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In Chapter 2 of this thesis, introduction of four flexible malonate substituents on the highly negatively charged iron(III) porphyrin, (P8-)FeIII increases the pKa1 of coordinated water in (P8-)FeIII(H2O)2. The electronic effects are reflected in partial depopulation of the iron dx2-y2 atomic orbital, leading to ca. 26% contribution of the S = 3/2 spin state in the spin-admixed (P8-)FeIII(H2O)2 system. This contribution is relatively high in comparison with those of other (P)FeIII(H2O)2 species and correlates with the high lability of coordinated water, as is indicated by rapid water-exchange and NO coordination rates observed for the (P8-)FeIII(H2O)2 complex. The solution pH determines the nature of the axial ligands in (P8-)FeIII and thereby controls the coordination number, spin state, and reactivity of the iron(III) center toward NO. The predominantly five-coordinate, purely high-spin (P8-)FeIII(OH) formed at pH > 9 binds NO according to an associative mechanism, in contrast to a dissociatively activated process observed for six-coordinate (P8-)FeIII(H2O)2 at lower pH. In the case of (P8-)FeIII(OH), however, the NO binding step is no longer controlled by the lability of the metal center. Other (most probably electronic and structural) factors involved in the rate-limiting Fe-NO bond formation and breakage (for the “on” and “off” reactions, respectively) apparently account for the slower rate of NO binding and release observed at high pH for (P8-)FeIII(OH). Correlations between spin/ligation states and reactivity inferred on the basis of spectroscopic and kinetic data for other water-soluble iron(III) porphyrins studied to date further support this conclusion. As suggested by preliminary studies on other water soluble iron(III) porphyrins, the pH-reactivity pattern reported here for water-exchange and reversible NO binding to (P8-)FeIII is a common phenomenon for diaqua- and monohydroxo ligated (P)FeIII species in aqueous media. In Chapter 3, the results of kinetic and mechanistic studies on the reversible binding of NO to (P8+)FeIII indicated that the reactivity of model water-soluble iron(III) porphyrins toward nitric oxide is tuned by the pH of the solution. In this context, the pH-induced change from diaqua- to monohydroxo-ligated porphyrin slows down the binding and release of NO and results in a mechanistic changeover in the coordination of NO to the iron(III) center. A comparison of experimental data obtained for (P8+)FeIII with those reported for other water-soluble (P)FeIII porphyrins clearly shows that the nature and charge of substituents in the porphyrin periphery affect the dynamics of both the binding and release of NO (extremely negatively charged porphyrin complex, (P16−)FeIII for Chapter 5). These effects are particularly evident in the reactions of diaqua-ligated porphyrins, where the effective electron density on the metal center influences the lability of coordinated water (which determines the rate of the “on” reaction) and the strength of the Fe–NO bond in the {FeNO}6 product (as evidenced by variation in koff values determined for the complexes studied). Other (mainly steric and electrostatic) factors are likely to influence the binding and/or release of NO in specific porphyrin systems. Despite a rather limited set of studied porphyrins, it can be concluded on the basis of the available data that reversible binding of NO to iron(III) porphyrins is very sensitive to the nature of axial ligands and the porphyrin periphery. The NO+ ligand in the {FeNO}6 nitrosyl formed by (P8+)FeIII undergoes subsequent reductive nitrosylation to form (P8+)FeIINO ({FeNO}7 nitrosyl) as the final reaction product. This reaction is addressed in detail along with data for related complexes in a following report. Chapter 4 presents that the results of kinetic and mechanistic studies on NO2- binding to (p8+)FeIII as a function of solution pH, in which pH-induced changes from diaqua- to monohydroxo-ligated porphyrin slows down and results in the mechanistic changeover in NO2- coordination to the iron(III) center. This observation is in line with our results for the reactivity of model water-soluble iron(III) porphyrins toward nitric oxide, as supported by previous studies. The rate of reductive nitrosylation consequently correlates with the formation of ferric nitrosyl porphyrin complex (P)FeII(NO+) from the binding of NO toward Fe(III) center depending upon the basicity of the porphyrin ring linked electrophilicity of the metal center. It can be concluded on the basis of the available data that electronic features of the Fe(III) center is primarily controlled by the nature of substituent (i.e., electron-releasing or electron-withdrawing meso substituents) and steric effect, overall charge of porphyrin ring and in addition electrostatic effects. In these studies, we report that the rate of reductive nitrosylation is varied over two orders of magnitude through appropriate modification of the porphyrin structure and investigated the nitrite catalysis on the observed reaction. There is still controversy regarding the separation of the inner- and outer- sphere mechanisms. Finally in Chapter 6, presents the spectrophotometric, kinetic and mechanistic studies for the reversible binding of NO toward the thiolate ligated iron(III) porphyrin, (SRP4+)FeIII, soluble in aqueous solution, as a function of pH and temperature. It can be concluded on the basis of the results obtained that the stopped flow kinetic measurements indicate that initial NO binding occurs at the ferric heme and is followed by heme reduction with nitrosothiol formation (SNO). The second order rate constants determined for the formation of (SRP4+)FeIII(NO) follow a dissociative mechanism, which involves the release of bound H2O and OH– axial ligands. Subsequently, rapid formation of the nitrosyl product, (SRP4+)FeIII(NO) undergoes subsequent reactions that involve direct attack of a second NO molecule on the thiolate ligand, and reaction with higher nitrogen oxides lead to the nitrosyl-nitrite complex are also involved. Lastly, reductive nitrosylation occurs over a longer time scale.