Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump
Ligand exchange of hydrated metal complexes is common in chemical and biological systems. Using the ultrafast T-jump, we examined this process, specifically the transformation of aqua cobalt (II) complexes to their fully halogenated species. The results reveal a stepwise mechanism with time scales varying from hundreds of picoseconds to nanoseconds. The dynamics are significantly faster when the structure is retained but becomes rate-limited when the octahedral-to-tetrahedral structural change bottlenecks the transformation. Evidence is presented, from bimolecular kinetics and energetics (enthalpic and entropic), for a reaction in which the ligand assists the displacement of water molecules, with the retention of the entering ligand in the activated state. The reaction time scale deviates by one to two orders of magnitude from that of ionic diffusion, suggesting the involvement of a collisional barrier between the ion and the much larger complex.
Additional Information© 2008 by The National Academy of Sciences of the USA. Freely available online through the PNAS open access option. Contributed by Ahmed H. Zewail, July 15, 2008 (sent for review June 15, 2008). Published online before print August 25, 2008, doi: 10.1073/pnas.0806869105. This work was supported by the National Science Foundation. Author contributions: H.M., C.W., and A.H.Z. performed research; and H.M. and A.H.Z. wrote the paper. The authors declare no conflict of interest.
Published - MAHpnas08.pdf