Alkyltin clusters are exploited in nanolithography for the fabrication of microelectronics. The alkyltin Keggin family is unique among Keggin clusters across the periodic table; its members appear to favor the lower-symmetry beta and gamma isomers rather than the highly symmetrical alpha and epsilon isomers. Therefore, the alkyltin Keggin family may provide important fundamental information about the formation and isomerization of Keggin clusters. We have synthesized and structurally characterized a new butyltin Keggin cluster with a tetrahedral Ca2+ center, fully formulated [(BuSn)(12)(CaO4)(OCH3)(12)(O)(4)(OH)(8)](2+) (beta-CaSn12). The synthesis is a simple one-step process. Extensive solution characterization including electrospray ionization mass spectrometry, small-angle X-ray scattering, and multinuclear (H-1, C-13, and Sn-119) nuclear magnetic resonance shows beta-CaSn12 is essentially phase-pure and stable. This differs from the previously reported Na-centered analogues that always form a mixture of beta and gamma isomers, with facile interconversion. Therefore, this study has clarified prior confusion over complex spectroscopic and crystallographic characterization of the Na-centered analogues. Density functional theory calculations showed the following stability order: gamma-CaSn12 < gamma-NaSn12 < beta-CaSn12 < beta-NaSn12. The beta analogue is always more stable than the gamma analogue, consistent with experiment. Notable outcomes of this study include a rare tetrahedral Ca coordination, a Na-free alkyltin cluster (important for microelectronics manufacturing), and a better understanding of Keggin families built of different metal cations.