Infrared activity of symmetric bridging ligand modes in pyrazine-bridged hexaruthenium mixed-valence clusters

A fully symmetric (A(g)) vibrational mode of pyrazine is observed in the infrared spectrum of four pyrazine-bridged hexaruthenium mixed-valence complexes with varying degrees of electronic coupling between clusters. Deuteration of the bridging pyrazine ligand and the accompanying shift in frequency confirm the assignment of this mode. Previous observation of infrared line coalescence in the carbonyl stretching region assigns all of these complexes to Robin-Day class II (partial localization of charge) on the picosecond time scale. The infrared activity of the fully symmetric bridging ligand mode could provide a complementary assignment of these complexes to class II on a faster, femtosecond time scale. However, the extinction coefficient for this band is much greater than that observed in similar asymmetric, non-mixed-valence complexes and suggests that its strong IR activity is due to vibronic enhancement rather than electronic asymmetry.