SYNTHESIS, CHARACTERIZATION, ELECTRONIC-STRUCTURE, AND BONDING OF HETEROATOMIC DELTAHEDRAL CLUSTERS - NA49CD58.5SN37.5, A NETWORK STRUCTURECONTAINING THE FIRST EMPTY ICOSAHEDRON WITHOUT A GROUP 13 ELEMENT ANDTHE LARGEST CLOSO-DELTAHEDRON

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 determi ned 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 o f cadmium and tin that contains the following: (a) the first empty ico sahedra built without atoms of an element of group 13; (b) 12-bonded I s-atom closo-deltahedra, the largest closo-deltahedron ever made; (c) 20-atom spacers. The sodium cations are located between the clusters a nd also center the 18-atom cluster. Band calculations on the network a nd molecular orbital calculations on the 12-bonded 18-atom cluster sho w that the latter is hyperelectronic with 2n + 4 skeletal electrons, a nd also that its six lone-pair orbitals are not occupied. Magnetic mea surements show small temperature-independent paramagnetism.