Contribution of a δ-Orbital Electron to a Quadruple Metal-Metal Bond. A Direct Experimental Measure from Vibrational Fine Structure in the δ Ionization of MO₂(O₂CCH₃)₄

The first observation of metal-metal vibrational fine structure in a photoelectron band is reported. Attention is focused on the predominantly metal “δ” ionization band (arising from production of the ²B_2g positive ion state) of Mo₂(O₂CCH₃)₄. From our high-resolution and high signal-to-noise He I ionization data collection techniques, we have observed within this band approximately 20 members of the vibrational progression predominantly corresponding to the totally symmetric (a_lg) metal-metal stretching mode in the ²B_2g positive ion state. The metal-metal stretching frequency is found to be lower in the ²B_2g state than in the ground state, indicating an appreciable reduction of metal-metal bond strength upon the loss of the δ-bonding electron. This stretching frequency is also lower than that observed in the ¹A_2u (δδ*) electronic excited state of the neutral molecule as obtained from absorption techniques. A normal Franck-Condon analysis of the vibrational progression in the ²B_2g positive ion state reveals that the equilibrium metal-metal bond distance is substantially longer than in the ground state of the molecule and, in addition, is longer than in the ¹A_2u electronic excited state. The comparatively low metal-metal vibrational frequency and long bond length indicate that, in addition to the change in bond order upon ionization, the change in the metal oxidation state also serves an important role in determining the strength of the resultant metal-metal interaction.