Dissociative substitution reactions steric effects

There is also evidence supporting a as effect on the rate of square planar substitution reactions, although it is not quite as dramatic. However, steric factors can be very important in the c/s position. Steric congestion enhances the rate of a dissociative substitution reaction and slows the rate of an associative one. Because the bulky ligands are closer to each other in the c/s position than in the trans position. 

Successive sequential substitutions of cluster complexes can also occur by different mechanisms. In a classic study of the substitution reactions between Ir fCO) and PPhj (Equation 5.53), the initial substitution was shown to occur by an associative mechanism, but the second substitution was shown to occur by a dissociative mechanism. Under the conditions employed, the relative rates for the first, second, and third substitutions were 1 30 920. This trend has been ascribed, in part, to the presence of bridging carbonyl ligands in the substitution products. Steric effects also may be important. 

Ten years ago Rorabacher (13) observed the substitution rate constants for aquonickel(II) ion with different amines (Table II). There is a decrease in the rate constants by a factor of 14 in going from ammonia to dimethylamine. If nickel-(II) substitution reactions are dissociative, then why is the effect this large Is this a steric effect with some associative contribution or is it an outer-sphere effect There has been surprisingly little investigation of the nature of the entering ligand so far as its bulk or its nucleophilicity is concerned even for what have been generally considered as simple substitution reactions. 

Also tetracarbonylnickel which isn t sterically hindered but is d10, exchanges with radiocarbon monoxide by dissociative mechanism, and I think it could go to the same trigonal bipyramid without much steric difference. I wouldn t want these comments to swing everybody over to steric effects and say that really there is nothing interesting in substitution reactions—all the six-coordinate ones go one way, all the planar ones go the other way, and the electronic structure doesn t matter. This will drive people out of substitution mechanisms into the more interesting field of electron transfer mechanisms where electronic effects must be important. 

The scope and limitations of the Lewis acid-catalyzed additions of alkyl chlorides to carbon-carbon double bonds were studied.51 Since Lewis acid systems are well-known initiators in carbocationic polymerizations of alkenes, the question arises as to what factors govern the two transformations. The prediction was that alkylation products are expected if the starting halides dissociate more rapidly than the addition products.55 In other words, addition is expected if the initial carbocation is better stabilized than the one formed from the dissociation of the addition product. This has been verified for the alkylation of a range of alkyl-and aryl-substituted alkenes and dienes with alkyl and aralkyl halides. Steric effects, however, must also be taken into account in certain cases, such as in the reactions of trityl chloride.51... 

The drastic drop in the rate of epimerization of ortho-substituted derivatives compared to the unsubstituted or para-substituted complexes is ascribed to the steric effect of the ortho substituents. In an intramolecular isomerization, steric hindrance is expected to increase with the size of the ortho substituents (//, 92). For a dissociation reaction, or for chelate ring opening, a steric acceleration should be observed if the ortho substituents are bulkier. These substituent effects therefore support an intramolecular formulation of the epimerization reaction in Scheme 16. 

The RSE is calculated here as the difference between the homolytic C-C bond dissociation energy in ethane (5) and a symmetric hydrocarbon 6 resulting from dimerization of the substituted radical 2. By definition the C-C bonds cleaved in this process are unpolarized and, baring some strongly repulsive steric effects in symmetric dimer 6, the complications in the interpretation of substituent effects are thus avoided. Since two substituted radicals are formed in the process, the reaction enthalpy for the process shown in Equation 5.5 contains the substituent effect on radical stability twice. The actual RSE value is therefore only half of the reaction enthalpy for reaction 5.5 as expressed in Equation 5.6. 

A convenient system for studying substituent effects is the equilibrium between meta- and />ara-substituted benzoic acids and their corresponding anions (reaction 3.5). The acids are straightforward to synthesize, and the acidities in water at 25 °C are readily determined the pH of a solution containing equal molar quantities of the acid and its fully ionized sodium salt will be equal to — log A, where KA is the dissociation constant of the acid. Ortho substituents are not considered because of complications caused by steric effects. 

Kinetic studies of H2 dissociation and substitution rates show that the potential energy surfaces for these reactions vary dramatically even with minor changes in ancillary ligands or for isomers (Table 7.7).5,69 Electronic effects, especially the influence of the trans ligand, appear to be more important than steric factors. For the Ir system, the ds isomer with H2 trans to Cl has a strongly bound H2 (dHH = 1.11 A) while the trans isomer with H2 trans to H contains a weakly bound H2 that dissociates nearly 10s times faster (see also Section 4.7.1). One of the few comprehensive quantitative studies of H2 substitution reactions shows displacement of H2 by L (MeCN, PhCN, ) fromCMHCHjXP) (M = Fe, Ru, Os) is first-order in concentration of complex and zero order in L, Le., a dissodative mechanism.70... 

For cw-Mo(CO)4(L)2 complexes, steric effects seem to dominate the substitution reactions. For L = PPhj, the rate of L displacement by CO is -200 times larger than that for L = PMePhj. The ground-state structures show that the P—Mo—P angle is distorted from 90 to 104.6 in the former and only to 92.5 in the latter. The distortion of the structure is relieved in the dissociative transition state. It should be noted that the Mo—P bond lengths are very similar, 2.577 A for PPhj and 2.555 A for PMePh2. 

---