The synthesis and characterization of the amine-oxime complex [Ni(meso-HMPAO)-H]center dot ClO4 (1) and its oxidized Schiff-base product [Ni(meso-HMPAO)-5H] center dot ClO4 (2), where HMPAO is hexamethyl propylene amine oxime, are presented. Complex 2 results from the oxidation of 1 by molecular oxygen in basic aqueous solution. The structural change of the amine complex I to the Schiff-base complex 2 was investigated by the means of NMR spectroscopy and X-ray crystallography. (c) 2007 Elsevier Ltd. All rights reserved.