Alfred Werner

Alfred Werner (December 12, 1866 - November 15, 1919) was a Swiss chemist who was a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry. He was the first inorganic chemist to win the Nobel prize, and in fact the only one prior to 1973.

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He was born in 1866 in Mulhouse, Alsace (which was then part of France, but which was annexed by Germany in 1871). He went to Switzerland to study chemistry at Zurich where he obtained his doctorate in 1890. After postdoctoral study in Paris, he returned to Zurich to teach in 1892, and became a professor as well a Swiss citizen in 1895.

Coordination chemistry

In 1893, Werner was the first to propose correct structures for coordination compounds containing complex ions, in which a central transition metal atom is surrounded by neutral or anionic ligands.

For example, it was known that cobalt forms a "complex" with formula CoCl₃•6NH₃, but the nature of the association indicated by the dot was mysterious. Werner proposed the structure [Co(NH₃)₆]Cl₃, with the Co³⁺ ion surrounded by six NH₃ at the vertices of an octahedron. The three Cl^- are dissociated as free ions, which he confirmed by measuring the electrical conductivity of the compound in aqueous solution.

For complexes with more than one type of ligand, Werner succeeded in explaining the number of isomers observed. For example, he explained the existence of two isomers of "Co(NH₃)₄Cl₃", one green and one purple. Werner proposed that these are two geometric isomers of formula [Co(NH₃)₄Cl₂]Cl, with one Cl^- ion dissociated as confirmed by conductivity measurements. The Co atom is surrounded by four NH₃ and two Cl ligands at the vertices of an octahedron. The green isomer is "trans" with the two Cl ligands at opposite vertices, and the purple is "cis" with the two Cl at adjacent vertices.

Werner also prepared complexes with optical isomers, and in 1914 he reported the first synthetic chiral compound lacking carbon, known as hexol with formula [Co(Co(NH₃)₄(OH)₂)₃]Br₆.

Nature of valence

Before Werner, chemists defined the valence of an element as the number of its bonds without distinguishing different types of bond. However in complexes such as [Co(NH₃)₆Cl₃] for example, Werner considered that the Co-Cl bonds correspond to a "primary" valence of 3 at long distance, while the Co-NH₃ bonds which correspond to a "secondary" or weaker valence of 6 at shorter distance. This secondary valence of 6 he referred to as the coordination number which he defined as the number of molecules (here of NH₃) directly linked to the central metal atom. In other complexes he found coordination numbers of 4 or 8.

On these views, and other similar views, in 1904 Richard Abegg formulated what is now known as Abegg's rule which states that that the difference between the maximum positive and negative valence of an element is frequently eight. This rule was used later in 1916 when Gilbert N. Lewis formulated the “octet rule” in his cubical atom theory.

Today Werner's primary valence corresponds to the oxidation state, and the secondary valence is always called coordination number. The Co-Cl bonds (in the above example) are now classed as ionic, and each Co-N bond is a coordinate covalent bond between the Lewis acid Co³⁺ and the Lewis base NH₃.

References

• W. Gregory Jackson, Josephine A. McKeon, Silvia Cortez (2004). "Alfred Werner's Inorganic Counterparts of Racemic and Mesomeric Tartaric Acid: A Milestone Revisited". Inorg. Chem. 43 (20): 6249 -6254. doi:10.1021/ic040042e.
• Kristin Bowman-James (2005). "Alfred Werner Revisited: The Coordination Chemistry of Anions". Acc. Chem. Res. 38 (8): 671 -678. doi:S0001-4842(04)00071-8 10.1021/ar040071t S0001-4842(04)00071-8.