Nuclear Magnetic Resonance Studies of the Solution Chemistry of Metal Complexes. XIII. The Lability of the Metal–Nitrogen Bonds in the Cadmium, Zinc, and Mercury Complexes of Selected Polyaminocarboxylic Acids

Abstract

The lability of the metal–nitrogen bonds in the Cd(II), Zn(II), and Hg(II) complexes of (HO2CCH2)2N(CH2)nN(CH2CO2H)2 (n = 2, 3, 4) has been investigated by proton magnetic resonance spectroscopy. The acetate methylenic protons of each of the complexes give rise to AB multiplet patterns, indicating the metal–nitrogen bonds to be inert on the n.m.r. time scale. From the temperature dependence of the multiplet patterns, the relative lability of the metal–nitrogen bonds in each series of complexes has been established. In the Cd(II) and Zn(II) complexes, the metal–nitrogen bonds become more labile as the size of the internal chelate ring increases whereas it becomes more inert in the Hg(II) complexes. Also, the mercury–proton coupling constants increase as n is increased from two to four, which is interpreted as evidence that the increased kinetic stability results from structural changes to a more linear N—Hg—N arrangement. The metal–nitrogen bonds in the Cd(II) complexes having n = 3 and 4 is more labile than in the corresponding Zn(II) complexes. The thermodynamic stability patterns of the complexes are discussed in terms of their relative kinetic stabilities.