Credit: © 2008 National Academy of Sciences of the USA

Metal complexes are not just of interest to inorganic chemists: they serve as models for more complex biological systems that contain coordinated metal ions, including vitamins and cancer therapy. How they work is often governed by the exchange of ligands, with those already coordinated being replaced by ones in solution. These are often multi-step processes that are related to the hydration mechanism because they occur in water. Now, Ahmed Zewail and colleagues at the California Institute of Technology have studied1 the dynamics of the aquachloro series of cobalt(II) complexes using ultra-fast temperature jumps to control the equilibria between species.

The series ranges from an octahedral complex with six water ligands to a tetrahedral geometry with four chloride ions. The composition of the complex can be controlled by the temperature and chloride ion concentration, so changing these parameters results in the water and chloride ligands gradually being exchanged. Careful selection of reactant concentration and initial temperature enables the reaction to be tracked on a picosecond timescale. A brief laser pulse raises the temperature of the system by 5–10 °C in only 5–10 ps. The relaxation back to equilibrium is then investigated by measuring the absorption of another pulse of light, this time at the characteristic frequency for either the octahedral or the tetrahedral complex.

The sequential substitution process has a rate-limiting step when the configuration changes from an octahedral to a tetrahedral coordination. The high energy barrier creating this bottleneck is attributed to the large desolvation enthalpy resulting from removing two extra water molecules, as well as the entropy penalty of re-arranging the complex. Zewail and co-workers hope to extend the ultra-fast temperature jump method to examine the dynamics of biomolecular metal complexes, such as those involving proteins.