1,4,8,11-tetraazacyclotetradecane (cyclam) is widely known as an ideal ligand for chelating heavy metal ions such as Ni 2+ and Cu 2+ in aqueous media. In this work the consequences of chelation on the preference for high spin or low spin configuration were investigated for Fe 3+ , Ni 2+ , Cu 2+ and Cr 3+. Two methods were used to determine the number of unpaired electrons in the solid and dissolved complex. First the change of magnetic susceptibility of the crystalline powder as a function of temperature was measured. In the second case Evans NMR-based method was used to obtain information about the number of unpaired electrons in the dissolved complexes. In some cases, such as Fe 3+ and Ni 2+ , a discrepancy between the two methods was noticed. This behaviour is discussed in terms of the preference of the large high spin Ni 2+-ion to switch to the smaller low spin Ni 2+-ion in the aqueous environment, which better fits the cavity of the ligand in the square planar geometry. The results are influenced by the geometry of the complex, the aggregation state and the dimensions of the ions.

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Thestabilityconstants of the macrocyclic 1 -thia-4,7-diazacyclononane-and 1 -thia-4,8-diazacyclodecane-NN-diacetic acid ( H,L1 and H2L2) and of the open-chain 2,5-diazahexane-N,N'-diacetic acid ( H2L3) with Mg", Call, Sr", Ball, Mn", Co", Ni", Cull, Zdl, Cd", Pb" and Lam1' have been determined in aqueous solution (25 "C, 0.1 mol dmP3 KNO,) by pH potentiometry, and in some cases in combination with visible absorption spectrophotometry. The complexation enthalpies with Cull have been determined by adiabatic calorimetry. The electronic absorption spectra of the complexes of Cull and Nil1 were also recorded. All metal-ion complexes with H2L1 are stronger than with H,L3, even for the harder metal ions. The presence of the thioether donor particularly enhances covalent bonding with Cull as indicated by a higher heat of complexation, a more favourable entropy change and a stronger ligand-field strength. The change in stability of the complexes upon replacing H2L1 with a five-membered chelate ring between the metal ion and the two tertiary nitrogens, by H2L2 having a six-membered one, is dependent on the metal-ion size. The larger metal ions are destabilised relative to the small metal ions: the metal ion size-based selectivity for this pair of ligands is controlled by the chelate-ring size. Binding of Cull by H2L2 is sterically much more efficient than by H2L1 and is evidenced by a higher heat of complexation and a stronger ligand field.There is an extensive amount of data concerning metal-ion complexation in solution by polyaminocarboxylate complexones, ' and the interest, particularly in macrocyclic N-acetatesubstituted macrocyclic polyamines, is still g r ~w i n g . ~-' ~

The trans-dioxopentaaza macrocycles 2,6-dioxo-1,4,7,10,13-pentaazacyclopentadecane (L 1 ) and 2,6-dioxo-1,4,7,11,14-pentaazacycloheptadecane (L 2 ) have been prepared. Their protonation and metal-binding properties with Cu 2ϩ and Ni 2ϩ have been investigated in 0.10 mol dm Ϫ3 KNO 3 (aq) at 25.0 ЊC by potentiometric pH-metry, calorimetry (in part), electronic absorption and IR spectroscopy. Metal-ion complexation promotes the deprotonation and the co-ordination of the amide nitrogens, resulting in neutral complexes with four nitrogen donors and a MLH Ϫ2 stoichiometry at pH 8. Additional complexes with stoichiometry MHL, ML and MLH Ϫ1 complete the complexation set. The overall stability constants of the complexes were calculated. The electronic spectra support a tetragonal co-ordination of Cu 2ϩ and a square-planar co-ordination of Ni 2ϩ in the MLH Ϫ2 complexes leaving one amino group unco-ordinated. It is suggested that the planar trigonal nature of the coordinated amido nitrogens in trans position to each other in the equatorial plane around Ni 2ϩ or Cu 2ϩ structurally prevents the fifth nitrogen donor from binding at an axial site. The NiL 2 H Ϫ2 complex exhibits a yellow-to-blue conversion equilibrium.