A study of the D-2h to C-2h pseudo-Jahn-Teller distortion in the edge-sharing bioctahedral complex Mo-2(DXyIF)(2)(O2CCH3)(2)(mu(2)-O)(2) is presented. We have performed extensive density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations. For both the full target complex and a model derived by replacing xylyl and methyl groups with hydrogens we observe that the central Mo-2(mu(2)-O)(2) motif displays C-2h rather than D-2h symmetry. Analytical CASSCF frequency calculations prove that the rhomboidal distortion of the complex from D-2h to C-2h is due to a vibronic mixing of the ground electronic state and a low-lying pi delta* excited state.