Rate constants for Ru

Table 5.6 Calculated Values for the Self-Exchange Rate Constant for Ru(H20)6+ + using (5.35) and Data for a Number of Cross-Reactions (from Ref. 43)...

Figure 41 (A) Rate constants for Ru(bpy)2+ luminescence decay versus concentration...

Figure 2.10. Correlation of theoretical and experimental maximum rate constants for Ru-modified cytochrome c derivatives. The numbers on the figure are related to the number of modified His groups. (Tezcan et al. 2001) Reproduced with permission.

Second, the surface chemistry, microstructure, and electronic properties can influence the electrode reaction kinetics and mechanisms for redox systems to differing extents [1-5, 12-15]. Good electrical conductivity is essential for all electrodes, so the electronic properties affect the electrode reaction kinetics of all redox systems. The surface chemistry, on the other hand, can influence the kinetics and mechanisms for some redox systems more than others. For example, surface carbon-oxygen functionalities on sp carbon electrodes increase the heterogeneous electron-transfer rate constant for aquo Pe-l-3/-i-2 exert little influence on the rate constant for Ru(NH3)6" /+ [40]. It is important to note that if the goal is to understand structure-function relationships at carbon electrodes, then there needs to be a judicious choice of redox systems to probe this relationship with. 

Similar experiments were performed to measure the ET rate constants for the reaction between ZnPor in benzonitrile and aqueous reductants, Ru(CN)g, Mo(CN)g and FeEDTA (where EDTA denotes ethylenediaminetetra-acetic acid). Although the... 

Fig. 8 Long range charge transport between dppz complexes of Ru(III) and an artificial base, methyl indole, in DNA. The methyl indole is paired opposite cytosine and separated from the intercalating oxidant by distances up to 37 A. In all assemblies, the rate constant for methyl indole formation was found to be coincident with the diffusion-controlled generation of Ru(III) (> 107 s )> indicating that charge transport is not rate limiting over this distance regime...

For CO, this choice gives a rather different value of 2.20 x 10 11 cm3 molecule rs 1 (the rate constant for the addition of CO to 3Fe(CO)3 (53,58)), which is approximately five times smaller than the value calculated using TST. For H2, the value chosen for add is 1.03 x 10 n cm3 molecule-1 s-1, which corresponds to the rate constant for addition of H2 to the Ru(dmpe)2 complex (76). In this case, the variational TST value was not very different. In both cases, the error involved in determining add can reliably be estimated to be smaller than that involved in calculating other properties. 

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 rate constant for aquation of the 4,4-dithiodipyridine complex [Ru(NH3)5(dtdp)]2+, = 4.5x10 5s is almost exactly the same as that for [Ru(NH3)5(py)]2+, and only slightly slower than that for dissociation of [Ru(CN)5(dtdp)]2+ in aqueous DMSO. Dissociation of [Ru(CN)5 (dtdp)]2+ is, unusually, only 10 times slower than that of its iron(II) analogue [Fe(CN)5(dtdp)]2+ (159). Rate constants for formation and dissociation of [(H3N)5Ru(NCpy)Fe(CN)5] were given and referenced in Table IV (Section II.D.5) a useful summary of rate constants for formation and dissociation of pentacyanoruthenates (D mechanism in all cases) forms part of a review of pentacyanometallates(II) [M(CN)5L]", M = Fe, Ru, and Os (134). 

The reactions of [Ru(edta)(H20)] with adenine, and with adenosine and its phosphate derivatives amp, adp, and atp, involve ring closure in a reversibly-formed intermediate containing a unidentate incoming ligand. Both formation (cf. above) and aquation of the intermediates are, on the evidence of the AS values for the amp, adp and atp systems, associative. Rate constants for ring closure are between 0.6 and 4.4 s-1 (165). 

A suggestion for the existence of at least three populations of adsorbed Ru(II) comes from the time evolution of the transient UV-vis absorption spectra. These spectra show that the recovery of the initial Ru(II) spectra occurs with two parallel (fast and slow) second-order components. The rate constants for these two components show remarkably little dependence on the nature of the coordinating ligands. Both of these components are attributed to recombination of the adsorbed Ru(III) with the injected electrons. Thus there is a small luminescent population of Ru(II) that does not engage in electron injection, a non-luminescent population that injects and recombines rapidly, and a third population that injects rapidly and recombines slowly. A detailed picture of the nature of the ligand/semiconductor interaction and how it affects the behavior of these systems awaits further study. 

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H20)6 +-Fe(H20)6 +> Ru(NH3)62+-Ru(NH3)63+ and Ru(bpy)32+-Ru(bpy)33+ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 

It is too early to draw any conclusions about the insensitivity of the rate constants to the nature of the dipeptide. Differences among the peptides seem to be revealed more in the temperature dependencies of the rate constants for intramolecular electron transfer than in the magnitude of the rate constant itself. Work is in progress on the synthesis of other di-, tri-, and tetra-peptides separating Co(III) and Ru(II) in order to examine the temperature dependence of the intramolecular rate... 

The self-exchange rate constant for reaction 14, when M is Ru and when an excited Ru(II) product is formed, has been esti-... 

---