As part of our continuing studies of the stereochemistry of transition-metal complexes, cobalt(II1) complexes with the tetraamine ligand 1 ,lO-diamino-2,9-dimethyl-4,7-diazadecane were prepared. The two asymmetric carbon centers and two asymmetric nitrogen centers admit the possibility of six unique trans isomers. Two trans-dichloro complexes were isolated and characterized by IH and I3C NMR and by vibrational and electronic spectroscopy. Strain energy minimization calculations indicate the steric requirement for axial methyl substitution exceeds the induced ring strain leading to a high degree of stereoselectivity. Significant distortions in the chelate ring conformations are reflected in increased C-C-C and N-C-C angles of one of the six-membered chelate rings of one isomer. These conformational differences are apparent in both the 13C and proton NMR spectra.