Inorganic Metal Ions for forming Ligand Complexes to Isolate Organic Intermediate Compounds

 Chromium (III), Cobalt (III) and Iron Transition Metal Complexes

Cobalt

Cobalt (III) complexes are exceedingly numerous. Because they generally undergo ligand exchange reactions slowly, but not too slowly, they have, from the days of Werner and Jørgensen, been extensively studied and a large fraction of our knowledge of the isomerism, modes of reaction, and general properties of octahedral complexes as a class is based upon studies of Co^III complexes.

[Advanced Inorganic Chemistry 1966 pg. 873]

Chromium

Chromium ( III) is the most stable and important oxidation state of the element in general and particularly in the aqueous chemistry. “The foremost characteristic of this state is the formation of a large number of relatively kinetically inert complexes. Ligand displacement reactions of  Cr ^III complexes are about 10 times faster than those of Co ^III with half-times in the range of several hours. It is largely because of this chemical inertness that so many complex species can be isolated as solids and that they persist for relatively long periods of time in solution, even under conditions where they are thermodynamically quite unstable.” That is, the times for practical formation and degradation of these complexes are convenient- not too slow and not too fast.

[Advanced Inorganic Chemistry 1966 pg. 823]

Iron

Iron (III) forms a large number of complexes, mostly octahedral ones, and the octahedron may be considered its characteristic coordination polyhedron. When contemplating possible complex formation, ian important consideration is that the affinity of iron (III) for amine ligands is very low. No simple amine complexes exist in aqueous solution; addition of aqueous ammonia only precipitates the hydrous oxide. Chelating amines such as EDTA are slightly exceptional and do form some definite complexes among which is the 7 coordinate [Fe (EDTA):H₂O] ion. Also, those amines such as 2,2’-dipyridyl and 1,10-phenanthroline which produce ligand fields strong enough to cause spin-pairing form fairly stable complexes, isolable in crystalline form with large anions such as perchlorate but these too are exceptional and should not encourage organic chemists to try to make amine complexes with iron ion.

Transition Metals

Transition metals are now extensively used as catalysts in organic chemistry and indeed nickel, palladium and platinum complexes catalyze numerous reactions for which there is no uncatalyzed equivalent. Consequently an extensive chemistry has been established concerning the practical question of the recovery and recycling of  noble metal catalysts, mainly palladium and platinum, since these represent expensive inputs into a process.

My Perspective

Kilomentor is interested in potential complexes as a special means for isolation and purification. I look for areas where inexpensive transition metal complexes can simplify the work-up of chemical process steps.