Dissociation kinetics with simple ligands

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 ... 

Kinetic Scheme. Generally, metal ions in a solution for electroless metal deposition have to be complexed with a ligand. Complexing is necessary to prevent formation of metal hydroxide, such as Cu(OH)2, in electroless copper deposition. One of the fundamental problems in electrochemical deposition of metals from complexed ions is the presence of electroactive (charged) species. The electroactive species may be complexed or noncomplexed metal ion. In the first case, the kinetic scheme for the process of metal deposition is one of simple charge transfer. In the second case the kinetic scheme is that of charge transfer preceded by dissociation of the complex. The mechanism of the second case involves a sequence of at least two basic elementary steps ... 

This diagram illustrates many important points. First of all, it shows the mechanistic complexity that may be anticipated for even an apparently simple reaction, reductive elimination of acetone from 2.5. Second, it shows that RhCl(PPh3)3 is thermodynamically more stable than complex 2.5 by about 40 kJ/mol. However, complex 2.5 does not undergo spontaneous conversion to RhCl(PPh3)3 because it has sufficient kinetic stability (>92 kJ/mol). Third, the high free energy of activation is associated with a ligand dissociation step that precedes the reductive elimination step. The five-coordinated intermediate, once... 

There is a simple dissociative mechanism for [Fe(phen)3] + dissociation in acid, which is independent of acid concentration. However, when the ligand is the more flexible bipy, a monodentate protonated intermediate is kinetically significant. Here, the observed rate of dissociation increases with increasing acid concentration up to about 2moll, as shown in Scheme 3, and the reaction rate is given by equation (5). 

Both the onset of binding, when the radioligand is first applied, and offset, when dissociation is promoted, can be studied directly. The relevant kinetic equations relating to the simple bimolecular interaction of ligand with receptor are presented in Chapter 1, Section 1.3. 

It was found that CO exchange in (diphosphine)Rh(CO)2H complexes proceeds via the dissociative pathway [60], The decay of the carbonyl resonances of the (diphosphine)Rh(13CO)2H complexes indeed followed simple first-order kinetics. The experiments with ligand 20 at different 12CO partial pressure show that the rate of CO displacement is independent of the CO pressure. Furthermore, the rate is also independent of the (diphosphine)Rh(13CO)2H complex concentration, as demonstrated by the experiments with ligand 18. It can therefore be concluded that CO dissociation for these complexes obeys a first-order rate-law and proceeds by a purely dissociative mechanism. 

The decay of the carbonyl bands of the HRh(diphosphine)("CO)2 complexes with time follows simple first-order kinetics in all experiments. Plots of ln[HRh (diphosphine)("CO)2] vs. time are linear for at least two half-lives. Comparison of the rate constants, kj, obtained for ligands 32 and 33 [54] with those obtained for other xantphos ligands [52] shows that the CO dissociation rate for ligand 32 is in the same range as other ligands. The CO dissociation rate for ligand 33, however, proves to be four to six times higher. 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

In theory, K (i.e., kjki) should be the same whether determined by kinetic or equilibrium approaches. In practice, however, moderate differences arise that are often attributed to technical problems associated with separating bound from free rapidly without losing a significant proportion of receptor-toxicant complex. This problem is troublesome, particularly when estimating the amount bound at early time points in association or dissociation experiments, when the amount of bound ligand is changing rapidly. Large differences between the KD as determined in saturation and kinetic experiments, however, may indicate that the reaction is more complex than a simple bimolecular reversible reaction. 

The rates of the overall reactions can be related to the rate law expressions of the individual steps by using the steady state approximation. However simple kinetic data alone may not distinguish a mechanism where, for example, a metal and an olefin form a small amount of complex at equilibrium that then goes on to react, from one in which the initial complex undergoes dissociation of a ligand and then reacts with the olefin. As a reaction scheme becomes more complex such steady state approximations become more complicated, but numerical methods are now available which can simulate these even for complex mixtures of reactants. 

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