Aragonite is a metastable polymorph of calcium carbonate. The calcareous exoskeletons of some organisms like corals or molluscs consist essentially of aragonite. The questions of how, and why these organisms prefer the thermodynamically unstable aragonite for the construction of their hard shells are discussed. The importance of the biomineralization process for the development of new materials is outlined. In the experimental part, a very simple synthesis of polycrystalline aragonite is performed, using carbonated mineral water available at the market. The synthesized aragonite is easily identified by its infrared spectrum.

Dois novos complexos zwitteriônicos de oxorrênio(V), [ReOCl2(Hdhp)(PPh3)] (1) e [ReOBr2(Hdhp)(PPh3)] (2) (H2dhp = 2,3-dihidroxipiridina), foram sintetizados e caracterizados por espectroscopia de absorção no infravermelho, ressonância magnética nuclear de ¹H e 31P, análise elementar e determinação da estrutura cristalina e molecular por difração de raios X em monocristais. Os complexos apresentam geometria de coordenação octaédrica bastante distorcida, com os dois ligantes haletos arranjados em posições cis equatoriais, o ligante trifenilfosfina em posição trans a um dos haletos e o ligante Hdhp- coordenado de forma bidentada através de seus átomos de oxigênio, sendo um em posição trans ao ligante oxo e o outro em posição trans com relação ao outro haleto. Este ligante tem seu átomo de nitrogênio protonado. Os compostos 1 e 2 apresentam empacotamento cristalino bastante diferente, influenciado em ambos os casos por ligações de hidrogênio intermoleculares dos tipos N-H<FONT FACE=Symbol>...</FONT>X (X = Cl, Br) e N-H...O.

Two novel zwitterionic oxorhenium(V) complexes, [ReOCl2(Hdhp)(PPh3)] (1) and [ReOBr2(Hdhp)(PPh3)] (2) (H2dhp = 2,3-dihydroxypyridine), were synthesized and characterized by infrared spectroscopy, ¹H and 31P nuclear magnetic resonance, elemental analysis and crystal and molecular structure determination by X-ray diffraction on single crystals. Both complexes show distorted octahedral coordination geometry, with the halide ligands arranged in equatorial cis positions, the triphenylphosphine ligand in a trans position to one of the halides and the Hdhp- ligand coordinated in a bidentate form through its oxygen atoms, one in trans position to the oxo-ligand and the other in trans position to the second halide. The nitrogen atom of this ligand is protonated. Compounds 1 and 2 show quite different crystal packing, both influenced by hydrogen bonds of the types N-H...X (X = Cl, Br) and N-H<FONT FACE=Symbol>...</FONT>O.