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.
Ma
Hairong
H
Physical Biology Center for Ultrafast Science and Technology and Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
Wan
Chaozhi
C
Zewail
Ahmed H
AH
D003035:Cobalt / Q000737:chemistry*; D004058:Diffusion; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D013053:Spectrophotometry; D013696:Temperature; D013816:Thermodynamics
D008024:Ligands; D003035:Cobalt