Structure of the Solvated Zinc(II), Cadmium(II), and Mercury(II) Ions in N,N-Dimethylthioformamide Solution Christina M. V. Sta ˚ lhandske, † Ingmar Persson,* ,† Magnus Sandstro 1 m,* ,‡ and Ewa Kamienska-Piotrowicz †,§ Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, S-750 07 Uppsala, Sweden, and Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden ReceiVed NoVember 6, 1996 X By means of large-angle X-ray scattering zinc(II) and mercury(II) ions in N,N-dimethylthioformamide solution are found to coordinate to four N,N-dimethylthioformamide molecules with Zn-S and Hg-S bond distances of 2.362(5) and 2.527(6) Å, respectively. The intermediate divalent ion in group 12, cadmium, is solvated by six N,N-dimethylthioformamide molecules with a Cd-S bond distance of 2.69(1) Å. Raman and far-infrared spectra have been recorded and assigned for the solvated ions both in solution and in the solid state. The character of the bonds to the metal ion is discussed in order to explain the lower coordination numbers of the zinc and mercury- (II) ions. Introduction The divalent d 10 ions of group 12, Zn 2+ , Cd 2+ , and Hg 2+ , display a large variety of coordination geometries. The flex- ibility of the zinc ion is of vital importance for its role in the active site of numerous metalloenzymes and in many other important biochemical processes. 1 A recent survey of crystal structures shows prevalent tetrahedral four-coordination for zinc compounds but octahedral six-coordination in the majority of cadmium structures. 2 Mercury(II) often displays linear two- coordination or tetrahedral four-coordination in its complexes. 3 With monodentate ligands, however, as is the case for solvated metal ions, octahedral six-coordination is common in zinc complexes and also occurs for mercury(II), e.g. in its hexasol- vates of water, dimethyl sulfoxide, pyridine 1-oxide, and pyridine. 4-8 A theoretical study has shown the energy gain to be small between four- and six-coordinated hydrated Zn 2+ ions. 2 Steric repulsion between the ligands is more important for the smaller zinc ion, and with the bulky monodentate oxygen-donor solvent tetramethylurea zinc forms four-coordinated solvates, although six-coordination is maintained with the larger Cd 2+ ion. 9 Mercury(II), cadmium, and in particular zinc ions often show tetrahedral four-coordination with soft sulfur-donor ligands, for example with sulfides and most alkyl xanthate, O,O-dialkyl dithiophosphate and N,N-dialkyldithiocarbamate ions. 3,10,11 A special feature of mercury(II) complexes with soft donor ligands is that two strong collinear bonds are frequently formed, often combined with weak equatorial contacts or secondary bonds to distant atoms, 12 sometimes giving rise to an axially compressed octahedron around the central mercury(II) ion in the solid state. 3 For mercury(II), a theoretical study shows that vibronic coupling destabilizes a regular octahedral coordination geometry. 13 This destabilization is found to be stronger with sulfur than with oxygen ligands, and no structure with regular coordination of six sulfur ligands around mercury(II) has been found experi- mentally. Evidently, increasing covalency of the metal-ligand bond strongly influences the coordination of the group 12 ions, and it was therefore of interest to study the effect of a soft donor ligand. The N,N-dimethylthioformamide molecule is a mono- dentate sulfur donor with a high dipole moment, µ ) 4.44 D. 14 It is also one of very few sulfur donor solvents with a sufficiently high permittivity, ǫ ) 47.5, 15 to sustain concentrated electrolyte solutions of divalent ions. Crystalline N,N-dimethylthioforma- mide solvates of zinc and cadmium ions have been prepared from saturated solutions with noncoordinating anions. 16 Single- crystal X-ray diffraction studies showed four- and six-coordina- tion around the Zn 2+ and Cd 2+ ions, respectively. For mercury- (II), however, only a two-coordinated crystalline solvate could be obtained when N,N-dimethylthioformamide was added to an acetonitrile solution; our attempts to obtain crystals from a saturated N,N-dimethylthioformamide solution of mercury(II) perchlorate failed. 16 However, in a discussion of how the chemical bonding influences the coordination of a metal ion, solution studies of the structure and of thermodynamic properties are preferable. In the crystalline state intermolecular interactions † Swedish University of Agricultural Sciences. ‡ Royal Institute of Technology. § Present address: Department of Physical Chemistry, Technical Uni- versity of Gdansk, ul. Gabriela Narutowicza 11/12, PL-80-952 Gdansk- Wrzeszcz, Poland. X Abstract published in AdVance ACS Abstracts, June 1, 1997. (1) Siegel, H.; Martin, R. B. Chem. Soc. ReV. 1994, 83. (2) Bock, C. W.; Kaufman Katz, A.; Glusker, J. P. J. Amer. Chem. Soc. 1995, 117, 3754. (3) (a) Prince, R. H. 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(13) Stro ¨ mberg, D.; Sandstro ¨ m, M.; Wahlgren, U. Chem. Phys. Lett. 1990, 172, 49. (14) Riddick, J. A.; Bunger, J. W. Techniques of Chemistry, Organic SolVents; Wiley-Interscience: New York, 1970. (15) Diggle, J. W.; Bogsa ´nyi, D. J. Phys. Chem. 1974, 78, 1018. (16) Stålhandske, C. M. V.; Stålhandske, C. I.; Sandstro ¨m, M.; Persson, I. Inorg. Chem. 1997, 36, 3167. 3174 Inorg. Chem. 1997, 36, 3174-3182 S0020-1669(96)01339-0 CCC: $14.00 © 1997 American Chemical Society