Single crystals of 1 suitable for single crystal X-ray diffraction were grown by slow diffusion of either pentane or diethylether into an acetonitrile solution of 1 at -20 °C and confirmed the target structure (Figure ). The solid-state structure shows onedimensional coordination polymers of [K(18-crown-6)] + cations capping each end of the [NCCCO] -anion (Figure ). 1 crystallizes in the monoclinic space group P21/c, and the central carbon C2 sits on an inversion centre, so only half an equivalent of 1 makes up the asymmetric unit. This feature enforces linearity on the C1-C2-C3 moiety although the thermal ellipsoids for the NCCCO moiety suggest that the linearity may be an artifact of crystal packing. We note that similar deviation from linearity was also suggested for the isoelectronic carbon suboxide, where irregular thermal ellipsoids for the OCCCO unit were also seen. The end-for-end disorder precludes a detailed discussion of the bond metrics, and thus the structure of 1 has been further probed using computational methods. ), N1-C1-C2: 171.5( ), C1-C2-C3: 180.0 (C2 on inversion centre), C2-C3-O3: 173.9(5). Structures generated through -x,-y,-z symmetry element. DFT calculations performed at the M06-2X-D3(benzene)/Def2-TZVP//M06-2X-D3/Def2-SVPP level of theory were used to probe the structure of 1 further (see SI for full details). The optimized structure of the free anion was shown to have a linear geometry (Figure ). This is in agreement with observations for the isoelectronic system of carbon suboxide, in which only high levels of computational theory could replicate the non-linear structure that was observed in the solid state. Inclusion of the K counterion in the calculations result in a contraction of the bond angles around the carbon atoms. Due to the polymeric network observed in the solid state, structures with the K bound through the C-N bond (1') and the C-O bond (1'') separately were optimized (Figure ). 1'' is marginally higher in energy than 1', but within the error of the computational method (ΔE = +1.1 kcal/mol), in accord with the observation of the polymeric network. In both cases the coordination of the K ion results in a considerable contraction of the central C1-C2-C3 angle to 149.2° (1') and 166.3° (1''), and of the angle around the carbon atom involved in K coordination to 166.9° (N-C1-C2 in 1') and 167.8° (C2-C3-O in 1''). This produces a W-shaped geometry, reminiscent of that observed with carbon suboxide, again highlighting the similarity between these isoelectronic species. These deviations from linearity are more pronounced than those observed in the solid-state structure; however, interactions with the cation and packing forces in the solid state are thought to override the small barriers between these two computed bonding geometries. The electronic structure of 1 was explored using NBO analysis performed at the M06-2X/Def2-TZVP level of theory (see SI for full details). In all the calculated structures the NPA charges alternate across the structure in the format N δ--C δ+ -C δ--C δ+ -O δ-. In the free anion the most negative charge is calculated to reside on the oxygen atom, but this value is close to that of the central carbon atom. Inclusion of the K counterion changes this, as the NPA charges in 1' for C2 and O are -0.80 and -0.54, respectively, and in 1'' they are -0.61 and -0.71, respectively. As these two structures represent the two extremes of the K coordination an average of these charges could represent the experimental observations that would occur due to the polymeric framework. The average NPA charges on C2 and O are therefore -0.71 and -0.63, respectively, which suggests that C2 could be the site of nucleophilic reactivity in 1.