Trigonal Bipyramidal Clusters [Tl ₅ ] ⁷⁻ : Synthesis and Characterization of the <i>Zintl</i> Phase K _7−x <i>A</i> _x Tl ₅ ( <i>A </i> = Rb, Cs; 0 &lt; x ≤ 2.35)

In alkali metal thallides, a huge variety of thallium clusters can be realized, which are surrounded only by alkali metals. In most cases, different cluster types are present in the crystal structures at the same time. One example is the [Tl₅]^7- trigonal bipyramid, which has only been observed beside [Tl₉]^9-, [Tl₄]^8-, [Tl₃]^7-, [Tl]^5-, or [Tl₈Cd₃]^10-. In the new ternary alkali metal thallide phase K_7-xA_xTl₅ (A = Rb, Cs; 0 < x ≤ 2.35), exclusively [Tl₅]^7- clusters are present, and the compound can be described as a salt-like Zintl-phase, where the electron count is balanced in terms of the anionic moiety and counter cations. This phase crystallizes in the orthorhombic noncentrosymmetric space group Ama2. Different temperature programs applied during synthesis suggested metastability, which was subsequently proven by DSC measurements. DFT calculations reveal a minimum in the DOS at E_F and support the salt-like description. Dissolution experiments in the style of well-known group 14 and group 15 solution chemistry in liquid ammonia were performed and showed oxidation of the alkali metal thallide. Initial ²⁰⁵Tl NMR studies in liquid ammonia allow for the detection of the first signal of a dissolved thallium species emerging from a thallide Zintl phase.