| Abstract
| - By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study todetermine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbenecomplexes. These compounds can be ionized under ESI conditions only in the presence of additivessuch as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESIsource an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to themetallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of ahydrogen radical to form an allenylchromium anion that is detected as the [M − H]- ion in themass spectrum. The preference for this mechanistic pathway could be rationalized by DFTcalculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate,additive, and solvent demonstrate that the previously proposed allene−anion carbene complex isnot formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusionof a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M − H − CO]-.
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