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Three symmetric Mo-2 dimers [Mo-2(DAniF)(3)](mu-E2CCE2) [Mo-2(DAniF)(3)] (DAniF = N,N'-di(p-anisyl)-formamidinate) with oxalate (E = O) or the thiolated derivatives (E = O or S) as bridging ligands have been synthesized, and the optical properties of the mixed-valence (MV) derivatives obtained by one-electron oxidation studied within the framework of the vibronic two-state model. These Mo-2-Mo-2 systems are effective models for testing electron transfer theories, with the delta electrons of the Mo-2 fragments that are responsible for the redox and optical properties of the MV complex being well isolated from other metal d and ligand pi-type orbitals. This in turn gives rise to unique, well-resolved metal to ligand (MLCT) and intervalence charge transfer (IVCT) absorption bands that permit accurate analyses based on band shape. In the series [Mo-2(DAniF)(3)] (mu-E2CCE2)[Mo-2(DAniF)(3)], the extent of electron delocalization between the Mo-2 cores increases with increasing number of sulfur atoms, E, in the bridge. Higher-energy IVCT absorption bands are observed for the more strongly coupled complex, but in contrast to the predictions from the two-state model, the IVCT band becomes more symmetric in shape as the electronic coupling constant increases beyond the Class III border and 2H(ab)/lambda >> 1. Thus, the oxalate-bridged complex (E-2 = O-2) is situated on the Class II-III borderline, while the two thiolated species are well placed deep into Class III, where novel optical behavior can be observed. The electronic coupling matrix elements (H-DA) estimated from the transition energy E-IT (H-DA = E-IT/2, 2000-2500 cm(-1)) are in excellent agreement with data (H-ab, 2400-3000 cm(-1)) calculated from the modified Mulliken-Hush expression for Class III systems. DFT calculations show that linear combinations of the delta orbitals of the Mo-2 centers generate the HOMO (out-of-phase, delta-delta) and HOMO-1 (in-phase, delta + delta), with the energy difference corresponding to the E-IT. This study illustrates a systematic transition from a strongly coupled MV complex near the Class II-III border, to Class III, and to systems in which the underlying ground state is better described in terms of simple delocalized electronic states rather evolving from strongly coupled diabatic states which define Class III.
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