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-crystal X-ray diffraction methods found to crystallize in the monoclinic space 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)degrees for the iodide, a = 9.406(2), b = 15.208(3), c = 10.226(2) Angstrom 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 the chloride. Discrete mercury(II) complexes with pseudotetrahedral coordination are held together in layers by weak intermolecular interactions. An intramolecular hydrogen bond between one halide and the-CHS group of an N,N-dimethylthioformamide ligand makes the two mercury-halide bond lengths significantly different with 2.762(1) and 2.724(1) Angstrom for the iodide, 2.619(3) and 2.565(3) Angstrom for the bromide, and 2.509(5) and 2.452(5) Angstrom, for the chloride compound. Large-angle X-ray scattering of a saturated HgI2 solution in N,N-dimethylthioformamide shows HgI2(SCHN(CH3)(2))(2) complexes to be formed with the Hg-I bond length 2.740(8) Angstrom. The vibrational mercury-halide stretching frequencies are used to discuss correlations between bond distances and strengths. Raman and infrared vibrational spectra of N,N-dimethylthioformamide ligands in zinc, cadmium, and mercury complexes show the shifts of the C-S and C-N vibrational frequencies to be useful indicators of the metal ion-sulfur bond strength. Calorimetric measurements give strongly exothermic heats of solvation of HgI2 and HgBr2 in liquid N,N-dimethylthioformamide, -146 and -156 kJ.mol(-1), respectively. From the corresponding heat of solvation of HgI2 and HgBr2 by N,N-dimethylthioformamide in dilute benzene solution the enthalpy contribution from the resonance-induced C-H ... S hydrogen bonding in liquid N,N-dimethylthioformamide is estimated to be about 12 kJ.mol(-1). The overall 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.