Optically probing the localized to delocalized transition in Mo-Mo mixed-valence systems.

Four thienylene (CHS) bridged Mo dimers, [Mo(DAniF)](μ-OOCCHSCOO) (DAniF = N,N'-di(p-anisyl)formamidinate), [Mo(DAniF)](μ-N(H)SCCHSCN(H)S), [Mo(DAniF)](μ-OSCCHSCSO) and [Mo(DAniF)](μ-SSCCHSCSS), have been synthesized and studied in terms of electronic coupling. The subtle structural differences between these compounds vary largely the extent of electron delocalization; consequently, a systematic transition from Class II to Class III via Class II-III is achieved, which is probed using spectral parameters of intervalence charge transfer (IVCT) absorption (band energy, intensity and shape) for the mixed-valence complexes. Significantly, the species in Class II-III displays a low energy, half cut-off and solvent-dependent IVCT band, while a high energy, less asymmetrical IVCT band is observed for the complex in Class III. These results give fresh and detailed understanding of the system transition.