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 S_N1 mechanism as a reaction path. The equilibrium quotient $[\mathrm{Co}({\mathrm{NH}}_3{)}_5{\mathrm{H}}_2{\mathrm{O}}^{\text{18+++}}\mathrm{](}{\mathrm{H}}_2\mathrm{O}\mathrm{)/(}\mathrm{Co}({\mathrm{NH}}_3{)}_5{\mathrm{H}}_2{\mathrm{O}}^{\mathrm{+++}}\mathrm{)\times (}{\mathrm{H}}_2{\mathrm{O}}^{18})$ was measured as 1.019±0.001.