The 1 JP,C coupling constant varies more significantly and decreases from 218.9 to 133.8 Hz in the order of 3b > 3a > 3c > 3d ≈ 3e. This suggests a reduced s-character with increasing electron donation from the R group, thereby favoring the keteniminyl structure 3 (sp 2 hybridization) over the ynamide structure 3' (sp hybridization). This interpretation is further supported by infrared spectroscopy, which revealed characteristic bands for the C=C=N stretching vibration between 1968 and 2054 cm -1 . These values are in-between the regions reported for ketenimines (around 2000 cm -1 ) and ynamines (around 2100 cm -1 ), and increase with the electron-withdrawing ability of the Nsubstituent. Thus, compound 3b, with the 2,6dichlorophenyl substituent, exhibits the largest value, indicating the highest ynamide character. A comparison of the NMR and IR spectroscopic data of the keteniminyl anions with those of the corresponding ketenyl anion 4 and diazomethanide 5 reveals that the 13 C{ 1 H} NMR shift of the C1 carbon atom in 3 is deshielded relative to the other anions, indicating a reduced charge accumulation at this position. In contrast, the value for C=C=NR vibration falls in between the stretching frequency for the ketenyl and diazomethanide. Single crystals of the crown ether complexes of the keteniminyl anions were obtained by vapor diffusion of pentane (3a, 3b and 3c) or hexane (3d) into concentrated THF solutions at room temperature (Figure ). In case of 3e[K•(18-c-6)], single crystals were obtained by slow evaporation of a saturated pentane solution. X-ray diffraction (XRD) analysis revealed that all compounds form monomeric structures, except for 3e[(K•(18-c-6)], which features two molecules in the asymmetric unit, which are weakly bound to each other through intermolecular hydrogen bonding between the keteniminyl nitrogen and a para-CH moiety. In the aryl-substituted keteniminyl anions 3a-3c as well as in 3e, the potassium cation is coordinated exclusively by the phosphinoyl group, whereas it is bound by both the phosphinoyl moiety and the C1 carbon atom in 3d[(K•18-c-6)], reflecting the more pronounced keteniminyl character (3) in the alky-substituted systems. Indeed, the K-C distances in the monomeric complexes follow the same trend as observed by the NMR and IR spectroscopic studies, with the distances decreasing from 4.218 Å in the electron-poor dichloroaryl system 3b to 3.048 Å in the cyclohexyl-substituted keteniminyl 3d. These pronounced differences in bond lengths clearly support the change in the bonding situation, with complex 3d being best described as a keteniminyl anion, where the potassium cation is coordinated through the carbanionic center C1. In contrast, the structure of 3b argues for a pronounced ynamide character with no bonding interaction between the cation and C1. The structure parameters further reflect this trend in the bonding situation (Table ). Accordingly, the C-C bonds shorten in the order 3e ≈ 3d > 3c > 3a ≈ 3b, whereas the C-N bonds lengthen in the same sequence.