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Inorganic Chemistrytriazole)copper(II) ( J / k = -17.9 to -19.O"K) than in ligand atoms of dichloro(l,2,4-triazole)copper(II).13 copper(I1) benzoate trihydrate ( J / k = -12.7'K), deThus, although normal magnetic moments have been spite the fact that the Cu-Cu distance of 3.40 A in the observed for these compounds a t room temperature, former is longer than 3.15 A in the latter. Therefore, the spins are not free from one another, but antiferrothe superexchange interaction rather than the intermagnetic interaction exists between neighboring spins action of direct nature is predominant. This conchin a chain. sion is supported by the existence of spin densities in the (16) M . Inoue and &I, x7torg. them., 5, 70 (1066).The repulsion between the lone electron pairs of the nitrogen atoms in the staggered hydrazine molecule influences the length of the nitrogen-nitrogen bond; diminution of the repulsion causes shortening of the bond in those derivatives of hydrazine where the lone pair is attracted to bonds. the shifts of v(h--N) are in agreement with this point of view. The band shifts from 880 toward 1000 cm-' when the repulsion between the lone pairs diminishes. I n metal complexes of hydrazine, shifts of the band are also observed, depending on the field effect of the metal cations on the lone pairs, even if the nitrogen-nitrogen bond distance remains unaltered.The infrared spectra of several hydrazine derivatives have been examined:The hydrazine molecule H2N-NH2 is composed of two tetrahedra in staggered configuration, 1, one corner in each being occupied by a lone pair. The nitrogennitrogen boiid distance is N-N = 1.46 A3 in solid hydrazine but it becomes shorter in the cation +H3N-

A complete thermodynamic study of the protonation and Cu(II) complex formation equilibria of a series of alpha- and beta-aminohydroxamic acids in aqueous solution was performed. The thermodynamic parameters obtained for the protonation of glycine-, (S)-alpha-alanine-, (R,S)-valine-, (S)-leucine-, beta-alanine- and (R)-aspartic-beta-hydroxamic acids were compared with those previously reported for gamma-amino- and (S)-glutamic-gamma-hydroxamic acids. The enthalpy/entropy parameters calculated for the protonation microequilibria of these three types of ligands are in very good agreement with the literature values for simple amines and hydroxamic acids. The pentanuclear complexes [Cu5L4H(-4)]2+ contain the ligands acting as (NH2,N-)-(O,O-) bridging bis-chelating and correspond to 12-metallacrown-4 (12-MC-4) which are formed by self-assembly between pH 4 and 6 with alpha-aminohydroxamates (HL), while those with beta- and gamma-derivatives exist in a wider pH range (4-11). The stability order of these metallomacrocycles is beta- >> alpha- > gamma-aminohydroxamates. The formation of 12-MC-4 with alpha-aminohydroxamates is entropy-driven, and that with beta-derivatives is enthalpy-driven, while with gamma-GABAhydroxamate both effects occur. These results are interpreted on the basis of specific enthalpies or entropy contributions related to chelate ring dimensions, charge neutralization and solvation-desolvation effects. The enthalpy/entropy parameters of 12-MC-4 with alpha-aminohydroxamic acids considered are also dependent on the optical purity of the ligands. Actually, that with (R,S)-valinehydroxamic acid presents an higher entropy and a lower enthalpy value than those of enantiopure ligands, although the corresponding stabilities are almost equivalent. Moreover, DFT calculations are in agreement with a more exothermic enthalpy found for metallacrowns with enantiomerically pure ligands.

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