We report the synthesis and structural characterization of two Structure I clathrates in the KSi and Rb-Si systems. The alkali-Si clathrates are fully stoichiometric at the framework sites, i.e., devoid of framework vacancies. This is in sharp contrast to the analogous K-Ge, Rb-Ge and Rb-Sn, Cs-Sn systems, where vacancies are formed at one-third of the crystallographic 6c tetrahedral sites. This is rationalized in terms of Zintl-Klemm rules to remove the tetrahedral atom of its hypervalency. The contrasting behavior is understood in terms of weaker Tt-Tt (Tt – tetrelide, Si, Ge, Sn) bonding as one descends the periodic table, and results in poorly metallic conductivities for vacancy-free K7Si46 and Rb6Si46, but semiconducting behavior of K8Ge44. The observation suggests tuning of the electronic properties of Tt clathrates by substitution of (Si,Ge,Sn) on framework sites, for thermoelectric applications. We describe preliminary results designed to synthesize “mixed” Si-Ge clathrate structures. Thermal decomposition of K2SiGe results in formation of a Structure I clathrate with mixing of Si and Ge on framework sites. The lattice constant ao = 10.523(6) Å, is intermediate between those of K8Si46 and K8Ge44.