Pseudo-first order kinetics, ligand substitution

The sequential equilibria in Equation (4) are characterized by 12 = 12/ 21 (often denoted as K0) and K23 = k23/k32, respectively. When Kn cannot be directly determined it is often estimated using the electrostatic Fuoss equation.215 Usually, it is only possible to characterize the kinetics of the second equilibrium of Equation (4) so that the overall equilibrium is expressed as in Equation (5) irrespective of the intimate mechanism of ligand substitution. The pseudo-first-order rate constant for the approach to equilibrium, kabs, is given by Equation (6)... 

Stopped-flow kinetics of the aqua-substitution reaction revealed that under pseudo-first-order conditions of an excess of the incoming nucleophile (8), the values of the observed rate constant ( obs) increase linearly with the concentration of the entering ligand (8). The plots exhibited no meaningful intercepts. 

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