Activation Dissociative

The Ru surface is one of the simplest known, but, like virtually all surfaces, it includes defects, evident as a step in figure C2.7.6. The observations show that the sites where the NO dissociates (active sites) are such steps. The evidence for this conclusion is the locations of the N and O atoms there are gradients in the surface concentrations of these elements, indicating that the transport (diffusion) of the O atoms is more rapid than that of the N atoms thus, the slow-moving N atoms are markers for the sites where the dissociation reaction must have occurred, where their surface concentrations are highest. 

Fig. 26. Dissociation of the a-amylase — Biocarb complex. N is the fraction of dissociated (active) a-amylase...

The hypothesis of dissociative activation in Co(III) reactions stands the available tests well. It is therefore profitable to attempt to distinguish the D from the /j pathways. Fig. 7 summarizes the two pathways consistent with d activation, and the general methods for establishing the stoichiometric mechanism 1 are illustrated by the example of Co(NH3)50H2 ". ... 

These observations are, indeed, consistent with an associative activation, generation of a seven-coordinated intermediate (easier in the case of Mo and W than for Cr because their larger sizes produce less steric hindrance) by attack taking place directly upon the metal atom, that is, with an A or a limiting 5 2 mechanism, accompanied by a reaction sequence involving dissociative activation similar to scheme (24) above, viz. 

It is apparent that the existence of a second-order term in the rate expression does not of itself offer any proof of associative or dissociative activation, for there are two possible alternative mechanisms compatible. These are ... 

Lanthanum and samarium show virtually no NO dissociation activity even in the presence of Pt. These supports are not reducible and have no OSC property. The intrinsic NO dissociation activity of platinum is very weak, probably in reason of the low metal dispersion. The behavior of terbium oxide is more surprising. Although it is reducible in H2, it is unable to dissociate NO except in the presence of Pt. 

All A mechanisms must be associatively and all D mechanisms must be dissociatively activated. The interchange mechanisms (I) include a continuous spectrum of transition states where the degree of bondmaking between the entering ligand and the complex ranges from very substantial (Ia mechanism) to negligible (Id mechanism) and inversely... 

Activation volumes for aquation of Schiff base complexes [Fe(C5H4NCH=NHR)3]2+ (R = Me, Et, nPr, nBu) are between +11 and +14 cm3 mol-1 (107), and thus within the range established earlier (108) for (substituted) tris-l,10-phenanthroline-iron(II) complexes, viz. +11 to +22 cm3 mol-1. These positive values are consistent with dissociative activation. Kinetic studies of the reaction of a CH2S(CH2)3SCH2 -linked bis(terpy) ligand (L6) with [Fe(terpy)2]2+ showed a very slow two-step process. The suggested mechanism consisted of slow loss of one terpy, rapid formation of [Fe(terpy)(L6)], and finally slow displacement of the second terpy as the partially-bonded L6 becomes hexadentate (109). 

A new aminocarboxylate chelator of potential therapeutic value, 77(2-hydroxybenzy -Al -benzylethylenediamine-A Al -diacetate, reacts as LH4 and LH3 with Fe(OH)2q by dissociative activation with rate constants of 770 and 13 300 M s-1, respectively. These rate constants are similar to those for reaction of Fe(OH)2q with edta and with nta. These formation reactions are, however, considerably faster than with simple ligands of identical charge thanks to the zwitterionic properties of ami-nocarboxyl ates (334). 

It has previously been concluded that even in strong acidic solution, the dioxotetracyanoosmate(VI) complex cannot be protonated to form the oxo aqua complex or even the corresponding hydroxo oxo complex. The pA i and pKa2 values have been estimated to be substantially less than -1, which is also supported by the relationship between pKa values and 170 and 13C chemical shifts (Table II). Extreme slow kinetic behavior, as expected in the case of a +6 charged metal center for a dissociative activation exchange process, has been observed, with only an upper limit for the oxygen exchange determined (Table II). 

The aqua oxo complexes exchange by a dissociative activation mode. The evidence in support of this conclusion is given below. 

In the Re(V) and W(IV) aqua oxo complexes, comparison of both the complex formation of the [MO(OH2)(CN)4], by NCS ions and the water exchange (k iq) shows a relative increase in reactivity of approximately 3 orders of magnitude (Table II), which is in direct agreement with the previously (1, 2, 50) concluded dissociative mechanism. The increase in Lewis acidity of the Re(V) center compared to that of W(IV) is expected to result in a much less reactive system in a dissociative activated mode. 

Based on the above discussion it is concluded that the aqua oxo complexes undergo oxygen exchange via dissociative activation, whereas the hydroxo oxo complexes undergo oxygen exchange via an interchange or even an associative activation mode. 

The above, together with the fact that a fourfold variation of [CN ] for the [WO(OH)(CN)4]3 complex showed zero-order dependence on free cyanide concentration further points to a dissociative activation for the cyanide exchange process, also in the case of the mono oxo (classic 16-electron) [MO(X)(CN)4]m species. 

The volume of activation is probably the easiest parameter to understand conceptually. Consider again the water exchange of Cr(lll), Sec. 2.3.3. The AF"" value of —10 cm mol for Cr(H20)g+ indicates that an associative process pertains (If) since CrflljO) " plus one H2O will occupy more volume than the activated complex which has seven waters associated with the Cr. The volume of coordinated water has been estimated as anywhere between 5 and 9 cm moC, so that AK < — 9 cm mol" for an associative mechanism. Conversely, the value of +2.9 cm moC for water exchange on Cr(H20)50H + suggests a dissociative activation mode for the exchange.More success in interpreting AK values is likely for reactions in which the is small, or at least a small component of the overall AFjj,. The... 

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