Selected Ion Flow-Tube Mass Spectrometry (SIFT-MS) of Mo(CO)₆ and W(CO)₆ Vapours With Positive and Negative Reagent...

Selected ion flow-tube mass spectrometry (SIFT-MS) was used to characterise the gas-phase ion-molecule reactions of the eight standard reagent ions H₃O⁺, NO⁺, O₂ ⁺˙, OH^-, O^-˙, O₂ ^-˙, NO₂ ^- and NO₃ ^- with molybdenum and tungsten hexacarbonyls, Mo(CO)₆ and W(CO)₆. These compounds, chosen as model volatile transition-metal carbonyls, were introduced directly into the SIFT at variable concentrations, and mass spectra were obtained for all reagent ions. Reactions with the positive reagent ions were fast and close to the collisional limit, proceeding mainly by proton transfer (H₃O⁺) or charge transfer (NO⁺ and O₂ ⁺˙) with little fragmentation. In contrast, negative-ion reactions were slower and chemically more diverse. O^-˙ and O₂ ^-˙ produced sequential oxidation and ligand-exchange products, MoO_x(CO)_6-n ^-˙ and WO_x(CO)_6-n ^-˙, while NO₂ ^- reacted slowly only with Mo(CO)₆. Relative rate coefficients normalised to H₃O⁺ proton transfer were determined. The results establish the feasibility of SIFT-MS for direct, real-time detection of transition-metal carbonyl vapours and clarify their underlying ion-molecule chemistry. Positive-ion channels offer the most sensitive analytical response, whereas negative-ion reactions reveal complementary oxidative pathways. This study thus extends SIFT-MS to a new class of metal-ligand compounds, which are relevant to environmental and occupational monitoring.