  Using a combination of X-ray absorption spectroscopy experiments with first
principle calculations, we demonstrate that insulating KCuO_2 contains Cu in an
unusually-high formal-3+ valence state, the ligand-to-metal (O to Cu) charge
transfer energy is intriguingly negative (Delta~ -1.5 eV) and has a dominant
(~60%) ligand-hole character in the ground state akin to the high Tc cuprate
Zhang-Rice state. Unlike most other formal Cu^{3+} compounds, the Cu 2p XAS
spectra of KCuO_2 exhibits pronounced 3d^8 (Cu^{3+}) multiplet structures,
which accounts for ~40% of its ground state wave-function. Ab-initio
calculations elucidate the origin of the band-gap in KCuO_2 as arising
primarily from strong intra-cluster Cu 3d - O 2p hybridizations (t_{pd}); the
value of the band-gap decreases with reduced value of t_{pd}. Further, unlike
conventional negative charge-transfer insulators, the band-gap in KCuO_2
persists even for vanishing values of Coulomb repulsion U, underscoring the
importance of single-particle band-structure effects connected to the
one-dimensional nature of the compound.
