Interchange mechanisms dissociative

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). 

Ij mechanism. This is the dissociative interchange mechanism and is similar to the previons one in the sense that dissociation is stiU the major ratecontrolling factor. Therefore, we are stiU dealing with an SnI process. Nevertheless, differently from the D mechanism, no experimental proof exists that an intermediate of lower coordination is formed. The mechanism involves a fast onter sphere association between the initial complex and the... 

One mechanistic study7S worth describing here concerns the photoreactivity of [Pt(diethylenetriamine)Br]+. Photolysis in the presence of N02 accelerates the substitution of Br to yield [Pt(diethylenetriamine)N02 ]+. The reaction was shown to proceed via [Pt(diethylenetriamine)OH2 ]2+ which is rapidly anated by either Br or N02 The essential evidence rests in the fact that photolysis in basic solution yields only [Pt(diethylenetriamine)OH]+ even in the presence of N02 This result prompts the postulate that a dissociative interchange mechanism obtains as proposed for Co(CN) -. 6S ... 

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. 

H. M. Marques, J. C. Bradley, and L. A. Campbell, J. Chem. Soc., Dalton Trans., 2019 (1992). Ligand Substitution Reactions of Aquacobalamin Evidence for a Dissociative Interchange Mechanism. 

As with the dissociative mechanism, there are very few clear examples of associative mechanisms in which the intermediate is detectable. Most reactions fit better between the two extremes, following associative or dissociative interchange mechanisms. The next section summarizes the evidence for the different mechanisms. 

The dissociative nature of the reaction is evident from the magnitudes of the volumes of activation (+6.4cm3mol-1 for the substitution of ImH by H20). As in another report above the certainty of Id character is clouded by the fact that the values ofAV are composites of a precursor complex component and a contribution from the interchange step itself. A full D mechanism could not be categorically ruled out based on the properties of the system, and since the precursor formation step could contribute a negative volume term. Nevertheless a dissociative interchange mechanism was proposed as the more likely and a thorough discussion of mechanistic features, available at the time, of related systems was included. 

Cobalt(II) and nickel(II) species reacted with XCH3 to produce intermediate species containing metal-carbon cr-bonds, a process characterised by a small positive A Vthat indicated a dissociative interchange mechanism.295,296 Similar reactions of aquated Cr(II) ions with a series of 10 different aliphatic radicals exhibited an average AV of +4.3+ 1.0cm3mol-1, also evidence for an Id mechanism.297... 

The determined activation volumes for complex formation (AVs in cm3 mol-1, between +3.4 and + 6.3 for the different alkyl groups) and the large volumes of reaction support a dissociative interchange mechanism for [(H20)5CrR]2+ formation. Similar reactions were studied for the [(HzO)2ML] transition metal complexes with the CH3 radical, where L = nitrilotriacetate (nta) for M = Co(II) and Fe(II) 1,4,8,11-tetraazacyclotetradecane (cyclam) for M = Ni(II) [37-39]. The activation volumes calculated for the [ML(H20)(CH3)] complex formation, A Vs, and the suggested mechanisms are in accordance with those found for the water exchange on these aquo ions. 

For the aqua- and chelate-complexes described as Fe NO (S=3/2, derived from high-spin metal centers), water-exchange measurements and activation parameters support a dissociative-interchange mechanism, with high values of and variable values of koff as a function of the type of chelate (i31). Dissociative mechanisms have been proposed for the prototypical low-spin [Fe(CN)5(NO)] complex (S=l/2, described as Fe NO"), showing much lower values of feoff at The... 

A association mechanism, I interchan mechanism, associative interchange mechanism, dissociative interchange mechanism, -Id not distinguishable between 1 and 1 mechanisms. 

Cobalamin and Cobaloxime Substitution Reactions.— The substitution kinetics of vitamin Biaa with 8-azaguanine and 6-mercaptopurine have been investigated in aqueous media at 25 °C and ionic strength 0.1 M. The complex pH dependences are interpreted in terms of reactions of the base on aquocobalamin with protonated and deprotonated forms of the ligands. Rate constants are listed in Table 7 and are consistent with a dissociative interchange mechanism. A similar mechanism is likely in the substitution reaction of cyanocobalamin with [Fe(CN)e] . Both base on and base off forms of the coenzyme are reactive. 

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