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The valence ionizations of the metal-metal quadruple bond was examined by variable photon energy photoelectron spectroscopy of Mo2(O2CCH3)4. The metal-based delta, pi and sigma ionizations of the quadruple bond are strongly enhanced relative to the acetate-based ionizations in the region of photon energies from 40 to 50 eV. The pi ionization has the largest contribution to the metal-metal bond strength. The delta and sigma ionizations, partly because of their overlap with ligand orbitals, have less of a contribution to the metal-metal bond strength.