Neuartige Isomeriefälle bei 1:2‐Cr III ‐ und Co III ‐Komplexen von o, o '‐Dihydroxyazoverbindungen: Pyramidal gebundener Stickstoff mit hoher Inversionsbarriere?

From the dimensions described in previously known X ‐ray structure analyses and from NMR.‐spectroscopical datas of a 1:2 Co III complex it is concluded that metal chelates of two and tri‐dentated azo compounds are present in the diketo or quinone mono hydrazone form, provided that such a form is possible. As the metal atoms replaces the hydrazone proton the coordinating nitrogen atom can be predetermined. For the first time in the case of 1:2 Cr complexes from o,o '‐dihydroxy azo compounds small amounts of byproducts have been observed which have the empirical composition of the main complexes. As the coordinated nitrogen atom is sp 3 ‐hybridized, its three bonds should be arranged in a pyramidal form. Therefore the ligands should be slightly bent. In the 1:2 Cr complexes where two of these ligands are coordinated to the metal atom perpendicularly to each other ( Drew ‐ Pfitzner ‐arrangement) 3 conformers (+ 3 mirror images) are possible. These conformers are ascribed to one group of the observed isomeric complexes. For another group it is supposed that at least one ligand is coordinated in the plane azo form. The postulated thrice bonded nitrogen atom possesses the requirements for a high inversion barrier: it is a member of two ortho condensed rings; moreover, it is bound to a Lewis acid. No such system seems to have been described, but in some published X ‐ray structure analyses, there is evidence to be found of its existence.