Kinetics and mechanism of nitric oxide disproportionation upon reaction with ruthenium(II) porphyrin carbonyls: Evidence for dinitrosyl intermediates

The kinetics of the reaction of nitric oxide with the ruthenium porphyrin c arbonyl complexes Ru(P)(CO) (P octaethylporphyrin (OEP) or tetra-m-tolylpor phine (TmTP)) to give the respective nitrosyl nitrito complexes Ru(P)(NO)(O NO) plus N2O were studied by stopped flow spectrophotometric techniques. Te mporal spectral changes proceed in two stages via formation of an easily di scernible intermediate X. The rates of both stages are first order in ruthe nium concentration. In the NO concentration range 40-400 mu M, the rate law of the first stage is first order in [NO], but the slower second stage dis plays a rate law second order in [NO]. In the presence of some added CO, it is possible to demonstrate that the equilibrium between Ru(P)(CO) and X ha s a second-order dependence on [NO]. As a consequence, it is concluded that X is the dinitrosyl complex Ru(P)(NO)(2). No other intermediates develop s ufficient concentrations to be observable by the stopped flow method. The k inetics behavior of this system is interpreted in terms of a rate-limiting step in the first stage involving CO dissociation from a reactive intermedi ate Ru(P)(CO)(NO) formed in very low concentrations by the reversible addit ion of NO to Ru(P)(CO). Several hypothetical mechanisms for the second stag e are considered, and it is concluded that the evidence is most consistent with a pathway where the Ru center serves as a template to assemble an olig omer of NO which decomposes to nitrous oxide plus coordinated nitrite.