Bonding Analysis of Square-Antiprismatic and Fused Square-Antiprismatic Copper(I)-Selenium Clusters Bachir Zouchoune, 1,2,3 Jean-Franc¸ ois Halet, 2 and Jean-Yves Saillard 2 Received January 13, 2004 The electronic structure of the Cu 2(4n+2) Se 4n+2 (PH 3 ) 8 (n = 1–4) D 4h series of model clusters has been analyzed by means of density functional theory calcu- lations. The fused square antiprismatic structure of the metal framework is found to be always preferred over the fused cuboctahedral one because it rein- forces the Cu-P bonds. Thus, the presence of the terminal phosphine ligands tends to strengthen the Cu…Cu (d 10 … d 10 ) bonding by mixing bonding combi- nations of the vacant Cu 4s and 4p orbitals into the occupied 3d combinations. The calculations indicate that the compounds corresponding to n = 3 and 4 should be easily two-electron-reduced, leading to stable dianionic species. KEY WORDS: Density functional theory; copper(I)-selenium clusters; d 10 … d 10 bonding. INTRODUCTION Contrarily to the octahedron [1] or the cube [2], the square antiprism (SA) is not a very common building block in fused polyhedral transition-metal clusters. The few known examples are encountered in the extremely rich chemistry of copper-chalcogen clusters developed by Fenske and collabo- rators [3], as exemplified by Cu 12 Se 6 (PEtPh 2 ) 8 and Cu 20 Se 10 (PPh 3 ) 8 [4] (see 1 and 2 in Fig. 1). Related sulfur species also exist [3, 4]. The former 1 Laboratoire de Chimie Applique´e et Technologie des Mate´riaux, Centre Universitaire Larbi Ben M’Hidi, 04000 Oum-El-Bouaghi, and Laboratoire de Chimie Mole´culaire, du Controˆle de l’Environnement et des Mesures Physico-Chimiques, Universite´ Mentouri de Constan- tine, 25000 Constantine, Alge´rie. 2 Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR CNRS 6511, Institut de Chimie, Universite´ de Rennes 1, 35042 Rennes Cedex, France. 3 To whom correspondence should be addressed. 267 1040-7278/04/0900-0267/0 Ó 2004 Plenum Publishing Corporation Journal of Cluster Science, Vol. 15, No. 3, September 2004 (Ó 2004)