When chemists think of the neutral radical ammonium, few think of it as behaving like an alkali metal. However, in many ways, particularly its electronegativity, which can be transferred from compound to compound, it acts like a pseudo-atom akin to sodium, according to computations by a team from the UK and the US.

Electronegativity is critical to explaining much of chemistry. It represents the ability of an atom to attract electrons from its neighbours and underpins the existence of polar covalent bonds. It also gives rise to the substitution pattern seen in aromatic compounds in which electronegative substituents redistribute charge. Electronegativity is usually considered a portable property of the atom in question. The presence of a sodium atom, for instance, will have the same effect due to its electronegativity regardless of the chemical environment in which it finds itself. But, can the same be said of pseudo-atoms, such as ammonium?

Maciej Gutowski and Alexander Whiteside of Heriot-Watt University and Sotiris Xantheas of the Pacific Northwest National Laboratory, set out to answer that question. They have now computed the electronegativity of ammonium in binary complexes. They investigated ammonium compounds of astatine and several borohydrides - analogues of simple alkali metal salts - and confirmed that the electronegativity is similar to the alkali metals. Ammonium most resembles a sodium atom having similar ionisation potential and electron affinity. The group point out that the value is relatively large with respect to the radius of ammonium. 'In many chemical environments the neutral ammonium radical will end up as the ammonium ion NH₄⁺, because the electronegativity of the neutral radical is low and it will give up an electron,' Gutowski says.

In order to account for the molecular properties, the team also considered the species' angular anisotropy, geometric relaxation and reactivity, properties which distinguish its molecular behaviour from its pseudo-atomic properties. Their predictions suggest that ammonium is very much akin to alkali metal atoms and, therefore, its electronegativity can be considered as being similarly portable between compounds. 'This work could be extended to functional groups, such as methyl, amino, hydroxyl, nitro etc,' Xantheas tells Chemistry World. 'The CN neutral radical, which typically forms CN^- ions, is a typical pseudohalogen, the neutral H₃O radical is another pseudoalkali, which typically becomes H₃O⁺,' Gutowski adds. These too might be similarly investigated.

'What I like most is that for the first time they put NH₄, a pseudo-alkali metal, under careful scrutiny and found its particular place in the alkali metals series,' says Alexander Boldyrev of Utah State University, US. 'This work opens up the opportunity to developing a comprehensive view on other pseudo-alkali metal species, pseudo-halogens and other pseudo-atoms.'

David Bradley

 

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