The constitution of aqueous tin(IV) chloride and bromide solutions and solvent extracts studied by 119Sn NMR and vibrational spectroscopy

Abstract

119Sn NMR spectra were used to investigate the aquohalostannate(IV) complexes formed by SnCl 4 and SnBr 4 in aqueous and halogen acid solutions. Characteristic chemical shifts were assigned to the complex cations [SnX(H 2O) 5] 3+, [SnX 2(H 2O) 4] 2+ and [SnX 3(H 2O) 3] +, [SnX 4(H 2O) 2], and the complex anions [SnX 5(H 2O)] − and [SnX 6] 2− (where X = Cl or Br); including the pairs of cis/trans or fac/mer isomers. The CPMAS spectra of crystalline solids containing [SnCl 6] 2− and cis- or trans-[SnCl 4(H 2O) 2] assisted these assignments. The NMR data yielded quantitative estimates of the amounts of the various species in solution as a function of the tin concentration from 0.1 mol dm −3, where the hydrated stannic ion, Sn 4+ (aq), is predominant, to near saturation where auto-complexing results in complex anion formation. Soluble hydrolysis products for which OH-bridged structures are proposed were detected in solutions without added HCl or HBr and increase in amount when NaOH is introduced. Solutions containing excess NaOH reveal [Sn(OH) 6] 2− which has a chemical shift of — 590 ppm and exhibits the Raman frequencies: 552 ( v 1), 425 ( v 2), 290 cm −1 ( v 5), appropriate for octahedral ( O h) symmetry. The 119Sn NMR spectra of tin(IV) mixed halide aqueous solutions consisted of signals ranging from — 600 ppm (H 2O and Cl − ligands) to —2000 ppm (Br − ligands), due to the series of species represented by SnCl x Br y (H 2O) z (where x+ y+ z = 6), and can be interpreted by relating the chemical shift of the tin nucleus to the sum of ligand electronegativities in a given complex. The solvent extraction of tin halides from aqueous acid solution by diethyl ether or isobutyl methyl ketone has been investigated and tin(IV) shown to be extracted in the form of both molecular [SnX 4(H 2O) 2] and ion-pair H +(aq)[SnX 5(H 2O)] − complexes. These species can also be generated in solution by adding small amounts of water to [SnX 6] 2− in acetone. Conjoint use of NMR and Raman spectra enables the major species in these systems to be determined and allows the v(SnX) symmetric stretching frequencies to be identified and assigned.