THE USE OF THE ELECTRONIC LOCALISATION FUNCTION AND PARTIAL ELECTRONIC DENSITY FOR INVESTIGATING THE ELECTRONIC STRUCTURE OF THE ZINTL ANIONS IN BA₃GE₄

Fabio Zürcher, Reinhard Nesper

Laboratorium für Anorganische Chemie; ETH Zürich; Universitätstrasse 6; CH-8092 Zürich; Switzerland

Keywords: Zintl Phases, Electronic Localisation Function, Partial Electronic Density

The Zintl phases are intersting materials because of the almost infinite structural variety of their Zintl anions. The Zintl phase Ba₃Ge₄ [1] crystallises in a new structure type in the orthorhombic space group Cmmm. It contains isolated butterfly anions (Fig.1) and butterfly anions that are polymerised to infinite chains (Fig.2). In a concerted way an intermolecular bond is formed while the central intramolecular bond is broken. The structure of Ba₃Ge₄ is one of the few example of a polymerisation in the solid state. One of the most interesting characteristic of this structure is the presence of a reversible polymerisation in a single crystal, i.e. the polymer and the isolated monomers cohexist in the same crystal. It is also one of the few example of bond length isomerisation in the solid state. It is interesting to note that the intermolecular bond that connects the butterfly anions has a very long bond distances (287 pm).

Fig.1: isolated anions	Fig.2: polymerised anions

The electron localisation function (ELF) has already demonstrated to be an extraordinary powerful tool for investigating bonding structures for many different chemical systems by giving an easily and directly understandable picture of the chemical bond [2,3].

The partial electron density (PED) is an electronic density calculated in a selected energy window and it can be drawn as a 3-dimensional isosurface. It is suitable for giving a picture of the occupied orbitals in the chosen energy window.

The ELF is used to investigate the bonding structure of the Zintl anions contained in Ba₃Ge₄ (Fig.3). For a better undestanding of the bonding structure near the Fermi level, the use of the PED is shown here (Fig.4).

Of particolary interest is the understanding of the intramolecular and intermolecular bond of the isolated and polymerised buttefly anions, respectively.

Fig.3: ELF of the anions	Fig.4: PED of the anions

1. F. Zürcher; Dissertation; ETH Zürich (1998), F. Zürcher, R. Nesper; in Prep.
2. A. Savin, R. Nesper, S. Wengert, T.F. Fässler; Angew. Chem.; 109; 1892 (1997); Angew. Chem. Int. Ed. Engl.; 36; 1808 (1997)
3. A. Savin, T.F. Fässler; Chemie in unserer Zeit; 3; 110 (1997)