In 1932, E. Zintl reported on the binary phase NaTl as the first example of a compound with formally negatively charged thallium, the so-called thallides. The formal electron transfer from the less electronegative metal sodium to the more electronegative thallium results in a diamondlike thallium substructure, which was interpreted in terms of the pseudo-element approach, introduced by W. Klemm. While located left to the so-called Zintl border between group 13 and group 14, introduced by F. Laves, text-book known NaTl is still referred to as the first Zintl phase and therefore set a milestone for the chemistry of Zintl compounds in general. NaTl can be prepared using classical high-temperature solid-state synthesis. At the same time, a stoichiometry range is given in the related binary phase diagram. In addition, E. Zintl himself also prepared NaTl in a low-temperature solution route by reducing thallium(I) iodide by sodium in liquid ammonia. While the heavier congeners of the alkali metals, potassium and cesium, can be prepared by high-temperature synthesis, the low-temperature route has not yet been reported for these heavier alkali metals. Interestingly, KTl and CsTl do not yield NaTl analogue compounds, but instead [Tl6] -octahedra are present in the unit cells of the crystal structures of the latter compounds. Applying Wade rules for these clusters, a lack of electrons can be identified compared to an octahedral closo cluster, which would afford an eightfold negative charge for the [Tl6] entity. In KTl and CsTl, (2n) skeletal electrons of [Tl6] 6-classify them as hypoelectronic. This affects the shape of the clusters, as it significantly deviates from an ideal octahedral shape. The observed compression was explained as a result of a Jahn-Teller distortion. 9, 10 The binaries KTl and CsTl crystallize in different orthorhombic space groups (KTl: Cmce; CsTl: Fddd), but both still include [Tl6] 6-clusters as anionic moiety. Recently, it was shown, that the combination of potassium and cesium in ternary Cs1-xKxTl approaches allows for the formation of pentagonal bipyramidal shaped [Tl7] 7-clusters in Cs3.45K3.55Tl7 and Cs7.29K5.71Tl13. In general, mixing alkali metals increases the variety of thallide compounds, which are not yet accessible in binary materials. Concerning the thallides in an alkali metal thallium ratio of 1:1, the absence of RbTl at ambient conditions is remarkable, especially as the remaining binaries LiTl, NaTl, KTl, and CsTl are longtime known. For RbTl, only a high-pressure phase with NaTl structure has been mentioned. The binary phase diagram of the Rb-Tl system contains RbTl3 (=Rb4Tl13), RbTl2 (=Rb15Tl27) and Rb4Tl6 (=Rb8Tl11) but does not include an equimolar compound. In addition, a further binary compound Rb49Tl109.67 was later reported, which also does not appear in the phase diagram. The phase diagram for the binary system Cs-Tl again does not show an equimolar compound, but Dong and Corbett reported on CsTl already in 1996. These examples nicely demonstrate, that the absence of a binary compound in a phase diagram does not contradict its existence.