While expected by chemical intuition, this trend is not consistently reflected in the structure parameters of the three ketenyl anions. For example, IIb with the most electron-donating substituent exhibits the most acute bending at the C1 carbon atom and the longest C1-C2 bond length. This suggest that trends expected due to electronic effects might be overruled due to the coordination of the metal cation or packing effects. To gain further insights into the bonding situation in anion 2 also in comparison to the thiophosphinoyl and phospinoyl system IIa and IIb, we performed density functional theory (DFT) studies (PBE0/def2tzvpp). In contrast to the structure in the solid-state, the energy optimized structure of 2 in the gas phase was found to be stronger bent with a S-C-C angle of 138.6° and a C-C-O angle of 173.9°. This more acute bending is observed independent of the functional used and also with inclusion of the encapsulated cation (see Table ). A scan of the energy of 2 with varying S-C-C bond angles revealed a shallow minimum of the potential energy surface with less than 5 kJ/mol energy difference for the bending between 120 and 180°. This confirms our conclusions made based on the structure parameters found in the solid state. Similar to the thiophosphoryl system IIa, ketenyl anion 2 features a bonding situation between that of a ketene and an ynolate. Accordingly, the Wiberg bond indices of the C-C and C-O bonds amount to 2.06 and 1.73, respectively (Table ). These values vary surprisingly little with changing R substituent, even in comparison to IIb with a π-donor substituent. The same holds true for the calculated NBO charges in the C-C-O linkage, albeit the charge at the C1 carbon atom in 2 -owing to the electron-withdrawing ability of the tosyl group -is slightly less negative than in the other systems (-0.91 e vs -1.02 e). The HOMO and HOMO-1 of 2 (Figure and SI) represent the π-bond of the C-C-O linkage, with a polarization towards the C1 end. In contrast the LUMO is mostly delocalized over the tosyl group. Given the lower NBO charge at C1 calculated for 2 and the large S-C-C angle in the crystal structure, we next addressed the reactivity of 2-Na focusing on a comparison with IIa and IIb and the question whether also an oxygen-centered reactivity can be observed. Indeed, differences in reactivity compared to IIa and IIb are found, which we however mostly attributed to the reduced steric demand of 2-Na. As such, reaction with TMSCl cleanly proceeds to the mono silylated, anionic cyclobutadione 3 which forms through nucleophilic attack of the ketenyl anion at the carbonyl carbon of the in situ formed ketene (Figure ). The latter could not be isolated independent of the chosen reaction conditions. It is important to note, that 2-Na itself does not undergo a "dimerization" reaction, demonstrating an increased stability of ketenyl anions compared to neutral ketenes towards dimerization. Cyclobutanone formation was also not observed for the reaction of IIa with TMSCl, presumably due to the increased steric demand of the thiophosphinoyl moiety. Compound 3 is highly stable and does not react with a further equiv. of TMSCl even at elevated temperatures, thus allowing an isolation in high yields. The crystal structure of 3 exhibits an almost planar C4-ring. The C1-C2 and C1-C11 bonds are elongated (1.582(2) Å and 1.569(2) Å) compared to the C2-C10 and C10-C11 bonds (1.434(3) Å and 1.429(2) Å) due to the delocalization of the negative charge in the OCCCO linkage.