Lead-207 chemical shift data for bivalent lead compounds: thermodynamics of the equilibrium Pb(O2CCH3)2⇌[Pb(O2CCH3)]++ O2CCH3– in aqueous solution in the temperature range 303–323 K

Abstract

Lead-207 n.m.r. chemical shift data have been recorded for the following bivalent lead compounds (solvent in parentheses) : Pb(NO₃)₂(water), Pb(ClO₄)₂·3H₂O (water), Pb(O₂CCH₃)₂·3H₂O (water), Pb(O₂CC₆F₅)₂(tetrahydrofuran, thf), Pb[S₂P(OR)₂]₂(R = C₂H₅ or i-C₄H₉)(thf and dichloromethane). The nature of the solution species is discussed. The very high-field chemical shifts exhibited by the hydrated lead(II) dication and the monoacetato- and mononitrato-lead(II) monocations are ascribed to efficient nuclear shielding resulting from the confinement of the lead(II) lone pair to the metal 6s orbital. Chemical shifts of the other species occur at much lower field, and structures in which the lone pair is stereochemically active and occupying a hybrid sp³dⁿ orbital are proposed. Analysis of the concentration-dependence data has been employed to evaluate the association constant at 298 K, K_assoc., of [Pb(NO₃)]⁺(1.233 dm³ mol^–1), and the dissociation constant at 298 K, K_dissoc., of Pb(O₂CCH₃)₂(0.0139 mol dm^–3), for the aqueous lead(II) nitrate and acetate systems, respectively. Data for the systems Pb(O₂CC₆F₅)₂(tetrahydrofuran) and Pb[S₂P(OEt)₂]₂(dichloromethane) have been interpreted in terms of a dimer ⇌ monomer equilibrium, with dissociation constants at 298 K of the dimer, K_dissoc., of 0.143 and 0.268 mol dm^–3, respectively. The concentration-independent chemical shift of the lead(II) dithiophosphoridates in tetrahydrofuran is ascribed to the formation of a stable thf solvate in solution. Thermodynamic data for the equilibrium Pb(O₂CCH₃)₂⇌[Pb(O₂CCH₃)]⁺+ O₂CCH₃^– in aqueous solution at 298 K have been evaluated as ΔH₂₉₈=–14.2 kJ mol^–1, ΔG₂₉₈= 10.5 kJ mol^–1, and ΔS₂₉₈=–82.95 J K^–1 mol^–1.