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The first example of a Cu(II) [12]-MC-4 hydroxamic metallacrown compound containing a carboxyl group as a supporting donor function is described. The solution equilibria of malonomonohydroxamic acid (MACZ, H 2 L) with Cu(II) are investigated in aqueous solution using a combination of potentiometry, UV-vis absorption spectrophotometry, EPR spectroscopy and ESI mass spectrometry. Among the four complexes fitting the best speciation model ([CuL], [Cu 5 L 4 H À4 ] 2À , [CuL 2 ] 2À and [CuL 2 H À1 ] 3À ), a pentameric metallacrown molecule of composition Cu : L = 5 : 4 predominates in solution over the pH 4 to 11 range, and the corresponding complex was isolated in the solid state. The crystallization of the complex [Cu 5 L 4 H À4 ] 2À in the presence of [Cu(en) 2 (H 2 O) 2 ] 2+ cations resulted in the isolation of [Cu(en) 2 (H 2 O) 2 ] n [Cu(en) 2 (H 2 O)(m-H 2 O){Cu 5 (L 4 H À4 )(H 2 O) 3 } 2 ] n Á 20nH 2 O (1), whose crystal structure has been determined by X-ray analysis. The structure of 1 consists of centrosymmetric complex cations [Cu(en) 2 (H 2 O) 2 ] 2+ , infinite complex anionic chains [Cu(en) 2 (H 2 O)-(m-H 2 O){Cu 5 (L 4 H À4 )(H 2 O) 3 } 2 ] n 2nÀ and solvate water molecules. Within the complex anionic chains, the decanuclear double-decked bis([12]-MC-4) complex anions {Cu 5 (L 4 H À4 )(H 2 O) 3 } 2 4À are united by the [Cu(en) 2 (H 2 O) 2 ] 2+ complex cations due to the bridging function of the axial water molecule O (5). The magnetic behaviour of 1, studied in the temperature range 1.8-300 K, suggests the presence of both antiferromagnetic and ferromagnetic contributions to the observed magnetic susceptibility, resulting in a ground state of S = 2 per formula unit.

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The title compound, (C2H10N2)2[Ni(C2HN2O4)2].2H2O, has an ionic structure containing a centrosymmetric complex 4- anion, charge-balancing ethylenediaminium dications and solvent water molecules. The oxalohydroxamate unit is triply deprotonated and forms five-membered chelate rings with the central Ni ion; the Ni ion lies on an inversion centre. The two hydroxamate O atoms in the complex anion are linked by short intramolecular hydrogen bonds.

The title compound, [Ni(C 2 H 8 N 2 ) 3 ][Ni(C 3 HN 3 O 2 ) 2 ]ÁH 2 O, appears to be a modular associate consisting of two complex counter-ions, containing bivalent nickel as the central atom in both cases, and a solvent water molecule. The Ni II ion in the complex cation lies on the C 2 crystallographic axis. Its coordination environment is formed by six N atoms of three ethylenediamine (en) molecules, representing a distorted octahedral geometry. The Ni II ion in the complex anion occupies a position at the center of inversion. It exhibits a distorted square-planar coordination geometry formed by four N atoms belonging to the deprotonated oxidoimine and amide groups of the two doubly charged 2-cyano-2-(oxidoimino)acetamidate anions, situated in trans positions with respect to each other. In the crystal packing, the complex anions are linked by water molecules via hydrogen bonds between the amide O atoms and water H atoms, forming chains translated along the a direction. The [Ni(en) 3 ] 2+ cations ®ll empty spaces between the translational chains, connecting them by hydrogen bonds between the oxime and amide O atoms of the anions and the amine H atoms of the cations, forming layers along the ac plane. The water molecules provide connection between layers through N atoms of the cations, thus forming a three-dimensional modular structure.