Alkaline Earth Metal Subnitrides

The structural principle of alkali metal suboxides has been unique for many years. Early attempts to synthesize cluster-type subnitrides of Ba resulted in the only compound, Ba2N, that is isotypic with Ca2N. Beyond these, the Sr analogue Sr2N, and most recently Sr4N3 [34], have been characterized. 

When reacting Ba dissolved in liquid Na with nitrogen at temperatures not too far from the melting point of the solvent, there was evidence for the formation of Ba-rich nitride clusters prior to the deposition of solid Ba2N [8]. Using both Na as well as a K-Na alloy as solvents, an extensive class of new subnitrides can be prepared. They contain [M6N] octahedra, either discrete or condensed into finite clusters and infinite arrays, respectively. 

Nevertheless, there is a puzzling feature with the structure of Ba3N. Whereas the distances between the chains in Til3 closely correspond to twice the van der Waals radius of I, the corresponding Ba-Ba distances in Ba3N are extremely long, 487 and 496 pm, hence exceeding by far the sum of metallic radii, 448 pm. 

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Alkali and Alkaline Earth Metal Suboxides and Subnitrides... 

The alkali metal suboxides are well understood in terms of formation, structure, and properties, yet we still face some unsolved problems that will also be addressed in the following. In contrast, the first suboxide of barium came to light only recently [24], and one has to see whether this compound is the tip of another iceberg, as turned out to be the case with the subnitrides of alkaline earth metals. 

Tab. 3.1-1. Structurally characterized suboxides and subnitrides of alkali and alkaline earth metals, respectively, Z refers to the structural formula.

The structural principle of combining salt-like bonded units with stoichiometric amounts of alkali metal in the structure of Ba2ONa is equivalent to that which characterizes the alkali metal suboxides as well as the subnitrides of alkaline earth metals, such that a variation at least according to a general formula [Ba20 Na% can be expected. Indeed, there are indications of the existence of further suboxides in... 

A. Simon, Alkali and alkaline earth metal suboxides and subnitrides. In M. Driess and H. Noth (eds.), Molecular Clusters of the Main Group Elements, Wiley-VCH, Weinheim, 2004, pp. 246-66. 

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