Identification of the geometrical isomers of some tris-chelate cobalt(III) complexes by nuclear resonance

The proton resonance spectra of tris-(N-R-salicylaldimine) Co(III) complexes (R=methyl, ethyl, p-tolyl) and a new series of tris-(N-R-pyrrole.2-aldimine) Co(III) complexes (R=methyl. ethyl, isopropyl. sec-butyl) have been examined. The spectra provide unequivocal evidence for the existence of the trans isomer. In no case was any cis isomer detected. The apparent exclusive formation of the trans isomer is ascribed to significantly reduced steric interactions among the nitrogen substituents compared to the cis configuration. The more general formation of octahedral complexes in the pyrrole-2-aldimine than in the salicylaldimine series is concluded to be due to somewhat reduced steric interactions among these substituents. A general preparative method for tris(pyrrole-2-aldimine) Co(III) complexes is given