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The H+- and Cl--assisted dissociation kinetics and the stabilities of the complexes [Hg(sar)](2+) and [Hg((NH2)(2)-sar)](2+) (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane and (NH2)(2)-sar = 1,8-diamino-sar) were determined. The Hg2+ dissociation rates depend on both the proton and the chloride ion concentrations. H+ competes with the metal ion for dissociated amine groups, and Cl- competes with the amine for vacant coordination sites. The rate laws are complicated. For the [Hg(sar)](2+) system (0.1 less than or equal to [H+] less than or equal to 1.0 M, 0.01 less than or equal to [Cl-] less than or equal to 1.0 M, I = 2.0 M (NaO3SCF3), 25.0 degrees C) the observed rate law is upsilon(-Hg2+) = (a + b[Cl-])[H+][Hg(sar)(2+)]/(1 + c[Cl-]), with a = 35(3) M-1 s(-1), b = 2.9(4) x 10(3) M-2 s(-1), and c = 33(5) M-1. For the [Hg((NH3)(2)-sar)](4+) system (0.001 less than or equal to [H+] less than or equal to 1.0 M, 0.01 less than or equal to [Cl-] less than or equal to 1.0 M, I = 1.0 M (LiClO4), 25.0 degrees C) the observed rate law is upsilon(-Hg2+) = (a + b[H+] + c[H+](2))[Cl-][Hg((NH3)(2)-sar)(4+)])/((1 + d[Cl-])(1 + e[H+])), with a = 0.056(6) M-1 s(-1), b = 8(3) M-2 s(-1), c = 5(3) M-3 s(-1), d = 1.3(4) M-1, and e = 1.1(5) x 10(2) M-1. Intimate mechanisms for the dissociation reactions are proposed. Using iodide ion or sar ligand as competing ligands and the reported values for the stabilities of HeI3- and HgI42- the stability constants at 25.0 degrees C were determined for [Hg(sar)](2+) (10(28.1(1)) M-1), [Hg(sar)I](+) (10(29.1(1)) M-2), [Hg((NH2)(2)-sar)I](+) (10(28.5(1)) M-2), and [Hg(cyclam)I](+) (10(30.8(1)) M-2) (cyclam = 1,4,8,11-tetraazacyclotetradecane) with [OH-] = 0.1 M, I = 0.5 M (NaClO4) and for [Hg((NH2)(2)-sar)](2+) (10(26.4(3)) M-1) with [OD-] = 0.1 M, I = 0.1 M (NaOD).

The stability constants, log K1, have been determined by a potentiometric and u.v.-visible spectroscopic technique for the nickel(II) complexes of the tetra-aza macrocyclic ligands 1,4,7,10-tetra-azacyclododecane (L1) and 1,4,7,10-tetra-azacyclotridecane (L2). The value of log K1 in 0.1 mol dm–3 and 25 °C was found to be 16.4 ± 0.1 for L1 and 17.98 ± 0.05 for L2. The variation of log K1 for 12- to 16-membered ring tetra-aza-macrocycles as a function of metal-ion size is discussed. Two clear trends emerge. First, the larger the metal ion, the more strongly it prefers the smallest macrocycle, L1, and secondly, the smaller the metal ion, the more it prefers to complex with the 14-membered ring L3. This apparent paradox is explained in terms of molecular mechanics (m.m.) calculations, which show that the 12-membered macrocycle is more flexible than the 14-membered, and has effectively a larger cavity for accommodating the metal ion. A particularly puzzling feature is that low-spin nickel(II) complexes more strongly with L3 than with L2, even though the cavity size in L2 is a better fit for low-spin NiII as shown by m.m. calculations and numerous crystallographic studies. Possible causes of this are discussed.

On etudie le fait que les ions les plus grands se coordinent plus fortement au petit macrocycle 12-am-N 4 qu'au grand 14-am-N 4 . Le cycle du petit est plus flexible permettant une meilleure reponse au changement de taille de l'ion metallique. Constantes de stabilite des complexes de Cu(II), Pb(II) et Cd(II)