Clusters in Neat Solids and from Solutions

In a salt-like description of these compounds, AE, the alkali metal A transfers its valence electron to the tetrel , and - in agreement with the Zintl-Klemm-Busmann concept [9] - the tetrel elements form P4-analogous tetrahedral clusters [ 4] . These highly charged clusters [ 4] are retained in a liquid ammonia solution of RbPb, as shown by Korber et al. via the crystallization of the ammoniate Rb4Pb4(NH3)2 [10]. The isotypic compounds A4Sii4(N 143)2 were obtained for A = Rb and Cs from reactions of the element Sn, as well as from Cs with Sn(C6Hs)4 in liquid ammonia [10]. 

The structures of Djh-symmetric [Sns] and [Pbs] clusters were elucidated some time ago, in 1977 [11]. They form in ethylenediamine solutions of A-E alloys with different A E ratios in the presence of [2.2.2]crypt, and without any additional oxidizing agents. A similar route led to the discovery of [Ges] about 20 years later [12]. This anion was gained from reactions in liquid ammonia, and also characterized as an [A + ([2.2.2]crypt)] salt with A = K or Rb [13]. For the synthesis ofthe corresponding [Sis] , the binary compound Ki2Sii7 was extracted 

Intermetallic compounds of the general composition A12E17 contain both tetrahedral clusters [ 4 and [Eg] polyanions (A = Na, K, Rb, Cs and E = Si, Ge, Sn) [15, 16[. In these compounds, two [ 4] units and one anionic cluster [Eg] are counterbalanced by 12 positively charged alkali metal cations. 

Nonetheless, in most cases the potassium salts K4E9 (E = Ge, Sn, Pb) are selected to study the chemistry of Geg, Sng and Pbg clusters [2, 4], whereas the first crystal structure determination was presented for Na4Sng(en)7 by Diehl, Rummer and Strahle [7[. 

The structures of [Eg] clusters in alkali metal-crypt or rown-ether salts [2] were consecutively determined for E = Ge, Sn and Pb, and the series was recently completed by Korber, who presented the crystal structure of [Rb([18]crown-6)] Rb3Sig(NH3)4, crystals, obtained by dissolving the ternary phase K6Rb6Sii7 in liquid ammonia in the presence of [18]crown-6. The structure of the [Sig] anion deviated slightly from the expected monocapped square antiprism, and was coordinated by seven Rb cations [21]. 

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