mechanism could well operate during catalysis. 

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

---