The Exchange of Water between Co(NH3)5H2O+++ and Solvent

The rate of exchange of water between Co(NH3)5H2O+++ and solvent has been measured by a method involving isotopic assay of the bound water. The half-time for the exchange is independent of salt concentration and of acidity over a range of these variables and is 24.5 hr at 27°. The half-time is increased by SO4=, indicating that complex ion formation competes with the exchange process and thus supporting an SN1 mechanism as a reaction path. The equilibrium quotient [Co(NH3)5H2O18+++](H2O)/(Co(NH3)5H2O+++)×(H2O18) was measured as 1.019±0.001.