Anation dissociative mechanism

The reactions of [Co(CN)s(H20)]2 with Nf6768 and SCN-68 have been reinvestigated and important differences found which indicate that the evidence in favour of limiting kinetics is not as clear as originally supposed. In particular, deviations from the simple second-order rate law for anation, rate = /c[Co(CN)5(H20)2 ][Y ], appear to be small and the question of reaction mechanism remains somewhat open at present. The effect of solvent on reaction between [Co(CN)5-(H20)]2 and N 69 and the small positive activation volumes for anation by Br-, I- and SCN-70 are indicative of a dissociative mechanism, but the existence of a long-lived five-coordinate intermediate has not been definitively established. 

Robb and co-workers have studied the kinetics of the anation of [Rh(H20)Cl5]2 by a variety of monoanions (equation 286 X = Cl, Br, I, N02, N3 or NCS).1213 The rates of anation are relatively independent of the nature of the anating ligand (k 2.9 ( 1) x 10-2s-1 at 35 °C), consistent with a dissociative mechanism. The rates of equilibration of the hexachloro, pentachloro and m-tetra-chloro anions were studied as a function of chloride concentration and the data were fitted to the kinetic expression kobs = + fcanutCl ]- The effect of pressure (1-1500 bar) was also mon-... 

Dehydration-anation reactions of coordination compounds. House [174] has proposed a general mechanism for the solid state dehydrations and anations of coordination compounds. The generation of point defects, analogous to Schottky or Frenkel defects (Chapter 1), is considered as the transition state. Reactions with low , values may occur by an S l dissociative mechanism and reactivity is influenced by the availability of free space in the crystal structure. Corbella and Ribas [175] found support for such mechanisms [174,176],... 

Kinetic parameters for reaction of me50-tetrakis(4-sulfonato)-porphinatodiaquarhodate(II) with the uncharged entering ligand thiourea are 298 = 0.047 dm mol" s", AH = 85 kJ mol", AS = +15 J K" mol", and A V = +11 cm mol". These values indicate a dissociative mechanism here, as in the earlier study of anation of this complex by thiocyanate. ... 

Preliminary studies have shown that d-[Co(en)3]l3 cannot be racemized by application of high pressure (3900 atm over a period of 70 hr). However, this compound does undergo racemization when heated, perhaps indicating a dissociative mechanism in this case. A more detailed study of the thermal racemization of ti-[Co(en)3]l3 is underway. Studies are also underway on racemization of t/-ds-[Co(en)2(NH3)2]Cl3. If this later compound racemizes by a dissociative mechanism, anation would be expected. Racemization without anation could be interpreted as evidence for an intramolecular mechanism, possibly the trigonal (Bailar) twist. 

A dissociative mechanism involving a square-pyramidal transition state is proposed for the aquation and chloride-ion anation reactions of fac- and mer-[RhCl3-... 

Solvents. Solvent transfer parameters have been used from time to time in diagnosis of mechanisms of organic reactions. Now their first use in an inorganic system has been discussed. Comparisons between kinetic results and solvent transfer activity coefficients for solvolysis of [Cr(NCS)e] in DMSO, DMF, and dimethylacetamide have been considered in terms of a dissociative mechanism. The DMSO results can be accommodated by a dissociative model for the transition state, but those in DMF and dimethylacetamide fit less well. It is interesting to compare these conclusions with, for instance, the dissociative mechanism proposed by the same authors for anation of the [Cr(DMSO)e] + and [Cr(DMF)6] + cations, and the associative mechanism suggested on the basis of the determined activation volume for exchange of DMSO with [Cr(DMSO)e +. ... 

Both thermal and photochemical substitution of PhCN by MeCN in [Cp2Mo(I)(NCPh)] occurs by a dissociative mechanism. Anation and acid-induced cleavage of Cr(III) alkyl complexes has been reported. " ... 

Throughout our discussion, we still have not answered the fundamental question What is the preferred mechanism of substitution reactions of octahedral coordination compounds The answer is that dissociative mechanisms are preferred. In this section, we investigate some of the evidence that supports such a conclusion. We cite three different types of reactions (1) the rates of exchange of water molecules, (2) anation reactions, and (3) aquation reactions. 

To conclude this section, we have seen thatvarious pieces of data from (1) rates of water exchange, (2) anations, and (3) various aquation reactions all seem to generally favor a dissociative mechanism for substitution reactions of octahedral compounds. 

Cobalt(m).— Anation of [Co(NH3)6(OH2>] + by sulphate or chloride proceeds by a dissociative interchange mechanism, in which the reactive species are ion-pairs and ion-triplets, The activation enthalpy for anation of [Co(NH3)6(OHa)] + by glycine is 29 kcal mol again a dissociative mechanism is operative. An earlier study of oxalate anation of [Co(en)a(OH2)2] in acidic solution is complemented by a study of oxalate anation of [Co(en)a(OH2)(OH)] + in basic solution. Several examples of anation reactions of bridged dicobalt complexes have been mentioned in the section on aquation of these complexes. ... 

In the course of establishing the kinetics of ruthenium(n)-catalysed anation of ruthenium(m) it was found that the activation enthalpies of substitution of chloride, bromide, or iodide at [RuCOHale] were all equal, within experimental uncertainty, at 19.8 0.4 kcal mol. This strongly suggests a dissociative mechanism for these substitutions. The redox reaction between ruthenium(ii) and perchlorate ion also appears to involve rate-determining substitution at the [Ru(OH2)e] + cation. ... 

---