The binary silicides Eu 5 Si 3 and Yb 3 Si 5 were prepared from the elements in sealed tantalum tubes and their crystal structures were determined from single crystal X-ray data: I4/mcm, a = 791.88(7) pm, c = 1532.2(2) pm, Z = 4, wR2 = 0.0545, 600 F 2 values, 16 variables for Eu 5 Si 3 (Cr 5 B 3type) and P62m, a = 650.8(2) pm, c = 409.2(1) pm, Z = 1, wR2 = 0.0427, 375 F 2 values, 12 variables for Yb 3 Si 5 (Th 3 Pd 5 type). The new silicide Eu 5 Si 3 contains isolated silicon atoms and silicon pairs with a Si±Si distance of 242.4 pm. This silicide may be described as a Zintl phase with the formula 6± . The silicon atoms in Yb 3 Si 5 form a two-dimensional planar network with two-connected and three-connected silicon atoms. According to the Zintl-Klemm concept the formula of homogeneous mixed-valent Yb 3 Si 5 may to a first approximation be written as 6± . Magnetic susceptibility investigations of Eu 5 Si 3 show Curie-Weiss behaviour above 100 K with a magnetic moment of 7.85(5) l B which is close to the free ion value of 7.94 l B for Eu 2+ . Chemical bonding in Eu 5 Si 3 and Yb 3 Si 5 was investigated by semi-empirical band structure calculations using an extended Hu È ckel hamiltonian. The strongest bonding interactions are found for the Si±Si contacts followed by Eu±Si and Yb±Si, respectively. The main bonding characteristics in Eu 5 Si 3 are antibonding Si1 2 -p* and bonding Eu±Si1 states at the Fermi level. The same holds true for the silicon polyanion in Yb 3 Si 5 .