Low dimensional arrangements of the zintl ion [Ge-9-Ge-9](6-) and chemical bonding in [Ge-6](2-) , [Ge-9-Ge-9](6-), and (1)(infinity){[Ge-9]}(2-)

The compounds [Rb(Benzo-18-crown-6](2)Rb-4[Ge-9-Ge-9](en)(6) (1), [Rb(Benzo-18-crown-6](2)Rb-4[Ge-9-Ge-9](en)(4) (2), [K(18-crown-6)](2)K-4[Ge-9-Ge-9](en)(2) (3), and [K(18-crown-6)](3)Cs-3[Ge-9-Ge-9](en)(2) (4) have been synthesized by dissolving the phases of the formal composition LiRb3Ge9, NaRb3Ge9, K4Ge9, and K2Cs2Ge9, respectively, in ethylenediamine followed by the addition of the crown ethers under consideration. All four compounds contain a dimeric [Ge-9-Ge-9](6-) unit. Whereas the [Ge-9-Ge-9](6-) units in 1, 2 and 3 are connected via the alkali metal atoms to infinite chains of the composition (1)(infinity){A(6)[Ge-9-Ge-9]} (A = K or Rb) the connection via the Cs atoms in 4 leads to layers of the composition (2)(infinity){K2Cs3[Ge-9-Ge-9]}(1-). The increased number of dimers, which is now available, establishes the distortion of the clusters: The [Ge-9] clusters have the shape of a monocapped quadratic anti prism, which is compressed along one of the diagonal of the open rectangle. In each compound two idealized C-2nu symmetric [Ge-9] clusters are connected via an exo-bond between two Ge atoms of the open rectangle to form a dimeric [Ge-9-Ge-9](6-) anion. In all cases the exo-bond is oriented collinear to the shorter diagonal. Thus the square of the open rectangle of the cluster is compressed along the exo bond. The exo-bond lenghts are in the range of 2.471 Angstrom and 2.521 Angstrom. The analysis of the results of an Extended Huckel calculation using the electron localization function (ELF) reveals that a localized two-centre-two-electron bond connects the clusters of the dimeric unit. The results are compared to the ELF of an octahedral [Ge-6](2-) cluster and of the polymer (1)(infinity){[Ge-9]}(2-).