EXCITED-STATE DISTORTIONS AND ELECTRON DELOCALIZATION IN MIXED-VALENCE DIMERS - VIBRONIC ANALYSIS OF THE NEAR-IR ABSORPTION AND RESONANCE RAMAN PROFILES OF [FE-2(OH)(3)(TMTACN)(2)](2+)

The near-IR transition associated with valence delocalization in the c lass III mixed-valence dimer [Fe-2(OH)(3)(tmtacn)(2)](2+) is studied u sing variable-temperature (VT) electronic absorption and resonance Ram an (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption b and leads to dominant resonance Raman enhancement of totally-symmetric [Fe-2(OH)(3)](2+) core vibrational modes at 316 and 124 cm(-1), descr iptions of which are calculated from a normal coordinate analysis. Vib ronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provi des a description of the geometric distortions accompanying this excit ation. The excited-state distortion is dominated by expansion of the [ Fe-2(OH)(3)](2+) core along the Fe ... Fe axis, reflecting the signifi cant Fe-Fe sigma --> sigma character of this transition. The ground-s tate sigma-interaction between the two metals has been identified as t he orbital pathway for valence delocalization, and the sigma --> sigma distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, H-AB, associated with this pa thway. The dominant role of totally-symmetric nuclear coordinates in t he absorption and RR spectroscopies of [Fe-2(OH)(3)(tmtacn)(2)](2+) is also discussed in relation to the Q(-) vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q(-) coordi nate to the absorption and RR spectra of [Fe-2(OH)(3)(tmtacn)(2)](2+) are small relative to those of the totally-symmetric coordinates due t o the inefficient change-in-curvature mechanism by which the Q(-) coor dinate gains intensity, compared to the efficient excited-state displa cement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q(-) coordinate over other totally -symmetric coordinates observed in intervalence transfer (TT) absorpti on and RR spectroscopies of class II mixed-valence complexes.