Excited-state contributions to ground-state properties of mixed-valence dimers: Spectral and electronic-structural studies of [Fe-2(OH)(3)(tmtacn)(2)](2+) related to the [Fe2S2](+) active sites of plant-type ferredoxins

The electronic transitions of the S=9/2 class III mixed-valence dimer [Fe-2(OH)(3)(tmtacn)(2)](2+) are assigned using absorption, magnetic circular dichroism (MCD), and resonance Raman (RR) spectroscopies. This set of excited-state assignments allows the relative contributions of metal-metal and metal-ligand interactions to the D-3h molecular-orbital energy splittings to be estimated. From this analysis the pathway for valence delocalization in this dimer is determined to involve a significant direct Fe ... Fe sigma-bonding interaction. The spectroscopic analysis is supported by electronic-structure calculations, which predict a spectrum similar to that observed and provide descriptions of the dimer's molecular orbitals. These results are further supported by the observation of a significant increase in the Fe ... Fe internuclear separation with sigma-sigma* excitation, determined by vibronic analysis of the sigma-sigma* absorption band shape and associated RR excitation profiles. Combined, these results provide a measure of the geometry dependence of the double-exchange electron-transfer parameter, B, that dominates the ground-state magnetic properties of this dimer. The excited-state assignments also identify the superexchange pathways active in this dimer and provide a spectroscopic measure of the ground-state Heisenberg exchange-coupling constant, yielding -23 <J < +2 cm(-1). The fact that J is small is confirmed by variable-temperature magnetic-susceptibility experiments, which provide an upper limit of similar to 70 cm(-1) for the magnitude of antiferromagnetic coupling in this dimer. The contributions of B, J, and vibronic coupling to the valence-delocalized S=9/2 ground state of [Fe-2(OH)(3)(tmtacn)(2)](2+) are related to the analogous properties of mixed-valence iron--sulfur and other non-heme iron dimers. From this comparison it is concluded that significant direct Fe-Fe electronic coupling also occurs in [Fe2S2](+) dimers, but the combination of an order-of-magnitude increase in Heisenberg exchange coupling relative to that of [Fe-2(OH)(3)(tmtacn)(2)](2+) and the greater influence of vibronic-coupling terms in S=1/2 vs S=9/2 states outweighs this electronic coupling and leads to the valence-trapped antiferromagnetically-coupled ground state observed in [Fe2S2](+) dimers. The difference in Heisenberg exchange between the [Fe-2(OH)(3)(tmtacn)(2)](2+) and [Fe2S2](+) dimers is related to specific differences in the superexchange pathways and covalencies of metal-bridge interactions.