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The kinetics of the binding and release of NO is studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The mechanistic interpretation is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature and pressure.

The article presents a study which examines the rate constants and activation parameters of water exchange mechanisms in Manganese (MN)IIIporhyrins with the use of 17Oxygen (O) Nuclear Magnetic Resonance (NMR) technique. The study shows that the water exchange rates are faster than the expected rates for trivalent metal ions. It also concludes that the faster water exchange rate is influenced by a Jahn-Teller effect and is independent of a poryphyrin and trans ligand.

Paramagnetic effects on the relaxation rate and shift difference of the 17O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [MnII(edta)(H2O)]2- (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMIR data. The results revealed a rapid water exchange reaction for the [MnII(edta)(H2O)]2- complex with a rate constant of kex = (4.1 ± 0.4) × 108 s-1 at 298.2 K and ambient pressure. The activation parameters ΔH‡, and ΔV‡ are 36.6 ± 0.8 kJ mol-1, +43 ± 3 J K-1 mol-1, and +3.4 + 0.2 cm³ mol-1, which are in line with a dissociatively activated interchange (Id) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [FeII(tmdta)(H2O)]2- are kex = (5.5 ± 0.5) × 106 s-1 at 298.2 K, ΔH‡ = 43 ± 3 kJ mol-1, ΔS‡ = +30 ± 13 J K-1 mol-1, and ΔV‡ = +15.7 ± 1.5 cm³ mol-1, and those for [MnII(tmdta)(H2O)]2- are kex = (1.3 ± 0.1) × 108 s-1 at 298.2 K, ΔH‡ = 37.2 ± 0.8 kJ mol-1,ΔS‡ = +35 ± 3 J K-1 mol-1, and ΔV‡ = +8.7 ± 0.6 cm³ mol-1. The water containing species, [FeIII(tmdta)(H2O)]- with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [FeIII(tmdta)]-. The kinetic parameters for [FeIII(tmdta)(H2O)]- are kex = (1.9 ± 0.8) × 107 s-1 at 298.2 K, ΔH‡ = 42 ± 3 kJ mol-1, ΔS‡ = +36 ± 10 J K-1 mol-1, and ΔV‡ = +7.2 ± 2.7 cm³ mol-1. The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [NiII(edta′)(H2O)]2- and [NiII(tmdta′)(H2O)]2- is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)]n+ and the aqua-pentadentate forms [M(L′)(H2O)]n+ of the NiII- edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [NiII(edta′)(H2O)]2- (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 ± 0.2) × 105 s-1 at 298.2 K and ambient pressure was measured, and the activation parameters ΔH‡, ΔS‡, and ΔV‡ were found to be 34 ± 1 kJ mol-1, -27 ± 2 J K-1 mol-1, and +1.8 ± 0.1 cm³ mol-1, respectively.… [ABSTRACT FROM AUTHOR]

The reaction of sodium nitrite with sodium dithionite was studied in the presence of cobalt(II) tetrasulfophthalocyanine, COII(TSPc)4-, in aqueous alkaline solution. The overall mechanism comprises the reduction of CoII(TSPc)4- by dithionite, followed by the formation of an intermediate complex between COI(TSPc)5- and nitrite, which undergoes two parallel subsequent reactions with and without nitrite as a reagent. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction was found to be ammonia. Contrary to those found for the catalytic reduction of nitrite, the products of the catalytic reduction of nitrate were found to be dinitrogen and nitrous oxide. The possible catalytic reduction of nitrous oxide was confirmed by independent experiments. The striking differences in the reduction products of nitrite and nitrate are explained in terms of different structures of the intermediate complex between CoI(TSPc)5- and substrate, in which nitrite and nitrate are suggested to coordinate via nitrogen and oxygen, respectively.