Reactions of water soluble iron(II) and cobalt(II) porphyrins with nitric oxide. Implications for the reactivity of NO and biologically relevant metal centers

The entropies and volumes of activation (Delta S double dagger(on) and Delt a V double dagger(on)) for the fast reactions (k(on) > 10(9) dm(3) mol(-1) s(-1)) reaction of NO with the water soluble iron(II) porphyrin complexes F e-II(TPPS) [TPPS = tetrakis(4-sulfonatophenyl)porphinato] and Fe-II(TMPS) [ TMPS = tetrakis(sulfonatomesityl)porphinato] are small and positive indicat ing that these reactions are dominated by the diffusion processes as is the analogous formation of the cobalt(II) complex Co-II(TPPS)(NO).