Structure and bonding of bisaquamercury(ii) and trisaquathallium(iii) trifluoromethanesulfonate

Abstract

The structure and bonding in bisaquamercury(II) trifluoromethanesulfonate, [Hg(OH₂)₂(CF₃SO₃)₂]_∞, and trisaquathallium(III) trifluoromethanesulfonate, [Tl(OH₂)₃(CF₃SO₃)₃], have been studied by means of single-crystal X-ray diffraction, EXAFS and vibrational spectroscopy. The crystal structure of bisaquamercury(II) trifluoromethanesulfonate shows an unusual connectivity pattern. The mercury(II) ion strongly binds two water molecules axially with the Hg–O bond distance 2.11 Å, and four oxygen atoms from four trifluoromethanesulfonate ions complete a tetragonally compressed octahedral coordination geometry, at the mean Hg–O distance 2.53 Å. Two trifluoromethanesulfonate ions form double bridges between the bisaquamercury(II) entities giving rise to infinite >Hg(OH₂)₂ <(CF₃SO₃)₂ >Hg(OH₂)₂< chains. The parallel chains are held together in layers by relatively strong hydrogen bonds with O(–H)⋯O distances in the range 2.688(9)–2.735(9) Å. The O–D stretching vibrational frequencies of the hydrogen bonds in the partly deuterated compound occur in a broad band at about 2400 cm⁻¹, bandwidth ca. 170 cm⁻¹. The layers are connected only via van der Waals interactions between the protruding CF₃ groups, consistent with the fragile sheet-like structure of the crystalline compound. Trisaquathallium(III) trifluoromethanesulfonate crystallises as molecular complexes where each thallium(III) ion binds three water molecules and three oxygen atoms from trifluoromethanesulfonate ions, with Tl–O bond distances in the range 2.18–2.24 Å. A hydrogen bond network between the water molecules and trifluoromethanesulfonate ions with O(–H)⋯O distances in the range 2.65(1)–2.80(1) Å holds the structure together. Raman and infrared spectra have been recorded and analysed. The changes in force constants and vibrational frequencies have been correlated with bond lengths for the S–O bond in the coordinated trifluoromethanesulfonate ion and for the Hg–O and Tl–O bonds, also including the hexaaquaions in the comparisons.