We report the synthesis and characterization of the cluster anions [Ge18Pd3{Si`Pr-3}(6)]2-(1) with a core of face fused twinned icosahedra, Ge18Pd3, and two sets of three 'Pr3Si-substituents positioned in "eclipsed" geometry. The new anion is a positional isomer of the recently reported "staggered" stannyl-ligated counterpart [Ge18Pd3{Sn'Pr-3}(6)](2-)(2), showing the possibility to find such positional isomerism in Zintl clusters. Both anions are characterized by single-crystal X-ray diffraction, H-1 and C-13 NMR, and negative-ion ESI-MS. Using relativistic DFT calculations, we elucidate and discuss the reasons for the different positioning of the ligands in the stannyl- and silyl-functionalized species.