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鈥淚onic compounds form because metals want to give up valence electrons and nonmetals want to gain valence electrons鈥� 鈥� a听convenient fiction for听students starting out in chemistry. But the part of the statement referring to metals is as fictitious as the听Tooth Fairy. Let us start by looking at the ionization energy and electron affinity of a gaseous sodium atom:

Na(g) 鈫� Na+(g) + e^鈭� 螖E = +496 kJ鈭檓ol^鈭�1
Na(g) + e^鈭� 鈫� Na鈭�(g) 螖E = 鈭�53 kJ鈭檓ol^鈭�1

Thus it takes nearly a half of a megajoule of energy per mole to remove an electron from gaseous sodium but addition of an听electron releases energy. In other words, sodium 鈥渨ants鈥� to听acquire an electron!

There was a nice analogy, for which I have lost the original听source, that: 鈥渟odium no more 鈥榳ants鈥� to donate an electron than听you would happily donate your purse or wallet to a mugger.鈥澨齌hat is, it is a competition for the electron, for example, with听fluorine for which the electron affinity is much higher. So ionic听bonding is not benign, but atomic 鈥渘ature red in tooth and claw,鈥澨齣n other words, a fight for those valence electrons.

F(g) + e鈭� 鈫� F鈭�(g) 螖E = 鈭�328 kJ鈭檓ol^鈭�1

In this article, we will focus on the importance of electron affinity,but as I have discussed previously, lattice energy plays a very听significant role in the formation of solid ionic compounds.¹

The table below shows the trend in electron affinities.听A plot of electron affinities of gaseous atoms听(credit: Wikipedia, DePiep)

听
It can be seen that the alkali metals have higher electron听affinities than听the alkaline earth metals. There are the three听noticeable 鈥渟pikes鈥� which can be accounted for by completion,听or half-completion of the valence levels as follows:

Alkali metals: ns¹ 鈫� ns²
Group 14 elements: ns²np² 鈫� ns²np³
Halogens: ns²np⁵ 鈫� ns²np⁶

The sodide ion

If the formation of the sodide ion, Na鈭�, is energetically favoured, then听compounds containing that ion should be feasible. It was in 1974 that听James L. Dye and his researchers at Michigan State University synthesized the first known compound containing the sodide ion.2 Dye realized that, in the solid phase, there was little energy needed for the formation of the sodium cation-anion pair:

2 Na(s) 鈫� Na⁺(s) + Na^鈭�(s)

The key, then, was to find a way of keeping the two ions听separated. To do this, he caged the sodium ion in a bicyclic diaminoether, commonly known as 2,2,2-crypt. The synthesis was successful and gold-coloured crystals of [Na⁺(C₁₈H₃₆N₂O₆)]Na鈭� were produced.

From the crystal structure, the radius of the sodide ion was calculated to be听217 pm, close to that of the iodide ion, and the sodide compound has a structure similar to that of the analogous听iodide: [Na⁺(C₁₈H₃₆N₂O₆)]I^鈭� .

In 1987, Concepci贸n and Dye synthesized a very simple听compound of the sodide ion: [Li(diaminoethane)₂]⁺Na^鈭�.³ It is a听concern of mine that the teaching of chemistry is so 鈥渇ossilized鈥澨齮hat a discovery of decades ago has still not altered the mindsets听of chemistry instructors. And it is not because the听information is obscure, for even Wikipedia has a page on the
alkalides.⁴

The auride ion

Looking at the plot of electron affinities, gold stands out as an听obvious candidate for anion formation. In fact, the first evidence听for the formation of an auride came in 1937 by the equimolar听mixing of cesium and gold.⁵ This transparent yellow compound听was shown in 1959 not to be an alloy, but to be Cs⁺Au^鈭�, with a听sodium chloride crystal structure. Since then, several other听auride compounds have been synthesized, including Cs₃AuO,
which has the perovskite crystal structure and contains听(Cs⁺)₃(Au^鈭�)(O₂^鈭�).

As can be seen from the plot, the element preceding gold, platinum, has a high electron affinity, too. Thus it should come听as no surprise that there is an increasing chemistry of the听platinide ion, Pt^2鈭� including cesium latinide, Cs₂Pt.

Commentary

For beginners in chemistry, the idea of give and take of听electrons resulting in ionic compound formation makes sense.听The difficulty arises in dissuading them of the erroneous concept听when they advance to a more in-depth study of bonding.听

References

1. G. Rayner-Canham, 鈥淐hemical compounds: Why some are nonexistent,鈥澨�Chem 13 News, pages 4-5, February 2012.
2. F.J. Tehan, B.L. Barnett and J.L. Dye, 鈥淎lkali Anions,鈥� Journal of American Chemical Society, 1974, 96, pages 7203-8
3. R. Concepci贸n and J.L. Dye, 鈥淟i+(en)2鈭橬a鈭�: A Simple CrystallinSodide,鈥� Journal of American Chemical Society, 1987, 109, pages 7203-4.
4. 鈥淎lkalide,鈥� http://en.wikipedia.org/wiki/Alkalide, accesses December 12, 2012.
5. See: M. Jansen, 鈥淓ffects of relativistic Motion of Electrons on the Chemistry of Gold and Platinum,鈥漇olid State Sciences, 2005, 7, pages 1464-74.听