Nevil Vincent Sidgwick proposed inert pair effect, explained bonding in coordination compounds

Nevil Vincent Sidgwick – a theoretical chemist

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Biography & contributions

Nevil Vincent Sidgwick was an English theoretical chemist born on May 08, 1873 – died on March 15, 1952. Sidgwick contributed his work to the understanding of chemical bonding, especially in coordination compounds.

Inert Pair Effect

Sidgwick proposed the inert pair effect. The inert pair effect is the tendency of the electrons in the outermost s atomic orbital to remain un-ionized or unshared in compounds of post-transition metals. The chemical inertness of the s electrons in the lower oxidation state is not always married to steric inertness (where steric inertness means that the presence of the s electron lone pair has little or no influence on the geometry of the molecule or crystal). A simple example of steric activity is that of SnCl2 which is bent in accordance with VSEPR theory.

Sidgwick’s work in organic nitrogen compounds, presented in his Organic Chemistry of Nitrogen (1910), was of enduring value. Sidgwick became absorbed by the study of atomic structure and its importance in chemical bonding. He explained the bonding in coordination compounds (complexes), with a convincing account of the significance of the dative bond.

In 1927, he proposed the inert pair effect which describes the stability of heavier p-block in an oxidation state two less than the maximum. In 1940 his Bakerian lecture with Herbert Marcus Powell correlated molecular geometry with the number of valence electrons on a central atom.

Sidgwick worked on kinetics (studying the rates of isomerisation of triphenylmethane dye intermediates and the hydration of carboxylic anhydrides), thermodynamics (investigating phase equilibria and the solubility of organic acids and bases), as well as investigating the colour of copper complexes. He worked on many other wartime projects, such as the production of phenol from benzene.

The anomalous low solubility and high vapour pressure of ortho-nitrophenol compared to the meta- and para- isomers was representative of a puzzle that Sidgwick had been wrestling with for some time, and in 1924 he published a paper which described the solution to the problem. The ortho-nitrophenol is able to make an intramolecular hydrogen bond. This is stronger than any hydrogen bond to water and so the solubility is low; there is little favourable energy available when ortho-nitrophenol dissolves in water in comparison to para- and meta-nitrophenol which can make hydrogen bonds to the water molecules.

The anomalously high vapour pressure of ortho-nitrophenol can be explained in a similar way; para- and meta-nitrophenol can make intermolecular hydrogen bonds which must be broken before molecules can evaporate into the vapour phase. These intermolecular hydrogen bonds are not present in the case of ortho-nitrophenol and so there are relatively more molecules of ortho-nitrophenol in the vapour phase (giving rise to a greater vapour pressure). Sidgwick book "The Electronic Theory of Valency" was published in 1927 as a culmination of many years' interest in the nature of covalent and (what we now call) dative bonds. This interest was no doubt sparked by discussions Sidgwick had had with Rutherford in 1914, and given further impetus by Bohr's publications on atomic structure.

He had brought together organic and inorganic chemistry by concluding that the covalency of the carbon-carbon bond was essentially no different to the co-ordinate link (dative bond) and so provided a firm basis for the general electronic theory of valency, including the concept of a lone pair of electrons.


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