The structural stabilities of binary Mg-X (X = Si, Ge, Sn) and 4d transition metal silicides Mo-Si and Ru-Si are investigated. The convex hulls of stable alloys are in overall good agreement with the known experimental phase diagrams. It is shown how the Si-rich Ru-Si structures have band gaps at the Fermi-level and how the Ru(2)Si(3) structure is stabilized compared to the corresponding Fe(2)Si(3) structure. We discuss the band structure of Ru(2)Si(3) and show how the anisotropic band masses lead to favorable calculated thermoelectric properties.