Proton resonance spectra of some nickel complexes in aqueous solution

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Abstract
The proton resonance spectra of some aminopolycarboxylate complexes of nickel have been measured. The assignments of the large contact shifts of the CH protons are discussed, and it is suggested that the two protons of the N—CH2 group in a chelate ring are often non-equivalent. This is ascribed to the puckering of the rings, and a consideration of the structures of the complexes indicates that the magnitude of the shift for an N—CH proton depends on the dihedral angle across the group Ni—N—C—H in a manner similar to that found for the vicinal proton-proton coupling in aliphatic groups in diamagnetic compounds. The ligands N-2-hydroxyethyl iminodiacetate and nitrilo β-propionate diacetate, which are tetradentate in the 1 : 1 complexes, are shown to be tridentate in the 2 : 1 complexes. With the 2 : 1 complexes of iminodiacetate and N-methyl iminodiacetate, spatial isomers appear to be present.

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