We rely on structure prediction algorithms coupled to largescale Density Functional Theory (DFT) calculations to explore the possible chemistry between K and CO. The procedures underlying these efforts are detailed in the computational methods section. Our predictions of the thermodynamic stability of various K:CO stoichiometries are summarized by a convex hull diagram in Figure . In this representation, stable compounds lie on the solid line, whereas those phases that are unstable or metastable with respect to decomposition into neighboring stoichiometries lie above the line. Our reference of pure CO, i.e., where xCO=1 in Figure , is a chain-like Pna21 phase identical to the predictions by Xia et al. Structural details (Section S2) and phonon band structures of all ground state phases (Section S4) used to construct the convex hull are provided in the Supporting Information. Our calculations predict a clear preference for homologation of CO into a variety of molecules and extended phases that are stable with respect to elemental K and pure CO. These structures range from one-and three-dimensional polymers to three-, four-and six-membered oxocarbon anions rings (see Figure for some selected examples of these structures). One way to rationalize the potential for unconventional K-CO coordination chemistry in such phases is to view K as a transition metal. Experiments and calculations 39 support a 4s→3d electronic configurational reordering of K at high pressure (ca 30-50 GPa). That such electronic transmutation might occur also at lower -even ambient -pressure conditions can be argued in terms of chemical precompression, the notion that chemical bonding effectively compress the constituent atoms of molecules and materials. We do not predict K 3d orbital participation in bonding in any of the ground state phases of our study, but we did not exhaustively analyze predicted metastable phases. Compounds in which molecular CO remains distinct as a ligand coordinated to K are here exclusively calculated to be unstable or metastable with respect to decomposition into other phases. We note, however, that Wu et al. have detected eight-coordinate carbonyl complexes with Ca, Ba and Sr in low temperature matrix isolation experiments. We predict six-membered oxocarbon anions to be the most thermodynamically favored, as was also calculated by Yamashita et al with Na as counterions, and Liam et al. We refer to these six-membered cyclic C6O6 n-anions as rhodizonates but note that they could alternatively be called cyclohexanehexoneates, hexaketocyclohexaneates, triquinoylates, or tetrahydroxybenzoquinones (THBQs). Several rhodizonatebased salts have been previously synthesized (see e.g., ), and we will return to discuss some of them in what follows.