The reaction of the water-soluble Fe-III(TMPS) porphyrin with CN- in basic solution leads to the stepwise formation of Fe-III(TMPS)(CN)(H2O) and Fe-III(TMPS)(CN)(2). The kinetics of the reaction of CN- with Fe-III(TMPS)(CN)(H2O) was studied as a function of temperature and pressure. The positive value of the activation volume for the formation of Fe-III(TMPS)(CN)(2) is consistent with the operation of a dissociatively activated mechanism and confirms the six-coordinate nature of the monocyano complex. A good agreement between the rate constants at pH 8 and 9 for the formation of the dicyano complex implies the presence of water in the axial position trans to coordinated cyanide in the monocyano complex and eliminates the existence of Fe-III(TMPS)(CN)(OH) under the selected reaction conditions. Both Fe-III(TMPS)(CN)(H2O) and Fe-III(TMPS)(CN)(2) bind nitric oxide (NO) to form the same nitrosyl complex, namely, Fe-II(TMPS)(CN)(NO+). Kinetic studies indicate that nitrosylation of Fe-III(TMPS)(CN)(2) follows a limiting dissociative mechanism that is supported by the independence of the observed rate constant on [NO] at an appropriately high excess of NO, and the positive values of both the activation parameters Delta S double dagger and Delta V double dagger found for the reaction under such conditions. The relatively small first-order rate constant for NO binding, namely, (1.54 +/- 0.01) x 10(-2) s(-1), correlates with the rate constant for CN- release from the Fe-III(TMPS)(CN)(2) complex, namely, (1.3 +/- 0.2) x 10(-2) s(-1) at 20 degrees C, and supports the proposed nitrosylation mechanism.