Werner, Alfred stereochemistry

As the present author was a student at the Eidgenossische Technische Hochschule in Zurich (1939-1945) and has been there as Professor since 1960, it may be mentioned that Arthur Hantzsch (1857-1935) was also a Professor at ETH (1885-1893) and that Emil Bamberger (1857-1932) was his successor (1893-1905). Hantzsch s stereochemical interpretation of the isomeric diazoates was based on his cooperation with Alfred Werner (University of Zurich, founder of stereochemistry, Nobel prize 1913), when Hantzsch was at ETH (see Sec. 7.1 and Zollinger, 1992). 

Chelate complexes of ethylenediamine provide many of the examples on which the theories of coordination chemistry have been founded. Co111 complexes of this ligand were studied by Alfred Werner and his students186 and the separation of or-CoCl(en)2(NH3)2+ into its optical enantiomers187 was a key factor in establishing octahedral stereochemistry. 

The early history of stereochemistry has been explored.174 An account of stereochemistry from the mid-19th century to 1960 surveys the proposals of van t Hoff and Le Bel as well as those of later workers in the field, such as Odd Hassel and D. H. R. Barton (1918-1998), and J. W. Comforth and V. Prelog (1906-1998).175 G. J. W. Bremer s experiments, which gave the initial experimental support to van t Hoff s stereochemical views, have been re-assessed through an analysis of the correspondence between the two chemists.176 There is a paper arguing that, in effect, van t Hoff and Le Bel produced two different theories177 and another one suggesting that John Dalton (1766-1844) was the first stereochemist.178 There is a brief analysis of the independent contributions of Alfred Werner and William Jackson Pope (1870-1939) to stereochemistry.179... 

The extensive chemistry of this oxidation state has two facets. The classical complexes containing a-bonded ligands have been known for nearly two centuries. At the beginning of the twentieth century, they were widely used by Alfred Werner in his fundamental research into the constitution, stereochemistry, and isomerism of coordination compounds. 

The semiquantitative stereochemical observations of Alfred Werner and his students, as well as later contributions by J. C. Bailar, Jr., J. P. Mathieu, P. Pfeiffer, M. Delephine, and others, have been ably reviewed by Fred Basolo 16, 17, 21, 22). Reviews emphasizing more recent investigations have also appeared 18, 68, 103), as have short reviews emphasizing aquation (acid hydrolysis) 47) and base hydrolysis 48, 90), both in terms of stereochemistry and kinetics. 

Tn a brilliant paper published in 1893 (38), Alfred Werner established octahedral stereochemistry for the 6-coordinate transition metal ion. In his deliberations Werner also considered both the hexagonal-planar and trigonal-prismatic structures but successfully eliminated these in the cases he investigated. It is a great tribute to Werner that in the 70-odd years since his historic work, countless structural investigations have failed to reveal a truly nonoctahedral, molecular, 6-coordinate complex. It has only been in the past year that an authentic example of nonoctahedral stereochemistry has been found. 

The work of Alfred Werner led to a considerable understanding of the stereochemistries of arrangements of ligands. This understanding was reinforced by X-ray diffraction studies of metal complexes, for example, in the determination of the structures of ammonium hexachloroplatinate and chloropalladite, shown in Figure 15.4. In these there are six groups around the metal ion, at an equal distance, rather like a small portion of the sodium chloride structure. 

The stereochemistry of coordination compounds is as old as coordination chemistry itself. The Swiss chemist Alfred Werner (1866-1919) derived the theory of coordination, published in 1893, to a large extent from stereochemical arguments. 

We emphasize almost completely , because there are important exceptions, e. g., Alfred Werner (1866-1919, Nobel Prize 1913) whose work on the stereochemistry of metal complexes was influenced by van t Hoff s and Le Bel s concept of the asymmetric carbon atom. 

The demonstration of the optical activity of octahedral complexes was important in confirming Alfred Werner s intuitive ideas about coordination chemistry. Early work involved the resolution of complexes characterized by optical rotations. Modem instmments for optical rotatory dispersion were developed first, but circular dichroism (CD) spectra proved to be more useful. CD has been a powerful tool for detailed studies of the stereochemistry of octahedral complexes. Contributions to rotational strength of chelate ring conformational, configurational, and vicinal contributions are additive. Chiral metal complexes are now used in enantioselective synthesis of chiral pharmaceuticals. 

Alfred Werner s ideas (1) were firmly based on his vision of the stereochemistry of metal coordination compounds. He was very successful in substantiating his ideas by the isolation of the number of isomers expected for octahedral complexes. This approach did not eliminate all other possibilities. 

This paper traces the development of optical activity in inorganic compounds to the point where Alfred Werner was able to use optical resolution to such striking effect in proving the octahedral stereochemistry of several metal ions by resolving chelated compounds, in particular the fully inorganic cation now called "Werner s hexol". This achievement came to fruition (1) in the years just prior to World War I. 

The tetrahedron is important in organic chemistry as representing the stereochemistry of the saturated carbon atom. It was first used in organic chemistry by Pasteur when he summarized his studies in 1862 on optical rotation of tartaric acid in solution. At about the same time Butlerov applied the tetrahedron concept to the carbon atom in connection with an assumed structure of ethane. Butlerov s paper influenced Kekule in the development of a tetrahedral carbon modeP useful for visualizing the links in acetylene H-C=CH and hydrogen cyanide, H-C=N. A few decades later Alfred Werner considered inorganic coordination compounds in a way analogous to that for carbon compounds. The octahedron was the key to a major portion of Werner s work which involved octahedral cobalt(III) ammines such as Co(NH3)6 + and their substitution products. 

In 1893, Alfred Werner proposed the coordination theory and demonstrated that stereochemistry is not limited to carbon compounds, but is a general phenomenon encompassing metal coordination compounds. This epoch-making theory not only succeeded in systematizing the transition-metal complexes but also afforded the foundation of modem structural inorganic chemistry. 

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