Activation parameters for 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). 

Further kinetic results relevant to the ligand replacement reactions discussed in this section are those for dissociation of lanthanide(m)-cydta complexes. Nearly all the tervalent rare-earth cations were included in this investigation, along with the closely related complexes of scandium(ra) and yttrium(m). Dissociation rates vary with pH, but are unaffected by the addition of copper(n) ions. Thus there is no direct transfer of cydta from Ln + to Cu + in the manner of equation (7) (next section). The rate constants for the acid-dependent dissociation path show a large variation with the nature of the lanthanide cation they vary from 1291 mol s (at 25 °C, fjb = 0.1 mol 1 ) for lanthanum(ra) to 0.0171 mols for lutecium(m) the trend of rate constants is one of regular decrease as the ionic radius decreases. The rate constant of 0.019 1 mol s for dissociation of the scandium(in) complex is, however, much higher than would be expected from the ionic radius of Sc +. Activation parameters for dissociation of these cydta complexes are reported for the La", Gd, Dy Tm ", Lu , and Y compounds. ... 

The kinetic pattern and mechanism for the aquation of [Co(bipy)3] + have been elucidated as a by-product of investigating the vanadium(n) reduction of [Co(bipy)s] +. The aquation mechanism for [Co(bipy)s] + bears a considerable resemblance to that for the much studied [Fe(bipy)a] + cation. In particular there is the same need to invoke intermediates containing unidentate, and unidentate monoprotonated, bipy for both complexes. The activation parameters for dissociation are SH = 12.4 kcal mol and = —10... 

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