Cysteamine and Its Homoleptic Complexes with Group 12 Metal Ions. Differences in the Coordination Chemistry of Zn^II, Cd^II, and Hg^II with a Small N,S-Donor Ligand

2-Ammoniumethanethiolate, -SCH2CH2NH3+, the first structurally characterized zwitterionic ammoniumthiolate, is the stable form of cysteamine (HL) in the solid state and in aqueous solution. Reactions of ZnCl2, Cd(Oac)2, and HgCl2 with cysteamine and NaOH in a 1:2:2 ratio, respectively, lead to the homoleptic complexes ML2. Their single-crystal X-ray structures demonstrate basic differences in the coordination chemistry of ZnII, CdII, and HgII. While chelating N,S-coordination modes are found for all metal ions, ZnII forms a mononuclear complex with a distorted tetrahedral Zn(N2S2) coordination mode, whereas HgII displays a dimer with Hg(N2S2) coordinated monomers being connected by two long Hg···S contacts. Solid-state 199Hg NMR spectra of HgL2 and [Hg(HL)2]Cl2 reveal a low-field shift of the signals with increasing coordination number. Strong and nearly symmetric Cd−S−Cd bridges in solid CdL2 lead to a chain structure, CdII displaying a distorted square pyramidal Cd(N2S3) coordination mode. The ab initio [MP2/LANL2DZ(d,f)] structures of isolated ML2 show a change from a distorted tetrahedral to bisphenoidal coordination mode in the sequence ZnII−CdII−HgII. A natural bond orbital analysis showed a high ionic character for the M−S bonds and suggests that the S−M−S fragment is best described by a 3c4e bond. The strength of the M···N interactions and the stability of ML2 toward decomposition to M and L−L decreases in the sequence Zn > Cd > Hg. Ab initio calculations further suggest that a tetrahedral S−M−S angle stabilizes ZnII against substitution by CdII and HgII in a M(N2S2) environment. Such geometry is provided in zinc-finger proteins, as was found by a database survey.