New metal-rich mixed nickel germanium chalcogenides, Ni_8.21(1)Ge₂S₂ and Ni_8.45(1)Ge₂Se₂, were synthesized by a high-temperature ceramic technique. Ni_8.21Ge₂S₂ decomposes incongruently at 798(5) °C, while Ni_8.45Ge₂Se₂ is stable only between approximately 600(5) °C and 813(5) °C. Single crystals were grown from a molten flux and by a recrystallization of presynthesized charge. The X-ray diffraction study revealed that Ni_8.21Ge₂S₂ and Ni_8.45Ge₂Se₂ are isostructural and crystallize in the tetragonal system (space group I4/mmm, Z = 2) with a = 3.578(1) Å, c = 24.687(5) Å, and a = 3.601(1) Å, c = 25.427(6) Å, respectively. Their crystal structures comprise heterometallic _∞²[Ni₆ Ge₂] slabs and are closely related to the Ni_8.54Ga₂Se₂ and Ni_8.93Sn₂S₂ structure types. The geometrical changes of structural units upon p-element substitution in all known Ni_10-δE₂Q₂ (E = element of group 13 or 14; Q = chalcogen) compounds are discussed. The relation to the intergrowth structure of Ni_5.46GeSe₂ is analyzed.