Interchange mechanisms steric effects

This reaction was followed by a second slower reaction that was not characterized in the report. An analysis of the overall results showed that the influence of steric or electrostatic effects on kinetic parameters is not necessarily minor. The dynamics of binding of NO to the diaqua species is mainly tuned by modulation of electron density on the iron center by the porphyrin macrocycle. A volume profile for NO binding based on values ofAV on and A V 0ff of + 1.5 and + 9.3 cm3/mol, respectively, maybe interpreted as an interchange mechanism for the on reaction, as the Fenl-H20 bond is decidedly stabilized (see Figure 7.15a). The volume of activation for the off reaction indicates a less dissociative mode of activation compared with NO release from other porphyrins. 

This reaction is sometimes called acid hydrolysis since reactions of this type are normally carried out in acidic solution to repress base hydrolysis. The published data on cobalt(III) complexes are consistent with an mechanism involving dissociative interchange. The bond between cobalt and the leaving group is considerably stretched, and the new bond between the metal and the entering group (water) is little formed (3.2). This view is consistent with the values of AS AV and the effects of sterically hindered ligands on the rates of reaction. 

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