Rh2(dimen)4 2+ (dimen = 1,8-diisocyanomenthane) exhibits very long Rh-Rh bond distances in the solid state (varying from 4.48 Å for the PF6 - salt to 3.861 Å for the B(C6H5)4 - salt), but the lowest dσ* → pσ excitation produces an excited state with a considerably shorter Rh-Rh distance (~3.2 Å). Several techniques (absorption, singlecrystal polarized absorption, emission, polarized excitation, resonance Raman, fluorescence, phosphorescence lifetime data, and MM2 calculations) help describe the ground- and lowest excited-state potential energy surfaces of this complex. The absorption corresponding to dσ* → pσ singlet-singlet excitation (1Ag1g → 3A2u) is markedly asymmetric, with a sharp maximum (420-440 nm) and a long tailing shoulder (~480 nm). A similarly asymmetric dσ* → pσ singlet-triplet feature (1Ag1g → 3A2u) is observed ~3000 cm-1 below the singlet-singlet band in the polarized singlecrystal spectra of the B(C6H5)4 - salt. In contrast, the corresponding emission bands (545-600 nm, fluorescence; 660-714 nm, phosphorescence) are nearly symmetric. The spectroscopic properties are interpreted in terms of ground- and excited-state potential surfaces that combine parameters derived from previous work on more conventional (Rh1)2 systems with the results of MM2 calculations of dimen deformations. The calculated ground-state surface is very shallow, with a minimum at 4.75 A, but nearly harmonic in the range of interest. The calculated excited-state surface is extremely anharmonic: after a well-defined minimum at 3.35 Å, it is broad and shallow at longer Rh-Rh distances. For Rh-Rh distances greater than ~4 Å, the calculations predict that the individual Rh(I) square planes of Rh2(dimen)4 2+ will be eclipsed; at shorter distances, these units are expected to twist (at a dihedral angle inversely proportional to the Rh-Rh separation) to relieve the substantial strain energy involved in distorting Rh2(dimen)4 2+ along the square planar, a2u bending coordinate. These predictions are consistent with the crystal structures of [Rh2(dimen)4] [PF6]2 (Rh-Rh = 4.48 Å; dihedral twist angle = 0°) and [Rh2(dimen)4] [Bi(C6H5)4]2 (Rh-Rh = 3.861 Å; dihedral twist angle = 16.2°) and provide an explanation for the dramatic rigidochromic effect (1000-cm-1 blue shift) on the emission as the temperature is lowered through the glass transition of solutions.