A study of the D2h to C2h pseudo-Jahn-Teller distortion in the edge-sharing bioctahedral complex Mo2(DXyIF) 2(O2CCH3)2(µ2-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 Mo2(µ 2-O)2 motif displays C2h rather than D 2h symmetry. Analytical CASSCF frequency calculations prove that the rhomboidal distortion of the complex from D2h to C2h is due to a vibronic mixing of the ground electronic state and a low-lying pd* excited state. © 2009 American Chemical Society.