  Essentially all undoped cuprates exhibit a quasi-planar, fourfold Cu-O
coordination responsible for the magnetically active antibonding
3d-(x^2-y^2)-like state. Here, we present an electronic structure study for
CuSb2O6 that reveals, in contrast, a half-filled 3d-(3z^2-r^2) orbital. This
hitherto unobserved ground state originates from a competition of in- and
out-of-plaquette orbitals where the strong Coulomb repulsion drives the
surprising and unique orbital ordering. This, in turn, gives rise to an
unexpected quasi one-dimensional magnetic behavior. Our results provide a
consistent explanation of recent thermodynamical and neutron diffraction
measurements.
