OUTER-SPHERE METAL-TO-LIGAND CHARGE-TRANSFER IN ORGANOMETALLIC ION-PAIRS

A series of organometallic salts which comprise a fac-(b)Re-I(CO)(3)(p y)(+) cation (b = 4,4',5,5'-tetramethyl-2,2'-bipyridine (tmb), 2,2'-bi pyridine (bpy), or 4,4'-dicarbomethoxy-2,2'-bipyridine (dmeb); py = py ridine) paired with the Co(CO)(4)(-) anion have been prepared and subj ected to photophysical study. In nonpolar solvents the salts feature a broad, low-intensity ion-pair charge transfer (IPCT) absorption band. The energy of the IPCT band decreases with the LUMO energy of the dii mine ligand, suggesting that the orbital basis of the transition is d (Co) --> pi (b). An X-ray crystal structure of [(bpy)Re-I(CO)(3)(py)( +)][Co(CO)(4)(-)] (2a) reveals that the anion occupies a lattice posit ion which is directly below (or above) the plant defined by the bpy li gand, which supports the d (Go) --> pi (b) IPCT assignment. Luminescen ce studies of the salts indicate that the d pi (Re) --> pi (b) metal- to-ligand charge transfer (MLCT) excited state is quenched by reductiv e electron transfer from Co(CO)(4)(-). Nominally IPCT and MLCT excitat ion of the organometallic ion pairs afford the same geminate radical p air, [(b(.-))Re-I(CO)(3)(py),Co(CO)(4)(.)]. However, laser flash photo lysis studies reveal that the rate of charge recombination within the geminate radical pair is significantly slower when MLCT excitation is applied. The slower rate of charge recombination is attributed to the fact that triplet state geminate pairs are produced via the triplet ML CT excited state manifold.