The synthesis and crystal structure of the first ternary A-Cu-Sn intermetallic phases for the heavier alkali metals A = Na to Cs is reported. The title compounds A(12)Cu(12)Sn(21) show discrete 33-atom intermetalloid Cu-Sn clusters {Sn@Cu-12@Sn-20}, which are composed of {Sn-20} pentagonal dodecahedra surrounding {Cu-12} icosahedra with single Sn atoms at the center. Na12Cu12Sn21 and K12Cu12Sn21 were characterized by single-crystal XRD studies, and the successful synthesis of analogous A-Cu-Sn compounds with A = Rb and Cs is deduced from powder XRD data. The isotypic A(12)Cu(12)Sn(21) phases crystallize in the cubic space group Pn (3) over barm (No. 224), with the Cu-Sn clusters adopting a face centered cubic arrangement. A formal charge of 12-can be assigned to the {Sn@Cu-12@Sn-20} cluster unit, and the interpretation of the title compounds as salt-like intermetallic phases featuring discrete anionic intermetalloid [Sn@Cu-12@Sn-20](12-) clusters separated by alkali metal cations is supported by electronic structure calculations. For both Na12Cu12Sn21 and K12Cu12Sn21, DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [Sn@Cu-12@Sn-20](12-) cluster is analyzed in consideration of the molecular orbitals obtained from hybrid DFT calculations (Gaussian 09) for the cluster anion. The [Sn@Cu-12@Sn-20](12-) cluster MOs can be classified with labels indicating the numbers of radial and angular nodes, in the style of spherical shell models of cluster bonding.