Studies on some Ni(II) and Zn(II) diaminomonocarboxylate complexes

Abstract

pH-metric, calorimetric, NMR and spectro-photometric studies were carried out on the Ni(II) and Zn(II) complexes of 2,3-diaminopropionic acid (dapa), 2,4-diaminobutyric acid (daba), 2,5-diaminopentanoic acid or ornithine (Orn) and 2,6-diaminohexanoic acid or lysine (Lys). It is concluded that the maximum number of coordinated ligands (with the exception of totally deprotonated Orn) is three in the case of Ni(II), but only two in the Zn(II) complexes. The ω-amino groups of dapa, daba and Orn take part in the coordination in the Ni(II) complexes. Hence, with Ni(II) both “diamine type” and “glycine-like” complexes are formed. The tendency to “diamine-like” coordination increases from Orn to dapa, so much so that dapa behaves much rather as a C-substituted 1,2-diamino ethane than a substituted glycine. Only Lys is coordinated to the Ni(II) ion exclusively in a “glycine-like” manner. Zn(II) complexes were studied only to a pH value of 9 (because of precipitation). In this pH region the terminal NH 2 groups take part in the coordination only in the cases of dapa and daba. With Orn and Lys “glycine-like”parent and mixed hydroxo complexes are formed.