[Co(trien)(Cl)₂]^{+
Triethylenetetramine (trien) is a linear tetradentate ligand in which the nitrogen donor atoms are connected by ethylene (two-carbon) linkges. In octahedral complexes, the trien ligand occupies four coordination sites, leaving two open. Three different geometric isomers are possible, differing in the way the ligand is wapped around the metal center; the symmerical-cis (alpha-cis), unsymmetrical cis (beta-cis) and trans. Furthermore, the symmetrical-cis and unsymmeytrical-cis have nonsuperimposable mirror images (not shown) giving a total of five stereoisomers.}