Anation step

This equation relates to the operation of the branched reaction path of Scheme 1, in which incoming nucleophile Y replaces resident ligand X. With weak nucleophiles and polar solvents, it is common to observe total dominance by the k solvation path, whereas examples dominated by the 2 nucleophile-dependent route are found when these factors are reversed. For this simple rate law to hold, it is necessary, of course, for the second step of the solvento pathway (often called the anation step when Y is an anion), in which Y replaces coordinated solvent, to be fast and many independent rate determinations of such anation reactions confirm that this is generally so. 

Similar kinetic rate laws have recently been found for the reaction between [PdCl(dien)]+ and inosine, i, k-i, and ks all being of similar magnitude (32). With adenosine instead of Ino, A3 is diminished to the extent that Ai and A 1 can be treated as a pre-equilibrium, and with the unreactive nucleobase uridine, even the reverse anation step, A 3, can again make a contribution. This type of complication has been found in reactions of palladium-dien complexes with the common buffering agent tris(hydroxymethyl)aminomethane (33), which acts as a poor nucleophile. 

Althou this route seems quite elegant, there are some disadvantages. From the point cd economic feasibility, it would be more attractive to start with a substrate that is a precursor of the optically piue a-disubstituted amino add. In the mettiod described, first the D,L-amino add has to be prepared and protected. After the enz)anatic step deprotection is necessary to obtain Ae D-amino add and the total process is circuitous. Furthermore, the unwanted isomer cannot be racemised. 

Complete volume profiles for the reactions of Eq. (6) have been obtained by separate examinations of the aquation and anation steps. Figure 5.1 shows the results. The activation volumes of -10 to -12cm mor are generally taken to... 

In proposed mechanism I, the loss of water from the complex is the rate-determining step, but removal of water from the coordination sphere of the metal ion should be independent of the nature of the anion that is not part of the coordination sphere of the metal ion. On the other hand, if mechanism II is correct, the entry of X into the coordination sphere of the metal would be dependent on the nature of the anion, because different anions would be expected to enter the coordination sphere at different rates. Because there is an observed anion effect, it was concluded that the anation reaction must be an Sn2 process. However, it is not clear how a process can be "second-order" when both the complex cation and the anion are parts of the same formula. As discussed in Chapter 8, it is not always appropriate to try to model reactions in solids by the same kinetic schemes that apply to reactions in solutions. 

For the low-spin t2g aqua ions [Ru(H20)6]2+, [Rh(H20)6]3+, and [Ir(H20)e]3+ a d-activation mode would a priori be predicted. The approach of a seventh water molecule towards a face or edge of the coordination octahedron is electrostatically disfavored by the filled t2g orbitals which are spread out between the ligands. Rate constants for anation reactions of Cl-, Br-, and I- on [Ru(H20)e]2+ are very similar, indicating identical steps to reach the transition state, namely the dissociation of a water molecule (130). An extension of this study to a large variety of ligands demonstrated clearly that the rate determining... 

The isomers cis- and trans-[Cr(CN)2(NH3)4]+ as well as m-[Cr(CN)(NH3)4DMS0](G04)2 have also been synthesized by the anation reactions shown in Scheme 29.325 While anation in DMSO is accompanied by stereochemical change, the reaction in H20 is stereoretentive. The dicyano complexes undergo H+-assisted thermal aquation, involving the successive loss of the CN- ligands. The trans complex is about ten-fold more reactive in the first step than the cis, an observation attributed to the trans labilizing effect of CN-. 

Monnard, P.A., and Deamer, D.W. 2002. Membrane self-assembly processes Steps toward the first cellular life. Anat. Rec. 268 196-207. 

The pH dependence of cytochrome c oxidation-reduction reactions and the studies of modified cytochrome c thus demonstrate that the coordination environment of the iron and the conformation of the protein are relatively labile and strongly influence the reactivity of the metallo-protein toward oxidation and reduction. The effects seen may originate chiefly from alterations in the thermodynamic barriers to electron transfer, but the conformation changes are expected to affect the intrinsic barriers also. One such conformation change is the opening of the heme crevice referred to above. The anation and Cr(II) reduction studies provide an estimate of 60 sec 1 for this process in Hh(III) at 25°C (59). To date, no evidence has been found for a rapid heme-crevice opening step in ferrocytochrome c. 

Logic would suggest that the second step of the solvento route, the fast anation reaction, should also be associative because it qualitatively resembles the other two reactions of Scheme 1. This appears to be the case. Several such reactions have been independently examined. Reactions (6) are typical (24, 25). 

It is not known if anation by the nitrite ion is the initial step or if the nitrosonium ion is formed by the equilibrium... 

Continuing with this system, consider the anation reaction of the aqueous intermediate, [Co(NH3)5H20]3+, equation (4.10). This step is also expected to be dissociative in nature. Experimentation shows, however, a surprising dependence on the incoming Y the rate = kf [Co(NH3)5H203+][Y-]- How can we account for this dependence on Y- There are several possible mechanisms ... 

Substitution reactions of [Ru(NH3)jOH2] have been reported (Table 3). Thermolysis of [Ru(NH3)jOH2]X3 in the solid state takes place via a two step process to give initially [RuX(N-H3)50H2] in a rate-determining step, with subsequent formation of [RuX(NH3)5] " (X = Cl", Br", 1", N03") the activation energies and entropies for these reactions have been asses-sed. In solution, anation of [Ru(NH3)sOH2)] shows second order kinetics.The photolysis of [Ru(NH3)5(OH2)] has been reported. ... 

---