CRYSTAL-STRUCTURE OF BIS(N,N-DIMETHYLTHIOFORMAMIDE)MERCURY(II) IODIDE, BROMIDE, AND CHLORIDE AND SOLVATION OF THE NEUTRAL MERCURY(II) HALIDE-COMPLEXES IN N,N-DIMETHYLTHIOFORMAMIDE, A RESONANCE-INDUCED HYDROGEN-BONDING SOLVENT AND LIGAND

The isostructural solvated mercury(II) halides HgI2(SCHN(CH3)(2))(2), HgBr2(SCHN(CH3)(2))(2), and HgCl2(SCHN(CH3)(2))(2) are by single-cryst al X-ray diffraction methods found to crystallize in the monoclinic sp ace group P2(1)/n (No. 14) with Z = 4 and the unit cell parameters a = 9.757(1), b = 15.546(3), c = 10.416(2) Angstrom and beta = 104.47(1)d egrees for the iodide, a = 9.406(2), b = 15.208(3), c = 10.226(2) Angs trom and beta = 104.66(1)degrees for the bromide, and a = 9.193(2), b = 15.094(2), c = 10.121(2) Angstrom and beta = 105.10(1)degrees for th e chloride. Discrete mercury(II) complexes with pseudotetrahedral coor dination are held together in layers by weak intermolecular interactio ns. An intramolecular hydrogen bond between one halide and the-CHS gro up of an N,N-dimethylthioformamide ligand makes the two mercury-halide bond lengths significantly different with 2.762(1) and 2.724(1) Angst rom for the iodide, 2.619(3) and 2.565(3) Angstrom for the bromide, an d 2.509(5) and 2.452(5) Angstrom, for the chloride compound. Large-ang le X-ray scattering of a saturated HgI2 solution in N,N-dimethylthiofo rmamide shows HgI2(SCHN(CH3)(2))(2) complexes to be formed with the Hg -I bond length 2.740(8) Angstrom. The vibrational mercury-halide stret ching frequencies are used to discuss correlations between bond distan ces and strengths. Raman and infrared vibrational spectra of N,N-dimet hylthioformamide ligands in zinc, cadmium, and mercury complexes show the shifts of the C-S and C-N vibrational frequencies to be useful ind icators of the metal ion-sulfur bond strength. Calorimetric measuremen ts give strongly exothermic heats of solvation of HgI2 and HgBr2 in li quid N,N-dimethylthioformamide, -146 and -156 kJ.mol(-1), respectively . From the corresponding heat of solvation of HgI2 and HgBr2 by N,N-di methylthioformamide in dilute benzene solution the enthalpy contributi on from the resonance-induced C-H ... S hydrogen bonding in liquid N,N -dimethylthioformamide is estimated to be about 12 kJ.mol(-1). The ove rall stability constants for the addition of N,N-dimethylthioformamide Ligands to HgX2 in benzene solution are determined to be beta(1) = (2 .9 +/- 0.8) x 10(4) and (8.2 +/- 1.0) x 10(4) mol(-1).dm(3) and beta(2 ) = (1.4 +/- 0.4) x 10(7) and (3.6 +/- 1.2) x 10(7) mol(-2).dm(6) for X = I and Br, respectively.