Low Dimensional Arrangements of the Zintl Ion [Ge₉-Ge ₉]^6- and Chemical Bonding in [Ge₆] ^2-, [Ge₉-Ge₉]^6-, and _∞¹{[Ge₉]}^2-

The compounds [Rb(Benzo-18-crown-6]₂Rb₄[Ge ₉- Ge₉](en)₆ (1), [Rb(Benzo-18-crown-6] ₂Rb₄[Ge₉-Ge₉](en)₄ (2), [K(18- crown-6)]₂K₄[Ge₉-Ge₉](en) ₂ (3), and [K(18-crown-6)]₃Cs₃[Ge₉- Ge₉](en)₂ (4) have been synthesized by dissolving the phases of the formal composition LiRb₃Ge₉, NaRb ₃Ge₉, K₄Ge₉, and K ₂Cs₂Ge₉, respectively, in ethylenediamine followed by the addition of the crown ethers under consideration. All four compounds contain a dimeric [Ge₉-Ge₉]^6- unit. Whereas the [Ge₉-Ge₉]^6- units in 1, 2 and 3 are connected via the alkali metal atoms to infinite chains of the composition _∞¹ {A₆[Ge₉-Ge₉]} (A = K or Rb) the connection via the Cs atoms in 4 leads to layers of the composition _∞²{K₂Cs₃[Ge ₉-Ge₉]}^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_2v symmetric [Ge₉] clusters are connected via an exo-bond between two Ge atoms of the open rectangle to form a dimeric [Ge₉-Ge₉]^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 Å and 2.521 Å. The analysis of the results of an Extended Hückel 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 [Ge₆]^2- cluster and of the polymer _∞¹{[Ge₉]}^2-.