The title compound has been synthesized and its structure refined by using powder diffraction data. The structure is orthorhombic, space group Immm, with cell constants (a=7.1156(9), b=10.2863) (18), (c=6.5756) (9) (\AA), and (Z=4). Two types of (\mathrm{Nb}) atoms are present in the structure: (\mathrm{Nb} 1) atoms in (\mathrm{Nb}{2} \mathrm{O}{8}) clusters with one of the shortest (\mathrm{Nb}-\mathrm{Nb}) distances known so far ( (2.560 \AA) ), and (\mathrm{Nb} 2) atoms forming layers of connected (\mathrm{NbO}{6}) octahedra. The electronic structure obtained from Extended HΓΌckel tight-binding band calculations suggests the coexistence of (\mathrm{Nb}(1+)) ions in the (\mathrm{Nb}{2} \mathrm{O}_{8}) clusters with quadruple (\mathrm{Nb}-\mathrm{Nb}) bonds, together with (\mathrm{Nb}(4.5+)) ions in the layers of octahedra. Deviations from this simplified model are analyzed. The electrical behavior, measured in the temperature range 4.2 (\mathrm{K}<T<300 \mathrm{~K}), is metallic at low temperatures and practically temperature-independent at room temperature. A semimetallic character is proposed based on the calculated band electronic structure, with the states around the Fermi level localized mainly on the layers of octahedra parallel to the (a c) plane. 1993 Academic Press, Inc.