GAS-PHASE OXIDATION OF GROUP-6 METAL-CARBONYL ANIONS

Oxidation of Cr(CO)5-, Mo(CO)5- and W(CO)5- was examined in a low-pressure (approximately 5 x 10(-8) mbar) and a high-pressure (approximately 3.5 mbar) environment using a Fourier-transform ion cyclotron resonance spectrometer equipped with a high-pressure external ion source. Although the oxidation chemistry displayed by the Cr and W systems was fairly similar, the Mo analogue had several unique features. In addition, remarkable differences were observed between high- and low-pressure oxidation. For example, MO5- ions were only generated under high-pressure conditions. This indicates that generation of MO5- ions requires termolecular reactions. Alternatively, it could be that the intermediate for generation of MO5- has a very short lifetime and dissociates prior to reaction with O2 under low-pressure conditions. Energy-resolved collision-induced dissociation was used to obtain structural information about the various product ions, and it allowed determination of bond strengths in these ions. The experiments revealed that for all the M(CO)(x)O2- (x = 3, 4) product ions the two oxygen atoms are bound as two separate oxo ligands and not as a dioxygen ligand. The MO5 ions contain two (or possibly one) O2 molecules bound as peroxo or superoxo ligands.