New complex silicides Sc₃<em>T</em>Si₃ (<em>T</em> = Ru, Rh, Ir, Pt) were prepared by arc-melting of the elements. They crystallize with a new orthorhombic structure type, space group <em>Pnma</em>. The four structures were refined from single crystal diffractometer data: <em>a</em> = 1167.6(3), <em>b</em> = 398.20(9), <em>c</em> = 1026.9(2) pm, <em>wR</em>2 = 0.0559, 923 <em>F</em>² values for Sc₃RuSi₃, <em>a</em> = 1166.8(1), <em>b</em> = 401.85(5), <em>c</em> = 1019.7(1) pm, <em>wR</em>2 = 0.0355, 819 <em>F</em>² values for Sc₃RhSi₃, <em>a</em> = 1160.7(2), <em>b</em> = 402.84(6), <em>c</em> = 1015.1(2) pm, <em>wR</em>2 = 0.0352, 905 <em>F</em>² values for Sc₃IrSi₃, and <em>a</em> = 1166.3(1), <em>b</em> = 403.18(5), <em>c</em> = 1019.3(2) pm, <em>wR</em>2 = 0.0465, 917 <em>F</em>² values for Sc₃PtSi₃ with 44 variables per refinement. The Sc₃<em>T</em>Si₃ structures contain complex three-dimensional [<em>T</em>Si₃] polyanionic networks with slightly distorted tetrahedral silicon coordination of the transition metal atoms, e.g. 237–254 pm Ru–Si in Sc₃RuSi₃. The RuSi₄ tetrahedra share common corners along the short unit cell axis and these rows are connected by Si–Si bonds, leading to Si₃ units (243–244 pm Si–Si) within the [RuSi₃] polyanion. Ab initio electronic structure investigations point to dominating Ru–Si interactions and show strong electron localization around the Si₃ units, underlining the polyanionic character. Significant differences in the ⁴⁵Sc quadrupolar interaction strengths between the three distinct scandium sites in the Sc₃<em>T</em>Si₃ silicide were detected by lineshape analyses of field dependent ⁴⁵Sc MAS as well as 2D TQMAS NMR spectroscopic data. By comparing the experimentally obtained quadrupole interaction parameters with theoretically calculated ones a site assignment of the equal intense signals is possible.< Réduire