Kinetics interchange mechanism

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

Chloride substitution kinetics of [NiniL(H20)2]3+, and its protonated form [NiniL(H20)(H30)]4+, where L = 14 -oxa-1,4,8,11 -tetraazabicy-clo[9.5.3]nonadecane, yield fyn20)2 = 1400 M 1s 1 and (h2o)(H3o+) = 142M 1s V The reverse, chloride dissociation, reactions have (h2o)ci = 2.7 s 1 (h3o+)ci = 0.22 s All four reactions occur through dissociative interchange mechanisms, like earlier-studied substitutions at nickel(III) (359). 

An intermediate situation between the A and D extremes is an interchange (/) mechanism. Here, the breaking of the E-Y bond and the formation of the E-Z bond occur simultaneously as a concerted process at no time is there an intermediate with both a fully-formed E-Y and an E-Z bond. The transition state is a complex which can be described as Y... EX ... Z. Like the A mechanism, the kinetics are second order, but the intermediate (or activated complex) has no real existence as a static entity in the reaction mixture unlike the intermediate in the A mechanism, there is no energy barrier to the breakdown of Y... EX ... Z, which appears as a maximum on the profile of energy... 

This reaction was followed by a second slower reaction that was not characterized in the report. An analysis of the overall results showed that the influence of steric or electrostatic effects on kinetic parameters is not necessarily minor. The dynamics of binding of NO to the diaqua species is mainly tuned by modulation of electron density on the iron center by the porphyrin macrocycle. A volume profile for NO binding based on values ofAV on and A V 0ff of + 1.5 and + 9.3 cm3/mol, respectively, maybe interpreted as an interchange mechanism for the on reaction, as the Fenl-H20 bond is decidedly stabilized (see Figure 7.15a). The volume of activation for the off reaction indicates a less dissociative mode of activation compared with NO release from other porphyrins. 

The rates of ligand binding and ligand dissociation kon and off) can be determined by stopped-flow methods. For example, the kinetics of the binding and release of NO with an iron(III) porphyrin complex was stndied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniqnes. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters and for the on and off reactions. 

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