Pyrophosphate complexation of Tin(II) in aqueous solutions as applied in electrolytes for the deposition of tin and tin alloys such as white bronze

摘要

Electrodeposition of tin and tin alloys from electrolytes containing tin(II) and pyrophosphates is an important process in metal finishing, but the nature of the tin pyrophosphate complexes present in these solutions in various pH regions has remained unknown. Through solubility and pH studies, IR and ~(31)P and ~(119)Sn NMR spectroscopic investigations of solutions obtained by dissolving Sn _2P _2O _7 in equimolar quantities of either Na _4P _2O _7· 10H _2O or K _4P _2O _7 the formation of anionic 1:1 complexes {[Sn(P _2O _7)]} _n ~(2n-) has now been verified and the molecular structures of the monomer (n = 1) and the dimer (n = 2) have been calculated by density functional theory (DFT) methods. Whereas the alkali pyrophosphates Na/K _4P _2O _7 give strongly alkaline aqueous solutions (pH ～13), because of partial protonation of the [P _2O _7] ~(4-) anion, the [Sn(P _2O _7)] ~(2-) anion is not protonated and the solutions of Na/K _2[Sn(P _2O _7)] are almost neutral (pH ～8). The monomeric dianion appears to have a ground state with C _(2v) symmetry with the Sn atom in a square pyramidal coordination and the lone pair of electrons in the apical position, while the dimer approaches C _2 symmetry with the Sn atoms in a rhombic pyramidal coordination, also with a sterically active lone pair. A comparison of experimental and calculated IR details favors the monomer as the most abundant species in solution. With an excess of pyrophosphate, 3:2 and 2:1 complexes (P _2O _7):(Sn) are first formed, which, in the presence of more pyrophosphate, undergo rapid ligand exchange on the NMR time scale. The structure of the 2:1 complex [Sn(P _2O _7) _2] ~(6-) was calculated to have a pyramidal complexation by two 1,5-chelating pyrophosphate ligands. Neutralization of these alkaline solutions by sulfuric or sulfonic acids (H _2SO _4, MeSO _3H), as also practiced in electroplating, appears to afford the tin(II) hydrogen pyrophosphates [Sn(P _2O _7H)] ~- and [Sn(H _2P _2O _7)] ~0. The molecular structures of the mononuclear model units have also been calculated and were shown to have an unsymmetrical complexation and to feature trigonal pyramidal (pseudotetrahedral) coordination. NMR observations have shown that, contrary to the results obtained for Sn ~(II) compounds, Sn ~(IV) as present in K _2SnO _3 or its hydrated form (K _2Sn(OH) _6) does not form a pyrophosphate complex in aqueous solution near pH 7. There is also no interference of sulfite.