Characterisation of the predominant low-pH lead(II)–hydroxo cation, [Pb4(OH)4]4+; crystal structure of [Pb4(OH)4][NO3]4 and the implications of basic salt formation on the transport of lead in the aqueous environment
A study of the reactions between PbII and hydroxide ions in aqueous nitrate solutions has identified three distinct basic salt phases. When the pH is raised, the first precipitate obtained from the Pb2+–NO–3–H2O system is the [Pb4(OH)4]4+ cluster cation. The crystal structure of [Pb4(OH)4][NO3]4 has been determined by single-crystal X-ray diffraction and refined by full-matrix least-squares analysis to R1= 0.0706, wR2= 0.1827. The complex crystallises in the monoclinic space group la, with a= 18.251(5), b= 17.206(5), c= 18.588(12)Å, β= 91.90(4)° and Z= 16. The PbII atoms are contained within discrete [Pb4(OH)4]4+ units. The four lead atoms are situated at the corners of a distorted tetrahedron with an average Pb–Pb distance of 3.785(4)Å and an average intracluster Pb–Pb–Pb bond angle of 60.0(1)°. The OH groups are situated above the faces of the Pb4 tetrahedron giving each Pb atom a trigonal-pyramidal arrangement of nearest-neighbour oxygen atoms. The average Pb–OH distance is 2.38(7)Å and the average OH–Pb–OH angle 71.8(2)°. The next-nearest Pb–O distances are to nitrate O atoms with the shortest of these being in the range 2.62–2.80 Å suggesting that the non-bonding electron pairs on Pb point outwards from the cluster and cannot take part in lead–lead cluster interactions. The Raman data for the [Pb4(OH)4]4+ cluster have been correlated with the crystal structure. The thermal decomposition of [Pb4(OH)4][NO3]4 has been studied and is rationalised in terms of a two-step dehydration of the [Pb4(OH)4]4+ ion, followed by a four-step decomposition of the nitrate-containing compounds. The formation of polynuclear complexes may also depend on the total lead(II) concentration but there is no evidence from the present work for the precipitation of any phase other than [Pb4(OH)4][NO3]4 in the initial precipitates. Under the conditions studied, increase of the pH above 5.25, after the precipitation of [Pb4(OH)4][NO3]4, resulted in the replacement of NO3– in the lattice by OH– and the formation of distinct phases at pH ca. 7 and 8.5 that are likely to be based on [Pb3(OH)4]2+ and [Pb6O(OH)6]4+ respectively. The implications of the formation of the Pb–OH clusters on the transport of lead in the aqueous environment have been discussed.