text: A further insight into the pre-equilibrium step and its equilibrium constant K comes from the Fuoss-Eigen equation proposed independently by Eigen and R. M. Fuoss:
  :K = (4πa/3000) x Nexp(-V/RT)
  Where a represents the minimum distance of approach between complex and ligand in solution (in cm), N is the Avogadro constant, R is the gas constant and T is the reaction temperature. V is the electrostatic potential energy of the ions at that distance:
  :V = zze/4πaε
  Where z is the charge number of each species and ε is the vacuum permittivity.
  A typical value for K is 0.0202 dmmol for neutral particles at a distance of 200 pm. The result of the rate law is that at high concentrations of Y, the rate approximates k[M] while at low concentrations the result is kK[M][Y].  The Eigen-Fuoss equation shows that higher values of K (and thus a faster pre-equilibrium) are obtained for large, oppositely-charged ions in solution.