On the structure of lead(II) complexes in aqueous solutions. Part IV. Binuclear clusters Martin Breza a, *, Stanislav Biskupic ˇ a , Alena Manova ´ b a Department of Physical Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia b Department of Analytical Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia Received 9 April 2003; accepted 1 July 2003 In the memory of Prof. Peter Pelika ´n (1939  /2002) Abstract Optimal geometries and corresponding electronic structures of [Pb 2 (m-OH) n ] q and of their hydrated [Pb 2 (m-OH) n (H 2 O) 2 ] q analogues with total charges q /4 /n , n /1 0/4, are investigated within Hartree-Fock, B3LYP and MP2 treatments. Direct Pb  /Pb and O  /O interactions are weakly antibonding in all the systems under study. The clusters are held together exclusively by Pb /O bonds that are approximately 2 /4 times stronger than Pb  /OH 2 interactions of van der Waals type. # 2003 Elsevier Ltd. All rights reserved. Keywords: Lead(II) clusters; Hydroxo-complexes; Molecular structures; Geometry optimizations 1. Introduction Lead compounds in aqueous systems play an impor- tant role in many environmental problems. Their structural characterization is important for understand- ing the mechanism of lead transport in natural systems. Precise potentiometric titration data [1  /7] on the hydrolysis of lead(II) in perchlorate and nitrate solu- tions over a broad concentration and pH range indicate significant formation of [Pb(OH)]  , [Pb 3 (OH) 4 ] 2 , [Pb 3 (OH) 5 ]  , [Pb 4 (OH) 4 ] 4 and [Pb 6 (OH) 8 ] 4 species. The previously assumed [2 /5] existence of [Pb 2 (OH)] 3 has not been confirmed in later studies [6,7]. Based on the study of dilead structure units in lead halide and hydroxo/oxo species, clusters and extended structures found in molten salts, aqueous solutions and the solid state, it has been assumed [8] that the [Pb 2 (OH) 2 ] 2 clusters are stabilized by partial Pb  /Pb bonding induced by bridging hydroxide ions. The strongest Pb /Pb bonds were deduced in complexes with predominantly ionic lead /anion interactions. Such a conception is used in chemistry textbooks [9] too. Semiempirical quantum-chemical studies [10  /13] of possible structures of trinuclear, tetranuclear and hexa- nuclear lead(II) hydroxo-complexes and of their hy- drated analogues indicate that the individual Pb atoms are bonded only via OH bridges due to vanishing Pb  /Pb bonds. The stability of individual isomers increases with the number of OH bridges. Inclusion of additional H 2 O molecules into the model system may significantly influence the relative stability of the systems even through the H 2 O ligands do not serve as bridges. There is only one ab initio MP2 study on such systems but this is restricted do the structure and vibrational spectra of Pb 4 (OH) 4 4 clusters [14]. The aim of this study is ab initio investigation of possible structures of binuclear Pb clusters with hydrox- yl bridges as well as of their bonding. This might help to explain why these clusters are not observed experimen- tally. * Corresponding author. Address: Faculty of Chemical Technology, Slovak Technical University, Radlinskeho 9, Bratislava 812 37, Slovakia. Tel.:  /421-2-5932-5482; fax:  /421-2-5249-3198. E-mail address: breza@cvt.stuba.sk (M. Breza). Polyhedron 22 (2003) 2863  /2867 www.elsevier.com/locate/poly 0277-5387/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0277-5387(03)00453-4