In conclusion, we have shown that the carbonylation of metallated ylides can either result in the formation of a ketenyl anion through displacement of the phosphine ligand or in the elimination of the anionic substituent (sulfinate) with concomitant formation of the phosphoranylideneketene. Investigation of the factors, that influence the carbonylation revealed that the selectivity can be completely shifted from one product to the other by choice of the reaction conditions. Selectivity for the formation of the ketenyl anion can be achieved by using electron-poor phosphines and reaction conditions, that prevent competing metal sulfinate formation through weakening the interaction between the anionic leaving group and the alkali metal cation: this includes the use of the less Lewis acidic heavier metal cations as well as polar solvents and additional ligands such as crown ethers or cryptands. These reaction conditions enabled isolation of the tosyl-substituted ketenyl anion 2 in high yields. The cryptand complex, [2•(Na•crypt)], features a free ketenyl anion in the solid state with an almost linear S-C-C-O linkage reminiscent for a pronounced ynolate character. However, further structure analyses together with computational studies showed a shallow energy surface for the bending of the S-C-C linkage and a significant contribution of the ketenyl anion structure. Accordingly, 2 predominantly reacted at the C1 carbon atom, but in contrast to known ketenyl anions led to the selective formation of stable cyclobutadiones as a consequence of the reduced steric bulk of the tosyl group. Overall, these findings provide a first understanding of the selectivity of the ligand exchange in metalated ylides, which will facilitate the selective formation of further ketenyl anions and their application as reagents in synthetic chemistry in the future.