ELECTRONIC DISTRIBUTION IN THE METAL-TO-LIGAND CHARGE-TRANSFER (MLCT)EXCITED-STATES OF O)(3)RE-I(4,4'-BPY)RE-I(CO)(3)(4,4'-(X)(2)BPY)](2- APPLICATION OF TIME-RESOLVED INFRARED AND RESONANCE RAMAN SPECTROSCOPIES() (X = H, CH3) )

Ground- and excited-state resonance Raman and infrared spectra (354.7 nm excitation, in acetonitrile at 298 K) have been measured for Re-I(4 ,4'-bpy)Re-I(CO)(3)(4,4'-(X)(2)bpy)](2+)(bpy is 2,2'-bipyridine; 4,4'- bpy is 4,4'-bipyridine; X = H, CH3). The lowest lying excited states i n these molecules are metal-to-ligand charge transfer (MLCT) in charac ter. The spectra provide answers to two questions: Is the acceptor lig and for the excited electron 4,4'-bpy or 4,4'-(X)(2)bpy? Are there loc alized (Re-II-Re-I) or delocalized (Re-1.5-Re-1.5 5) sites in the mixe d-valence excited states formed by MLCT excitation? Application of tim e-resolved resonance Raman (TR3) spectroscopy demonstrates that 4,4'-b py is the acceptor in the lowest lying MLCT excited state(s) of [(dmb) (CO)(3)Re(4,4'-bpy)Re(CO)(3)(dmb)](2+) (dmb = 4,4'-(CH3)(2)bpy) and t hat there is an equilibrium between 4,4'-bpy and bpy as acceptor in [( bpy)(CO)(3)Re(4,4'-bpy)Re(CO)(3)(bpy)](2+). For both complexes, the a ppearance of two sets of overlapping v(CO) bands in the excited state by time-resolved infrared (TRIR) measurements is consistent with the l ocalized description, Re-II-Re-I.