Two homologous and isostructural compounds Na5M2+xSn10-x (M = Zn, Hg) were obtained by direct reaction of the elements at high temperature. The crystal structures of these novel phases were determined from single-crystal X-ray diffraction data and represent a new structure type in tin chemistry. They crystallize in the space group Pbcn (No. 60, Z = 4) with a = 12.772(1), b = 10.804(1), and c = 12.777(1) angstrom, V= 1763.1(2) angstrom(3) for Na5Zn2.28Sn9.72(2) (I) and a = 12.958(1), b = 10.984(1), and c = 12.960(1) A, V= 1844.5(2) angstrom(3) for Na5Hg2.39Sn9.61(1) (II). The structures consist of an anionic 3D open framework of tetrahedrally coordinated Sri and M atoms interwoven with a cationic 2D array of interconnected {NaNa4} tetrahedra. The framework can be partitioned into fragments of realgar-like units {Sn8-xMx}(2x-) and twice as many {Sn-M}(2-) dimers. Formally, the compounds are charge-balanced Zintl phases for x = 0.5. As the structure refinements lead to x = 0.28 and 0.39 for I and II, respectively, both structures are electron-rich and expected to be metallic. Theoretical investigations at the density functional theory level reveal a deep minimum at the Fermi level for x = 0.5. According to rigid band analyses, the electronic structure of the phases with the experimentally observed compositions corresponds to heavily doped semiconductors, thereby meeting an important requirement of thermoelectric materials.