The d 10 ions MII (M = Zn, Cd, Hg) and four equivalents of the neutral zwitterion trimethylglycine (betaine, bet) aggregate to discrete six-coordinated cationic complexes. With nitrate as counter anions, solids of the composition [M(bet)4](NO3)2·H2O are obtained. Their structural characterization reveals close relationship: they crystallize in metrically similar unit cells adopting three different subgroups of a hypothetical aristotype. In all cases, the divalent cations exhibit a strongly distorted coordination sphere in which two betaine ligands bind with both carboxylato O atoms and the other two with only one oxygen to the cation. Non-coordinating O atoms of different betaine ligands act as hydrogen bond acceptors towards co-crystallized water molecules. At least for the Pearson-soft HgII cations, this coordination is unexpected: to the best of our knowledge, HgII complexes with four carboxylato ligands have not yet been characterized. Without the necessity for charge balancing, the composition of zwitterion complexes is dominated by steric requirements and possibly by competition with alternative dipole molecules such as solvent water.

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