Rate constants exchange

This is the origin of the various values for self-exchange rate constants. We may now attempt to rationalize some of these in terms of the /-electron configurations of the various oxidation states. Consider the self-exchange rate constants for some iron complexes. 

The above processes are related to the disproportionation reaction of Np(V) which has also been investigated. However exchange rate constants and activation parameters are different from those predicted from disproportionation data. ... 

Fig. 1. Mean lifetimes of a single water molecule in the first coordination sphere of a given metal ion, th2o> and the corresponding water exchange rate constants, h2o- The tall bars indicate directly determined values, and the short bars indicate values deduced from ligand substitution studies. References to the plotted values appear in the text.

Generally the magnitudes of solvent exchange rate constants increase in the sequence NH3 > H20 > DMF > MeCN > MeOH, which is largely independent of the nature of the metal ion (108,109). This sequence cannot be readily identified with specific characteristics such as dielectric constant, donor number, electric dipole moment, or stereochemistry, and it appears to reflect the overall solvent characteristic. There may be a correlation between the AH for solvent exchange on [M(solvent)6]2+ and the heat of dissociation of solvent from this species (110). 

Fig. 13. Absolute partial molar volumes, Vab8°, of [Ln(H20) P in aqueous LnCl3 solutions (301) (closed circles), compared with the calculated Vabs° values (4, 42) for [Ln(H20)8]3+ and [Ln(H20)9]3 indicated by the upper and lower solid curves, respectively. Interchange rate constants, kj (298 K) (310), for the substitution of S042 on [Ln(H20) ]3+ are shown as open squares, and water exchange rate constants, fcn2o (298 K) (311, 312), for [Ln(H20)8]3+ are shown as open circles. 

Fig. 5. Plot of apparent electron self exchange rate constants kf P, derived from polymer De values for films containing the indicated metals, mixed valent states, and ligands, all in acetonitrile, using Equation 2, vs. literature heterogeneous electron transfer rate constants k° for the corresponding monomers in nitrile solvents. See Ref. 6 for details. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

This means that there is a possibility of forming a hexa-coordinate intermediate. The finding that the exchange rate constant for Re(VII) is higher than that for Tc(VII) suggests that Re04 can expand its coordination number from 4 to 6 more readily [7,17]. 

Line shape analysis was performed for the binding of some dihydroxycholate ions to /1-cyclodextrin.205 The dihydroxycholates show different 18-CH3 signals for the complexed and free dihydroxycholate ions. To extract exchange rate constants from the NMR spectra, a complete line-shape simulation was performed. The simulation requires input of the chemical shift difference between the two sites, the line width in the absence of exchange, the residence time in each site (thg and Tg), and the relative population (fHG and fG) of each site (Equation (11)). The values were varied until the simulated and experimental spectra could be superimposed. The dissociation rate... 

Measuring the pressure dependence of the exchange rate constant leads to activation volumes, AV and this technique has become a major tool for the mechanistic identification of solvent exchange mechanisms (8,16,17). In the last 25 years high-pressure, high-resolution NMR probes were developed which allow the application of all NMR techniques described to pressures up to several hundreds of mega Pascals (18). 

The increase of the exchange rate constants for solvent exchange on [MS6]2+/3+ in the sequence MeOH < MeCN < DMF < H20 is independent of the metal ion and cannot be linked to specific properties like dielectric constant, donor number or size. It appears to reflect the overall characteristics of the solvent. 

Effect of Non leaving Ligands on the Solvent Exchange Rate Constants on First-Row Divalent Transition-metal... 

Rotzinger then evaluated and H t as a function of the distance between the two reactant metal centers. He used the Fuoss equation to calculate the ion-pairing equilibrium constant to form the precursor complex at these internuclear distances. Assembly of these data then allowed the calculation of the self-exchange rate constants as a function of the internuclear distance in the transition state, the maximum rate being taken as the actual rate. 

---