Cationic lead(II) hydroxide complexes in molten alkali-metal nitrate

Abstract

The complexation between lead(II) and hydroxide has been studied in molten (K,Na)NO₃ at 280 °C by various techniques. An inert platinized platinum wire bathed with a gaseous mixture of oxygen and water served as a hydroxide electrode in potentiometric determinations of the hydroxide activity in the melts. Only PbOH⁺ and Pb₂OH³⁺ can be detected at the low hydroxide concentrations used. Raman spectra and ¹H NMR experiments gave strong indications that hydroxide retains the proton also when coordinated to lead(II). Oxide ions added to the melts readily transform into hydroxide even at very low water activities. Studies of the solubility of Pb(NO₃)₂ in melts with various concentrations of dissolved PbO show the formation of [PbOH⁺]_q-species at fairly high hydroxide concentrations. Liquid X-ray scattering data yield q= 2 at these hydroxide concentrations, i.e. Pb₂(OH)²⁺₂ is the predominant complex ion. The Pb—O and Pb—Pb distances are 2.7 and 3.4 Å, respectively. The short Pb—Pb distance is the same as that observed in the halide species Pb₂X³⁺, and it is suggested that the complex is stabilized by a Pb—Pb interaction.