Isomers of Ir-2(dimen)(4)(2+) (dimen = 1,8-diisocyanomenthane) exhibit different Ir-Ir bond distances in a 2:1 MTHF/EtCN solution (MTHF = 2-methyltetrahydrofuran). Variable-temperature absorption data suggest that the isomer with the shorter Ir-Ir distance is favored at room temperature [K = similar to 8; Delta H degrees = -0.8 kcal/mol; Delta S degrees = 1.44 cal mol(-1) K-1]. We report calculations that shed light on M-2(dimen)(4)(2+) (M = Rh, Ir) structural differences: (1) metal metal interaction favors short distances; (2) ligand deformational-strain energy favors long distances; (3) out-of-plane (A(2u)) distortion promotes twisting of the ligand backbone at short metal metal separations. Calculated potential-energy surfaces reveal a double minimum for Ir-2(dimen)(4)(2+) (similar to 4.1 angstrom Ir-Ir with 0 degrees twist angle and similar to 3.6 angstrom Ir-Ir with +/-12 degrees twist angle) but not for the rhodium analogue (similar to 4.5 angstrom Rh-Rh with no twisting). Because both the ligand strain and A(2u) distortional energy are virtually identical for the two complexes, the strength of the metal metal interaction is the determining factor. On the basis of the magnitude of this interaction, we obtain the following results: (1) a single-minimum (along the Ir-Ir coordinate), harmonic potential-energy surface for the triplet electronic excited state of Ir-2(dimen)(4)(2+) (R-e,R-Ir-Ir = 2.87 angstrom; FIr-Ir = 0.99 mdyn angstrom(-1)); (2) a single-minimum, anharmonic surface for the ground state of Rh-2(dimen)(4)(2+) (R-e,R-Rh-Rh = 3.23 angstrom; FRh-Rh = 0.09 mdyn angstrom(-1)); (3) a double-minimum (along the Jr Jr coordinate) surface for the ground state of Ir-2(dimen)(4)(2+) (R-e,R-Ir-Ir = 3.23 angstrom; FIr-Ir = 0.16 mdyn angstrom(-1)).