Monoxo-bridged binuclear molybdenum(V) complex, $[Mo_2O_3;(bipy)_2;(NCS)_4]$ in aqueous acetone mixture produces the corresponding dioxo-bridged binuclear molybdenum(V) complex, $MoO_4(bipy)_2(NCS)_2$. The rate of conversion of $[Mo_2O_3(bipy)_2(NCS)_4]$ to $MoO_4(bipy)_2(NCS)_2$ has been measured by spectrophotometric method. The rate of formation of dioxo-bridged binuclear molybdenum(V) complexes with solvent water follows the rate law, rate = k$[Mo_2O_3(bipy)_2(NCS)_4];[H_2O]$. The reaction mechanism for the formation of dioxo-bridged complex is discussed. The observed negative volume of activation shows that the complex is strongly attracted to the solvent molecules at transition state.