NUCLEAR MAGNETIC RESONANCE STUDIES OF CONFIGURATION AND LIGAND CONFORMATION IN PARAMAGNETIC OCTAHEDRAL COMPLEXES OF NICKEL(II) VII. Bidentate Ligands Containing Oxygen Donors

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Abstract
The proton magnetic resonance spectra of the nickel(II) complexes of the ligands: ethylene glycol (eg), ethylene-glycol dimethyl ether (egde), ethanol amine (ea), 2-methoxy ethylamine (omea), and N-methyl ethanol amine (nmea) are reported. The contact shift of a proton adjacent to a coordinated oxygen atom is approximately 2/3 of the shift of a corresponding proton adjacent to a coordinated nitrogen atom. The contact shift is not sensitive to the stability constant of the complex. The complex formation constant rapidly decreases with increasing alkyl substitution on a coordinating oxygen atom. The formation constants for the complexes Ni(eg)(H2O)4 2+, Ni(eg)2(H2O)2 2+, Ni(egde)(H2O)4 2+, and Ni(ee)(H2O)4 2+ (ee = 2-ethoxyethanol) are 1.3 × 10−3, 2.9 × 10−5, 3.8 × 10−6, and < 10−6, respectively. The rate of oxygen inversion is slow in Ni(omea)(H2O)4 2+ (< 300 sec−1), and the OCH3 group shows essentially zero pseudo axial-equatorial conformational preference. Oxygen inversion occurs rapidly in Ni(ea)(H2O)4 2+ via an intermolecular proton exchange reaction involving hydrogen-bonded water molecules.