Source of information http://www.uncp.edu/home/mcclurem/cobalt/Cobalt.htm
Ligand is a molecule which has unshared electron pairs. Usually ligands contain the elements nitrogen, oxygen, or sulfur. A vast number of ligands are tetradentate, meaning that they have four donor atoms. When coordinated to Co(II), which has an octahedral geometry, six coordination sites are available. This means that tetradentate ligands occupy four of the six sites, leaving two sites open. These two open sites may be occupied by two monodentate groups such as chloride, or by a bidentate ligand such as carbonate or oxalate. The vast majority of tetradentate ligands fall under two categories: linear ligands and tetradentate ligands. Linear tetradentate ligands are exactly what their name implies; they are linear rather than branched. In contrast, tripodal tetradentate ligands have a central donor atom to which three separate arms are attached. The differences between these two types of tetradentate ligands and their geometries are discussed in the sections which follow.
Very early on it was recognized that octahedral complexes containing this ligand can adopt any one of three geometrical isomers, the geometry being entirely dependent upon the way in which the ligand coordinates to the cobalt center. These three isomers are the trans isomer, the symmetrical cis isomer, and the unsymmetrical cis isomer. In the trans isomer, all four of the donor atoms lie in one plane with the cobalt ion. The two monodentate groups are situated 180 degrees apart, and the complex possess a mirror plane of symmetry. The symmetrical cis isomer contains a C2 axis and the two monodentate ligands are 90 degress apart. The unsymmetrical cis isomer contains no symmetry elements at all.
The symmetry of these complexes are important from a spectroscopic standpoint. Since the symmetrical-cis isomer contains a C2 axis and the trans isomer contains a mirror plane, one would expect only three carbon signals arising from the triethylenetaramine. These two isomers could prove difficult to distinguish from their NMR standpoint. On the other hand, the trans isomer does not possess any symmetry elements, and so six carbon signals would be observed for any complex adopting this geometry.
One early attempt at producing a more sterospecific ligand was the symmetrical addition of methyl groups onto the ligand.
The first, studied by Robert Asperger and Chui Fan Liu, was reported to form the all three isomers. NMR spectroscopy was used to determine the isomerism of the isolated products. The symmetrical cis isomer was reported to be purple in color, whereas the trans isomer was reporetd to be green. A red color was reported for the unsymmetrical cis isomer.
The second ligand, studied by Yoshikawa, Sekihara, and Goto was found to adopt only the trans configuration. The complexes were found to preferentially form the unsymmetrical cis isomer when isolated as the dinitrite and oxalate species. Proton NMR helped to establish the identity of these isomers. As a symmetrical-cis or trans isomer, the methyl groups would have been equivalent and contributed to a single signal. However, two distinct signals were observed for the dinitrite and oxalate species, indicating the formation of the unsymmetrical isomer. A third chloro species was found to be a mixture of isomers.
Complexes of the third ligand are red in color and when isolated as the nitro salt, the results of NMR measurments indicate that this ligand coordinates to form only the unsymmetrical cis isomer.
The effect of placement of the methyl groups upon steroeochemistry is clearly illustrated by these studies. Placement of the methyl groups into the 2, 9 position yields has little effect upon the preferred isomer, whereas placement of methyl groups into the 3, 8, and 5, 6 positions yield and unsymmetrical cis isomers.
A second avenue of approach centered around changing the length of the carbon chains joining the donor atoms in triethylenetetramine. Would longer carbon chains between the nitrogen donors provide a ligand which would prefer one geometric isomer over the other? Considering only the symmetrical molecules, there are three possible variations called 2, 3, 2-tet, 3, 2, 3-tet, and 3, 3, 3-tet. The chemical formulas for these ligands are listed in the table below.
Dale Alexander and Hobart Hamilton studied the ligands 2, 3, 2-tet and 3, 2, 3-tet. Based upon the results of visible absorption spectroscopy, both complexes were reported to prefer the trans configuration. This is in contrast to the prototype triethylenetetramine, which is known to adopt all three geometric isomers as previously discussed. It was discovered that (3, 2, 3-tet) could be forced to to adopt a symmetrical cis geometry only if a bidentate ligand such as carbonate or oxalate was used to occupy the remaining two coordination sites.
Complexes with this ligand have been studied by Akamatsu, Komorita, and Shimura. Complexes with alanine, glycine, ethylenediamine, and several other bidentate ligands are reported. The various geometrical isomers were identified by visible absoprtion spectroscopy, circular dichroism spectra. The NMR was used only as a "fingerprint" to help in the identification of the various isomers.