Dissociative interchange

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

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). 

Kinetics and activation parameters for NO reactions with a series of iron(II) aminocarboxylato complexes have been obtained (Table II) in aqueous solution (31). Rate constants for these reactions ranged from 105 to 108M-1s-1 for the series of iron(II) complexes studied. The reactions of NO with Fen(edta) (edta = ethylenediaminetetraacetate) and Fen(Hedtra) (Hedtra = hydroxyethylenediaminetriacetate) yielded activation volumes of +4.1 and +2.8 cm3 mol-1, respectively and were assigned to a dissociative interchange (Id) mechanism (31b). All of the iron(II) aminocarboxylato complexes studied followed a similar pattern with the exception of the Fen(nta) (Nta = nitriloacetic acid) complex which gave a AV value of —1.5 cm3 mol-1. The reaction of this complex with... 

NO was proposed to occur through an associative interchange mechanism (Ia). A recent study of the formation of [Fe(H20)5(N0)]2+ from aquated ferrous ion (30) resulted in activation parameters similar to those for chelated ferrous ion (Table II). The small and positive activation volumes were used to assign the reaction mechanism as dissociative interchange in character. 

Pressure-decelerated water exchange reactions and evidence for dissociative interchange in corresponding net substitution reactions... 

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... 

In the Id mechanism, dissociative interchange, the transition state involves extensive elongation of the M-L bond, but not rupture. 

The LFMM reaction barriers correlate very well with experiment and even predict a mechanistic changeover from associative interchange for Mn(II) to dissociative interchange for Fe(II) and Co(II) in agreement with the change in signs of the volumes of activation (Fig. 33). 

For the much slower coordination reactions of several ligands L to [Fein(CN)5H20]2- (7 = ca. 10-4-10-7 M-1 s-1), a dissociative interchange (7d) mechanism has been suggested (3 5,49). The data for reaction (3), particularly the negative values of the entropy and activation volume do not agree with the I( mechanism, which predicts positive values in both cases. On the other hand, Fig. 2 describes the proposed mechanism. 

In coordinatively saturated metal hydrides, such as the HM(CO)s (M => Cr, Mo, W) derivatives, formation of the four-centered transition state for C02 insertion (Scheme 1) may proceed with or without CO loss and concomitant coordination ofC02 at the metal center. That is, C02 insertion may occur by means of dissociative (D) or dissociative interchange (Id) processes, or an associative interchange (Ia) process (47, 48). In either instance an acid-base interaction between the anionic hydride ligand and the electrophilic carbon center of carbon dioxide as represented in 6 may occur prior to formation of the four-centered transition state depicted in Scheme 1. An interaction of this type has been observed for these HM(CO)j derivatives with Lewis acids such as BH3 (49). 

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