Ultrafast Ligand Exchange: Detection of a Pentacoordinate Ru(II) Intermediate and Product Formation

The ultrafast kinetics of ligand exchange of cis-[Ru(bpy)(2)(CH3CN)(2)](2+) were measured in H2O and CH3CN. The formation of the (MLCT)-M-3 excited-state and a five-coordinate intermediate are observed in both solvents within 2 ps after excitation (310 nm, fwhm similar to 300fs). The (MLCT)-M-3 excited-state undergoes vibrational cooling (5-6 ps), then decays to regenerate the ground-state with a lifetime of similar to 50 ps. In CH3CN, ligand recombination takes place in 28 ps, while the formation of cis-[Ru(bpy)(2)(CH3CN)(H2O)](2+) in H2O takes place with tau = 77 ps.