Structure and bonding of bisaquamercury(II) and trisaquathallium(III) trifluoromethanesulfonate

The structure and bonding in bisaquamercury(II) trifluoromethanesulfonate, [Hg(OH2)(2)(CF3SO3)(2)](infinity), 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) trifluoromethane sulfonate shows an unusual connectivity pattern. The mercury(II) ion strongly binds two water molecules axially with the Hg-O bond distance 2.11 Angstrom, and four oxygen atoms from four trifluoromethanesulfonate ions complete a tetragonally compressed octahedral coordination geometry, at the mean Hg-O distance 2.53 Angstrom. 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) &BULL;&BULL;&BULL; O distances in the range 2.688(9)-2.735(9) &ANGS;. The O-D stretching vibrational frequencies of the hydrogen bonds in the partly denterated 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(m) ion binds three water molecules and three oxygen atoms from trifluoromethanesulfonate ions, with T1-O bond distances in the range 2.18-2.24 &ANGS;. A hydrogen bond network between the water molecules and trifluoromethanesulfonate ions with O(-H) &BULL;&BULL;&BULL; O distances in the range 2.65(1)-2.80(1) &ANGS; 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 T1-O bonds, also including the hexaaquaions in the comparisons.