Isonitroso-keto-imino Nicle (II) complexes-A study in ambidentate coordination.

Abstract

In conclusion, it may be said that the isolation of the present complexes reveals the wider occurrence of linkage isomerism in isonitroso-p-keto-imine ligand complexes. These belong to the rare type of complexes where the two moieties of an organic chelating ligand coordinate into two different isomeric forms in the same molecule of the complex. It is, however, difficult to explain why the complex prefers such an asymmetric mode of coordination. Although the factors determining the ambidentate coordination of isonitroso-p-keto-imine ligands are not yet clear, interestingly, in all the complexes, it may be noticed that the >C=NO carbon has the substituent X, where X = CH?, OCH?, OC?H?, OC?H?, NHC?H?, etc. The influence of this carbonyl substituent X in determining ambidentate coordination may possibly depend on its electron-withdrawing or donating ability. The acetyl group in HIAI, the ethyl ester group in HIEAI, the methyl ester group in HIMAI, the benzoyl group in HIBI, and the anilide group in HIANI are electron withdrawing and have closely similar Hammett ? values (45,46) (Table 3-5). In contrast, the methyl substituent in bis(3-imino-2-butanone oximato)nickel(II), where both the ligands are suggested to coordinate through the isonitroso nitrogen atoms (25), the substituent on the carbon is an electron-donating methyl group, Hammett ? value being -0.170. This electron-withdrawing or donating ability of the substituent X on the >C=NO carbon atom thus appears to play a significant role in determining the ambidentate coordination of isonitroso-p-keto-imine ligand complexes.