Two new boron-rich scandium borides were found with a structure closely related to tetragonal boron II (t-B192). Some black single crystals of ScB15 were obtained from a gallium melt besides rhombohedral ScB31. Its crystal structure (tP192; P41, a = 10.2995(15), c = 14.223(3) Å, 4603 refl., 438 param., R1(F) = 0.043, wR2(I) = 0.070) consists of a rod packing of B12-icosahedra and a new polyhedral B20+1 unit. Using a Cu-melt the main product is a ternary compound with the composition Sc2Cu1−xB45 (x ≈ 0.23) and a very similar crystal structure (tP192; P43, a = 10.2674(15), c = 14.261(3) Å, 5440 refl., 461 param., R1(F) = 0.032, wR2(I) = 0.080). In Sc2Cu1−xB45 one metal position is partially occupied by Cu, resulting in a different charge transfer by the metal atoms. This leads to slight differences in the B20+1 unit of the boron framework. Both single crystals were merohedrally twinned. The differences between ScB15 and Sc2Cu1−xB45 can be explained by the mnopq-rule of Jemmis.