Reaction rate substitution

In this spirit, an attempt will be made to account for the magnitude of pressure effects on ligand substitution reaction rates. Attention will necessarily be confined to a few simple model systems two recent reviews (1, 2) of pressure effects on reactions of transition metal complexes in solution may be consulted for more comprehensive surveys of the field. 

Figure C shows an extreme case of the dependence of a substitution reaction rate on the nature of the incoming group. This happens to be the hydrolysis of the trisacetylacetonate complex of silicon (IV), cationic species, which Kirchner studied first—the rate of racemization or rate of dissociation. We studied the base-catalyzed rate of dissociation and showed that a large number of anions and nucleophilic groups, in general, would catalyze in the dissociation process. We found that the reaction rates were actually for a second-order process, so these units are liters per mole per second. But the reaction rate did vary over an enormous range—in this case, about a factor of 109—and this is typical of the sort of variation in rates of reaction (that you can get) for processes that seem to be Sn2 bimolecular displacement processes.

The rate constants involved in the formation of larger clusters are described in terms of the RRK theory,180 which states that the substitution reaction rate for the addition of the strongly bound component is much faster than for the addition of the more weakly bound component. This gives rise to the experimental observation that the composition of the clusters does not reflect the composition of the vapor phase from which they are formed. Instead, during the formation stage of the clusters, a non-statistical enrichment toward the more strongly bound species occurs.181... 

The activation parameters and dependence on L are shown in Table 13. These data are fully consistent with an associative reaction. The 17-electron complex V(CO)6 has an associative substitution reaction rate that is > 10 ° more facile than for the 18-electron Cr(CO)6 complex. The vanadium complexes are among the most inert of the 17-electron complexes. Table 14 shows the rate constants for substitution of several complexes. As expected from size considerations, substituting a phosphine ligand for a CO decreases the rate for an associative reaction. 

As with nucleophilic substitution reactions, rates of dehydrohalogenation reactions will be dependent on the strength of the C-X bond being broken in the elimination process. Accordingly, it is expected that the ease of elimination of X will follow the series Br>Cl>F. The relative reactivities of Br and Cl toward elimination is evident from the hydrolysis product studies of 1,2-dibromo-3-chloropropane (DBCP Burlinson et al., 1982). DBCP has been used widely in this country as a soil fumigant for nematode control and has been detected in groundwaters (Mason et al., 1981) and subsoils (Nelson, et al., 1981). Hydrolysis kinetic studies demonstrated that the hydrolysis of DBCP is first order both in DBCP and hydroxide ion concentration above pH 7. Below pH 7, hydrolysis occurs via neutral hydrolysis however, the base-catalyzed reaction will contribute to the overall rate of hydrolysis as low as pH 5. Product studies performed at pH 9 indicate that transformation of DBCP occurs initially by E2 elimination of HBr and HCl (Figure 2.4). 

It is assumed that the ion-exchange rate in the aqueous phase is much higher than the substitution reaction rate in the organic phase. The effect of the ion-exchange reaction could be eliminated from the reaction controlling steps. 

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. 

A classic example of ligand steric bulk influencing ligand substitution reaction rates is... 

We wanted to determine whether the difference in olefin-substitution reaction rates observed in solution would translate into selective detection of certain olefins using the solid organoplatinum conplex on the surface of a SAW sensor. The results described here are for the pyridine-containing complex only. The effect of using various substltuted-pyridines on the reactivity of these conplexes is currently being explored. 

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