Ligand exchange, dissociation kinetics

If the metals bound in complexes exchange with biological ligands, the dissociation kinetics of these complexes, the ligand-exchange kinetics and the association kinetics with the biological ligands must be considered. Simple dissociation kinetics of complexes are related to their thermodynamic stability constants by the relationship ... 

A comparison with cross-linker 4a proves the underlying dynamics are controlled by metal-ligand dissociation. Ligand exchange kinetics for 4a are substantially faster than for 4b but the association thermodynamics are very similar, and the effect of those kinetics is dramatic. At 5% cross-linker, the dynamic viscosity of lOOmgmL 4a-PVP is only 6.7 Pa s, a factor of 80 less than that of the isostmctural network 4b PVP. Although the association constants are not identical, the effect of the thermodynamics would be to increase the viscosity of 4a PVP relative to 4b PVP, the opposite direction of that observed. The kinetics dominate even the extent of cross-linking 5% 4a PVP is less viscous by a factor of 5 than is 2% 4b PVP. 

The formation and dissociation kinetics for the complexation of Cu, Zn, Co and Ni with the quadridentate l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclotetradecane to give five-coordinate species have been reported.1203 The rate order (Cu > Zn > Co > Ni) is the same as that for H2O exchange, but the rates are much slower, probably owing to conformational changes occurring in the ligand. The dissociation is acid-catalyzed the five-coordinate species are found to be much less kinetically inert than four- or six-coordinate complexes. No macrocyclic effect was observed. 

The decline in optical rotation of the esters 2a and 2 b as a function of time is not affected by the addition of triphenylphosphine (Fig. 1). P(CgHs)3 in concentrations up to 10-fold excess with respect to complex concentration had no effect at all on the rate of racemization of 2a and 267. Deuteration experiments indicated that triphenylphosphine is indeed the ligand which dissociates. When the racemization of the manganese esters 2 is carried out in the presence of deuterated triphenylphosphine, P(C6D5)3 is incorporated into the complexes 16 [Eq. (14)]. A kinetic study showed that the rate of triphenylphosphine exchange is exactly equal to the rate of racemization8. ... 

Ligand Exchanges Ligand exchange mechanism may be associative (A), dissociative (D) or Interchange (Ia or Id). Kinetically inert complexes are formed by Cr3+ and Co3+ and by 4d- and... 

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