Werner Complexes

The history of modern coordination chemistry has been the subject of several books, of which perhaps the best known are those by George B. Kauffman. By necessity they all highlight Alfred Werner, "the Father of Coordination Chemistry" who in 1893, proposed the octahedral configuration of transition metal complexes and in 1913 received the first Nobel prize in Inorganic Chemistry.

In this series of experiments, some simple cobalt(III) complexes are to be prepared, which show some of the properties that Werner was able to interpret using the octahedral model. These include; optical, geometric and linkage isomerism.

Reaction Schemes as ISIS Draw .tgf file         Hexol.html

The oxidation of cobalt sulfate to the tetraamminecarbonate complex can be done by passing air through the solution for 2 hours, or more conveniently using hydrogen peroxide (although this is more costly, it allows the experiment to be easily completed in one session). The conversion of the bis-aqua complex to "hexol" given here is the method originally used by Jorgensen. By contrast, Werner added pyridine to a hot, dilute acetic acid solution. "Hexol" exists in several hydrated forms, all of them dark purple-black crystals. Air-drying the crystals has been found to result in the ennea-hydrate, whereas drying at 98C or over sulfuric acid results in the tetrahydrate.
The nitrite ion (NO2-) is an example of an ambidentate ligand. That is, it can form a bond to a metal via either of two non-equivalent sites, from the N or from an O.
In the second scheme, two linkage isomers of the type [Co(NH3)ONO]2+ and [Co(NH3)5NO2]2+ are prepared starting from [Co(NH3)5Cl]2+.
The actual isomer obtained depends on the pH of the solution. it should be noted that the two isomers are in equilibrium and that the nitro compound is the more stable. Whereas isomerisation takes several months at room temperature it is greatly accelerated by heating. Therefore the products should NOT be heated.

Kinetic Study

The nitrito isomer converts slowly at room temperature to the nitro isomer and the conversion can be conveniently followed by observing the disappearance of the nitrito band at approximately 1060 cm-1 in the IR spectrum.
Collect an IR spectrum of the penataamminechlorocobalt(III) chloride for comparison. The pellet should be kept in a dessicator in the dark at room temperature between readings. At the end of the experiment, the KBr pellet is placed in an oven at 100C to complete the isomerisation and to obtain an A(infinity) measurement.
Plot a graph of log (A-Ainf) versus time. The rate constant k for isomerisation is equal to 2.303*slope and the reaction is found to obey first-order kinetics.

References

"Classics in Coordination Chemistry - Part 1", G.B. Kauffman, Dover Publications Inc., N.Y., 1968.
"Alfred Werner - Founder of Coordination Chemistry", G.B. Kauffman, Springer-Verlag, Berlin, Heidelberg, New York, 1966.
"Inorganic Coordination Compounds", G.B. Kauffman, Heyden & Son, Ltd., London, Philadelphia, Rheine, 1981.
Cobalt(III) Ammines - "Werner" Complexes, A.M. Greenaway and R.J. Lancashire, J. Chem. Educ., 1982, 59, 419-420.
Coordination Complexes of Cobalt, G.M. Williams, J. Olmsted III and A.P. Breksa III, J. Chem. Educ., 1989, 66, 1043-1045.
The Resolution of a Completely Inorganic Coordination Compound, T. Yasui, T. Ama and G.B. Kauffman, J. Chem. Educ., 1989, 66, 1045-1048.

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