Synthesis, Characterization, Electronic Structure, and Bonding of Heteroatomic Deltahedral Clusters: Na49Cd58.5Sn37.5, A Network Structure Containing the First Empty Icosahedron without a Group 13 Element and the 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 determined by single-crystal X-ray diffraction (rhombohedral, R3hm, Z ) 3, a ) 16.034(1) Å, c ) 50.64(1) Å; R, Rw(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 18-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.