The intermetallic compound Na49Cd58.5Sn37.5 was obtained in nearly 100% yield after fusion of the elements in stoichiometric proportions in a Nb container and quenching of the mixture. The structure was determined by single-crystal X-ray diffraction (rhombohedral, R(3) over barm$, Z = 3, a = 16.034(1) Angstrom, c = 50.64(1) Angstrom; R, R(w)(F) = 3.3%, 4.2%). The covalently-bonded part of the structure is a network of cadmium and tin that contains the following : (a) the first empty icosahedra built without atoms of an element of group 13; (b) 12-bonded Is-atom closo-deltahedra, the largest closo-deltahedron ever made; (c) 20-atom spacers. The sodium cations are located between the clusters and also center the 18-atom cluster. Band calculations on the network and molecular orbital calculations on the 12-bonded 18-atom cluster show that the latter is hyperelectronic with 2n + 4 skeletal electrons, and also that its six lone-pair orbitals are not occupied. Magnetic measurements show small temperature-independent paramagnetism.