For Pd 2+ complex with MeCN, an associative mechanism has been suggested from NMR experiments and static electronic structure calculations. To determine the coordination number that corresponds to the TS, the fluctuations of the coordination number and the Pd-N bond lengths were analysed in the umbrella sampling trajectory that corresponds to the TS region in the PMF (Fig. ). The coordination number fluctuates around a mean value of 4.96, which yields a barrier of 15.7 kcal/mol in the PMF (black dot in Fig. ).