Polarographic Studies of the Geometric Isomers of the Bis(diethylenetriamine)Cobalt(III) and -cobalt(II) Cations in Acetone

Extensive studies have been made previously on the kinetically inert geometric isomers of [Co ^III(dien)₂]³⁺ (dien = the tridentate ligand diethylenetriamine); however, virtually no information has been available on the corresponding isomers of [CoII(dien)2]2+. Electrochemical reduction of the cobalt(III) complexes has been studied by dc polarography, differential pulse polarography, cyclic voltammetry, and controlled-potential electrolysis. Results have enabled equilibrium constants for the redox and isomer distribution to be calculated. Two reduction waves are found for each of the meridional (mer), symmetrical facial (s-fac), and unsymmetrical facial (u-fac) isomers. The step (mer, u-fac, s-fac) [CoIII(dien)2]3+ + e-⇄(mer, u-fac, s-fac) [CoII(dien)2]2+ occurs with retention of geometry on the electrochemical time scale. With respect to redox behavior, the s-fac-[CoIII(dien)2]3+ species is the hardest to reduce and the mer-[CoII(dien)2]2+ cation is the hardest to oxidize. In oxidation state II the complexes are kinetically labile and under conditions of controlled-potential electrolysis, the thermodynamically favored mer-[CoII(dien)2]2+ species is the only species found. Calculations suggest that compared with oxidation state III, the stability of the meridional form is considerably enhanced with respect to the facial isomers. On further reduction no cobalt(I) complexes are found and a two-electron step leading to formation of cobalt metal occurs: (mer, u-fac, s-fac) [CoII(dien)2]2+ + 2e- →Co + 2dien. With the N-methylated complex [CoIII(medien)₂]³⁺ (medien = NH₂CH₂CH₂N-(CH₃)CH₂CH₂NH2) stabilization of the s-fac isomer is enhanced remarkably and it is the only isomer detected in either oxidation state III or II.