An investigation into the substitution effects in Li₁₅Si₄, which is discussed as metastable phase that forms during electrochemical charging and discharging cycles in silicon anode materials, is presented. The novel partial substitution of lithium by magnesium and zinc is reported and the results are compared to those obtained for aluminum substitution. The new lithium silicides Li₁₄MgSi₄ (1) and Li_14.05Zn_0.95Si₄ (2) were synthesized by high-temperature reactions and their crystal structures were determined from single-crystal data. The magnetic properties and thermodynamic stabilities were investigated and compared with those of Li_14.25Al_0.75Si₄ (3). The substitution of a small amount of Li in metastable Li₁₅Si₄ for more electron-rich metals, such as Mg, Zn, or Al, leads to a vast increase in the thermodynamic stability of the resulting ternary compounds_. The ^6,7Li NMR chemical shift and spin relaxation time T₁-NMR spectroscopy behavior at low temperatures indicate an increasing contribution of the conduction electrons to these NMR spectroscopy parameters in the series for 1–3. However, the increasing thermal stability of the new ternary phases is accompanied by a decrease in Li diffusivity, with 2 exhibiting the lowest activation energy for Li mobility with values of 56, 60, and 62 kJ mol⁻¹ for 2, Li_14.25Al_0.75Si₁₄, and 1, respectively. The influence of the metastable property of Li₁₅Si₄ on NMR spectroscopy experiments is highlighted.

The novel ternary Zintl phase Li₃NaGe₂ comprises alkali-metal cations and [Ge₂]⁴⁻ dumbbells. The diatomic [Ge₂]⁴⁻ unit is characterized by the shortest Ge−Ge distance (2.390(1) Å) ever observed in a Zintl phase and thus represents the first Ge=Ge double bond under such conditions, as also suggested by the (8−N) rule. Raman measurements support these findings. The multiple-bond character is confirmed by electronic-structure calculations, and an upfield ⁶Li NMR shift of −10.0 ppm, which was assigned to the Li cations surrounded by the π systems of three Ge dumbbells, further underlines this interpretation. For the unperturbed, ligand-free dumbbell in Li₃NaGe₂, the π- bonding p_y and p_z orbitals are degenerate as in molecular oxygen, which has singly occupied orbitals. The partially filled π-type bands of the neat solid Li₃NaGe₂ cross the Fermi level, resulting in metallic properties. Li₃NaGe₂ was synthesized from the elements as well as from binary reactants and subsequently characterized crystallographically.

The novel ternary Zintl phase Li₃NaGe₂ comprises alkali-metal cations and [Ge₂]⁴⁻ dumbbells. The diatomic [Ge₂]⁴⁻ unit is characterized by the shortest Ge−Ge distance (2.390(1) Å) ever observed in a Zintl phase and thus represents the first Ge=Ge double bond under such conditions, as also suggested by the (8−N) rule. Raman measurements support these findings. The multiple-bond character is confirmed by electronic-structure calculations, and an upfield ⁶Li NMR shift of −10.0 ppm, which was assigned to the Li cations surrounded by the π systems of three Ge dumbbells, further underlines this interpretation. For the unperturbed, ligand-free dumbbell in Li₃NaGe₂, the π- bonding p_y and p_z orbitals are degenerate as in molecular oxygen, which has singly occupied orbitals. The partially filled π-type bands of the neat solid Li₃NaGe₂ cross the Fermi level, resulting in metallic properties. Li₃NaGe₂ was synthesized from the elements as well as from binary reactants and subsequently characterized crystallographically.

The reactivity of TiCp₂Cl₂ (d⁰) towards Zintl clusters was studied in liquid ammonia (Cp=cyclopentadienyl). Reduction of Ti^IVCp₂Cl₂ and ligand exchange led to the formation of [Ti^IIICp₂(NH₃)₂]⁺, also obtainable by recrystallization of [CpTi^IIICl]₂. Upon reaction with [K₄Sn₉], ligand exchange leads to [TiCp₂(η¹-Sn₉)(NH₃)]³⁻. A small variation of the stoichiometry led to the formation of [Ti(η⁴-Sn₈)Cp]³⁻, which cocrystallizes with [TiCp₂(NH₃)₂]⁺ and [TiCp₂(η¹-Sn₉)(NH₃)]³⁻. Finally, the large intermetalloid cluster anion [Ti₄Sn₁₅Cp₅]ⁿ⁻ (n=4 or 5) was obtained from the reaction of K₁₂Sn₁₇ and TiCp₂Cl₂ in liquid ammonia. The isolation of three side products, [K([18]crown-6)]Cp, [K([18]crown-6)]Cp(NH₃), and [K([2.2]crypt)]Cp, suggests a stepwise elimination of the Cl⁻ and Cp⁻ ligands from TiCp₂Cl₂ and thus gives a hint to the mechanism of the product formation in which [Ti(η⁴⁺²-Sn₈)Cp]³⁻ has a key role.

The reactivity of TiCp₂Cl₂ (d⁰) towards Zintl clusters was studied in liquid ammonia (Cp=cyclopentadienyl). Reduction of Ti^IVCp₂Cl₂ and ligand exchange led to the formation of [Ti^IIICp₂(NH₃)₂]⁺, also obtainable by recrystallization of [CpTi^IIICl]₂. Upon reaction with [K₄Sn₉], ligand exchange leads to [TiCp₂(η¹-Sn₉)(NH₃)]³⁻. A small variation of the stoichiometry led to the formation of [Ti(η⁴-Sn₈)Cp]³⁻, which cocrystallizes with [TiCp₂(NH₃)₂]⁺ and [TiCp₂(η¹-Sn₉)(NH₃)]³⁻. Finally, the large intermetalloid cluster anion [Ti₄Sn₁₅Cp₅]ⁿ⁻ (n=4 or 5) was obtained from the reaction of K₁₂Sn₁₇ and TiCp₂Cl₂ in liquid ammonia. The isolation of three side products, [K([18]crown-6)]Cp, [K([18]crown-6)]Cp(NH₃), and [K([2.2]crypt)]Cp, suggests a stepwise elimination of the Cl⁻ and Cp⁻ ligands from TiCp₂Cl₂ and thus gives a hint to the mechanism of the product formation in which [Ti(η⁴⁺²-Sn₈)Cp]³⁻ has a key role.

Reactions of the zinc(I) complex [Zn₂(Mesnacnac)₂] (Mesnacnac=[(2,4,6-Me₃C₆H₂)NC(Me)]₂CH) with solid K₃Bi₂ dissolved in liquid ammonia yield crystals of the compound K₄[ZnBi₂]⋅(NH₃)₁₂ (1), which contains the molecular, linear heteroatomic [BiZnBi]⁴⁻ polyanion (1 a). This anion represents the first example of a three-atomic molecular ion of metal atoms being iso(valence)-electronic to CO₂ and being synthesized in solution. The analogy of the discrete [BiZnBi]⁴⁻ anion and the polymeric [(ZnBi_4/2)⁴⁻] unit to monomeric CO₂ and polymeric SiS₂ is rationalized.

Die Natur der chemischen Bindungen in CaSi, einem Paradebeispiel für eine Zintl-Phase, wurde durch eine kombinierte Analyse der experimentellen und theoretischen Ladungsdichte untersucht, um das Zintl-Klemm-Konzept zu überprüfen. Das Vorliegen von kovalenten Si-Si-Wechselwirkungen, welche durch eine QTAIM-Analyse aufgedeckt wurden, unterstützt dieses fundamentale Bindungskonzept. Durch den Einsatz einer experimentellen Ladungsdichtestudie und theoretischer Bandstrukturanalysen konnten wir jedoch klare Beweise dafür finden, dass die Kation-Anion-Wechselwirkung nicht als rein ionisch beschrieben werden kann, sondern auch partiell kovalenten Charakter aufweist. Zudem weichen die integrierten QTAIM-Atomladungen der Ca-Kationen und der Si-Anionen mit ±1.28 e deutlich vom postulierten Idealwert von ±2 e ab, was in klarem Widerspruch zum ursprünglichen Zintl-Klemm-Konzept steht. Diese Beobachtungen liefern eine mögliche Erklärung für das unerwartete metallische Verhalten von CaSi.

The nature of the chemical bonds in CaSi, a textbook example of a Zintl phase, was investigated for the first time by means of a combined experimental and theoretical charge density analysis to test the validity of the Zintl–Klemm concept. The presence of covalent SiSi interactions, which were shown by QTAIM analysis, supports this fundamental bonding concept. However, the use of an experimental charge density study and theoretical band structure analyses give clear evidence that the cation–anion interaction cannot be described as purely ionic, but also has partially covalent character. Integrated QTAIM atomic charges of the atoms contradict the original Zintl–Klemm concept and deliver a possible explanation for the unexpected metallic behavior of CaSi.

Reactions of the zinc(I) complex [Zn₂(Mesnacnac)₂] (Mesnacnac=[(2,4,6-Me₃C₆H₂)NC(Me)]₂CH) with solid K₃Bi₂ dissolved in liquid ammonia yield crystals of the compound K₄[ZnBi₂]⋅(NH₃)₁₂ (1), which contains the molecular, linear heteroatomic [BiZnBi]⁴⁻ polyanion (1 a). This anion represents the first example of a three-atomic molecular ion of metal atoms being iso(valence)-electronic to CO₂ and being synthesized in solution. The analogy of the discrete [BiZnBi]⁴⁻ anion and the polymeric [(ZnBi_4/2)⁴⁻] unit to monomeric CO₂ and polymeric SiS₂ is rationalized.

Semiconducting Group 14 clathrates are inorganic host–guest materials with a close structural relationship to gas hydrates. Here we utilize this inherent structural relationship to derive a new class of porous semiconductor materials: noble gas filled Group 14 clathrates (Ng_x[M₁₃₆], Ng=Ar, Kr, Xe and M=Si, Ge, Sn). We have carried out high-level quantum chemical studies using periodic Local-MP2 (LMP2) and dispersion-corrected density functional methods (DFT-B3LYP-D3) to properly describe the dispersive host–guest interactions. The adsorption of noble gas atoms within clathrate-II framework turned out to be energetically clearly favorable for several host–guest systems. For the energetically most favorable noble gas filled clathrate, Xe₂₄[Sn₁₃₆], the adsorption energy is −52 kJ mol⁻¹ per guest atom at the LMP2/TZVPP level of theory, corresponding to −9.2 kJ mol⁻¹ per framework Sn atom. Considering that a hypothetical guest-free Sn clathrate-II host framework is only 2.6 kJ mol⁻¹ per Sn atom less stable than diamond-like α-Sn, the stabilization resulting from the noble gas adsorption is very significant.

A systematic approach to the formation of endohedrally filled atom clusters by a high-temperature route instead of the more frequent multistep syntheses in solution is presented. Zintl phases Na₁₂Ni_1−xSn₁₇ and K_13−xCo_1−xSn₁₇, containing endohedrally filled intermetalloid clusters [Ni@Sn₉]⁴⁻ or [Co@Sn₉]⁵⁻ beside [Sn₄]⁴⁻, are obtained from high-temperature reactions. The arrangement of [Ni@Sn₉]⁴⁻ or [Co@Sn₉]⁵⁻ and [Sn₄]⁴⁻ clusters, which are present in the ratio 1:2, can be regarded as a hierarchical replacement variant of the hexagonal Laves phase MgZn₂ on the Mg and Zn positions, respectively. The alkali-metal positions are considered for the first time in the hierarchical relationship, which leads to a comprehensive topological parallel and a better understanding of the composition of these compounds. The positions of the alkali-metal atoms in the title compounds are related to the known inclusion of hydrogen atoms in the voids of Laves phases. The inclusion of Co atoms in the {Sn₉} cages correlates strongly with the number of K vacancies in K_13−xCo_1−xSn₁₇ and K_5−xCo_1−xSn₉, and consequently, all compounds correspond to diamagnetic valence compounds. Owing to their diamagnetism, K_13−xCo_1−xSn₁₇, and K_5−xCo_1−xSn₉, as well as the d-block metal free binary compounds K₁₂Sn₁₇ and K₄Sn₉, were characterized for the first time by ¹¹⁹Sn solid-state NMR spectroscopy.

To gain more insight into the reactivity of intermetalloid clusters, the reactivity of the Zintl phase K₁₂Sn₁₇, which contains [Sn₄]⁴⁻ and [Sn₉]⁴⁻ cluster anions, was investigated. The reaction of K₁₂Sn₁₇ with gold(I) phosphine chloride yielded K₇[(η²-Sn₄)Au(η²-Sn₄)](NH₃)₁₆ (1) and K₁₇[(η²-Sn₄)Au(η²-Sn₄)]₂(NH₂)₃(NH₃)₅₂ (2), which both contain the anion [(Sn₄)Au(Sn₄)]⁷⁻ (1 a) that consists of two [Sn₄]⁴⁻ tetrahedra linked through a central gold atom. Anion 1 a represents the first binary AuSn polyanion. From this reaction, the solvate structure [K([2.2.2]crypt)]₃K[Sn₉](NH₃)₁₈ (3; [2.2.2]crypt=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) was also obtained. In the analogous reaction of mesitylcopper with K₁₂Sn₁₇ in the presence of [18]crown-6 in liquid ammonia, crystals of the composition [K([18]crown-6)]₂[K([18]crown-6)(MesH)(NH₃)][Cu@Sn₉](thf) (4) were isolated ([18]crown-6=1,4,7,10,13,16-hexaoxacyclooctadiene, MesH=mesitylene, thf=tetrahydrofuran) and featured a [Cu@Sn₉]³⁻ cluster. A similar reaction with [2.2.2]crypt as a sequestering agent led to the formation of crystals of [K[2.2.2]crypt][MesCuMes] (5). The cocrystallization of mesitylene in 4 and the presence of [MesCuMes]⁻ (5 a) in 5 provides strong evidence that the migration of a bare Cu atom into an Sn₉ anion takes place through the release of a Mes⁻ anion from mesitylcopper, which either migrates to another mesitylcopper to form 5 a or is subsequently protonated to give MesH.

The quaternary phases NaRb₇(Si_4–xGe_x)₂ (x = 1–3) crystallize with an NaRb₇Ge₈-type structure and contain tetrahedral four-atom clusters that consist of Si and Ge atoms. The quaternary phases possess larger cell volumes with increasing Ge amount. The formation of heteroatomic [Si_4–xGe_x]^4– clusters is indicated by chemically different Si environments, which were analyzed by means of solid-state ²⁹Si MAS (magic-angle spinning) NMR spectroscopy of a ²⁹Si-enriched NaRb₇(Si_4–xGe_x)₂ sample with x = 0.5 as well as quantum chemical calculations of the NMR coupling parameters. NaRb₇(Si_4–xGe_x)₂ represents another rare example of a phase with exclusively tetrahedral clusters that readily dissolves in liquid ammonia. The nature of the heteroatomic clusters was further investigated by dissolution of NaRb₇(Si_4–xGe_x)₂ with x = 2 in liquid ammonia. In the presence of MesCu (Mes = mesityl) and 18-crown-6 as a sequestering agent, crystals with the composition [Rb(18-crown-6)]₂Rb₂[(MesCu)₂(Si_4–xGe_x)](NH₃)₁₁ were isolated with x = 2.2(1). [(MesCu)₂(Si_4–xGe_x)]^4– represents another isomer of a MesCu-stabilized tetrahedral anion and supports the observation that Zintl phases, which contain solely tetrahedranide subunits, represent a new class of soluble Zintl phases.

The reaction of an ethylenediamine (en) solution of K₄Ge₉ with Zn₂(Mesnacnac)₂ {Mesnacnac = [(2,4,6-Me₃C₆H₂)NC(Me)]₂CH} in the presence of 18-crown-6 (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) leads to the formation of the new compound [K(18-crown-6)]₂{Zn[trans-μ₂(η³:η³-Ge₉)]}(en). A crystallographic structure determination revealed that the salt contains _∞¹[Zn(Ge₉)]^2– polyanions in which each Zn atom bridges two Ge₉ clusters by coordinating to opposite triangular faces of the Ge₉ deltahedra. The polymeric chain can be formally described as consisting of a trans Zn^II complex with two [η³:η³-Ge₉]^4– ligands. ¹H NMR spectroscopic investigations indicated that the protonation of the Mesnacnac^– ligand by the solvent ethylenediamine plays a crucial role in the disproportionation of the Zn^I starting material and thus in the formation of the polymeric Zn^II complex. In contrast, the reaction of ZnPh₂ instead of Zn₂(Mesnacnac)₂ under the same conditions leads to the known complex [PhZn(η⁴-Ge₉)]^3–.

A new type of Zintl phase is presented that contains endohedrally filled clusters and that allows for the formation of intermetalloid clusters in solution by a one-step synthesis. The intermetallic compound K_5−xCo_1−xSn₉ was obtained by the reaction of a preformed CoSn alloy with potassium and tin at high temperatures. The diamagnetic saltlike ternary phase contains discrete [Co@Sn₉]⁵⁻ clusters that are separated by K⁺ ions. The intermetallic compound K_5−xCo_1−xSn₉ readily and incongruently dissolves in ethylenediamine and in the presence of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (2.2.2-crypt), thereby leading to the formation of crystalline [K([2.2.2]crypt)]₅[Co₂Sn₁₇]. The novel polyanion [Co₂Sn₁₇]⁵⁻ contains two Co-filled Sn₉ clusters that share one vertex. Both compounds were characterized by single-crystal X-ray structure analysis. The diamagnetism of K_5−xCo_1−xSn₉ and the paramagnetism of [K([2.2.2]crypt)]₅[Co₂Sn₁₇] have been confirmed by superconducting quantum interference device (SQUID) and EPR measurements, respectively. Quantum chemical calculations reveal an endohedral Co¹⁻ atom in an [Sn₉]⁴⁻ nido cluster for [Co@Sn₉]⁵⁻ and confirm the stability of the paramagnetic [Co₂Sn₁₇]⁵⁻ unit.

The introduction of a mesityl (Mes; 2,4,6-Me₃C₆H₂) ligand to a Ge₉ polyanion is accomplished by the reaction of [Ge₉]^4– solutions with Ag₄Mes₄. The crystal structure investigation of its [K([2.2.2]crypt)] salt ([2.2.2]crypt: 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) shows that [Ge₉Mes]^3– comprises one exo-bonded aryl ligand in accordance with the ¹H NMR spectroscopic data. The formation of mono-, bis-, and tris-substituted Zintl Ions [Ge₉R_n]^(4–n)– (n = 1, 2, and 3; R = CHCH₂) is investigated by ¹H NMR spectroscopy. The mono- and bis-vinylated Ge₉ clusters, [Ge₉(CHCH₂)]^3– and [Ge₉(CHCH₂)₂]^2–, were obtained by the reaction of K₄Ge₉ with Me₃SiC≡CSiMe₃ in ethylenediamine. In the presence of [18]crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) and K(C₅H₅) crystals containing both cluster species were isolated and structurally characterized as[K([18]crown-6)]₂{(η⁵-C₅H₅)[K([18]crown-6)]₂}[Ge₉(CHCH₂)]and [K([18]crown-6)]{(η⁵-C₅H₅)[K([18]crown-6)]₂}[Ge₉(CHCH₂)₂], respectively. ¹H NMR experiments hint for the tris-vinylated cluster [Ge₉(CHCH₂)₃]^–.

For a long time, Zintl ions of Group 14 and 15 elements were considered to be remarkable species domiciled in solid-state chemistry that have unexpected stoichiometries and fascinating structures, but were of limited relevance. The revival of Zintl ions was heralded by the observation that these species, preformed in solid-state Zintl phases, can be extracted from the lattice of the solids and dissolved in appropriate solvents, and thus become available as reactants and building blocks in solution chemistry. The recent upsurge of research activity in this fast-growing field has now provided a rich plethora of new compounds, for example by substitution of these Zintl ions with organic groups and organometallic fragments, by oxidative coupling reactions leading to dimers, oligomers, or polymers, or by the inclusion of metal atoms under formation of endohedral cluster species and intermetalloid compounds; some of these species have good prospects in applications in materials science. This Review presents the enormous progress that has been made in Zintl ion chemistry with an emphasis on syntheses, properties, structures, and theoretical treatments.

Lange Zeit betrachtete man die Zintl-Ionen der Elemente der 14. und 15. Gruppe für bemerkenswerte, in der Festkörperchemie angesiedelte Spezies, die einerseits durch ihre unerwarteten Zusammensetzungen und faszinierenden Strukturen auffielen, andererseits aber von nur eingeschränkter Bedeutung waren. Neue Aufmerksamkeit erfuhren die Zintl-Ionen, als man feststellte, dass diese in festen Zintl-Phasen vorgeformten Spezies aus ihrem Kristallverband extrahiert, in geeigneten Solventien gelöst und damit als Reaktanten und Bausteine in der Lösungschemie verfügbar gemacht werden können. Der steile Anstieg von Forschungsaktivitäten in diesem Gebiet führte zu einer Fülle neuer Verbindungen, beispielsweise durch Funktionalisierung dieser Zintl-Ionen mit organischen Gruppen und metallorganischen Fragmenten, durch oxidative Kupplungsreaktionen unter Bildung von Dimeren, Oligomeren und Polymeren oder durch Einlagerung von Metallatomen, wobei endohedrale Cluster und intermetallische Verbindungen gebildet werden, von denen einige vielversprechende Perspektiven für Anwendungen in den Materialwissenschaften zeigen. Dieser Aufsatz berichtet über die enormen Fortschritte in der Chemie der Zintl-Ionen mit Hauptaugenmerk auf deren Herstellung, Eigenschaften, Strukturen und theoretischer Beschreibung.

Three novel Cu-capped Ge₉ clusters were synthesized from K₄Ge₉ and phosphane-stabilized copper(I) compounds CuCl(PR₃) with R = isopropyl (iPr) or cyclohexyl (Cy). Reactions in N,N-dimethylformamide (dmf) at ambient temperature resulted in the isolation of [Cu(η⁴-Ge₉)(PCy₃)]^3– as a [K([2.2]crypt)] salt ([2.2]crypt: 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane). From solutions of [Cu(η⁴-Ge₉)(PiPr₃)]^3– in liquid ammonia, the anion was isolated when the reaction mixture was stored at –70 °C. Under the same reaction conditions but at a temperature of –40 °C the cluster [Cu(η⁴-Ge₉)(η¹-Ge₉)]^7– crystallized in the presence of [2.2.2]crypt(4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosan)or [2.2]crypt. The ligand-free intermetalloid cluster [Cu(η⁴-Ge₉)(η¹-Ge₉)]^7– is a unique example in which two differently bonded Ge₉ units are linked by a transition-metal atom. This cluster complex remarkably demonstrates the ability of homoatomic polyanions to bind in different modes to the same transition-metal atom. One Ge₉nido cluster binds to the copper atom by means of its open square face, whereas the second Ge₉nido cluster is coordinated through one lone pair as a two-electron σ donor. The key role of the flexible electron-donor properties of [Ge₉]^4– in the reaction path towards the formation of intermetalliod clusters is discussed.

The endohedral stannaspherene cluster anion [Ir@Sn₁₂]³⁻ was synthesized in two steps. The reaction of K₄Sn₉ with [IrCl(cod)]₂ (cod: 1,5-cyclooctadienyl) in ethylenediamine (en) solution first yielded the [K(2,2,2-crypt)]⁺ salt (2,2,2-crypt: 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) of the capped cluster anion [Sn₉Ir(cod)]³⁻. Subsequently, crystals of this compound were dissolved in en, followed by the addition of triphenylphosphine or 1,2-bis(diphenylphosphino)ethane and treatment at elevated temperatures. [Ir@Sn₁₂]³⁻ was obtained and characterized as the [K(2,2,2-crypt)]⁺ salt. The isolation of [Sn₉Ir(cod)]³⁻ as an intermediate product establishes that the formation of the stannaspherene [Ir@Sn₁₂]³⁻ occurs through the oxidation of [Sn₉Ir(cod)]³⁻. Among the structurally characterized tetrel cluster anions, [Ir@Sn₁₂]³⁻ is a unique example of a stannaspherene, and one of the rare spherical clusters encapsulating a metal atom that is not a member of Group 10. Single-crystal structure determination shows that the novel Zintl ion cluster has nearly perfect icosahedral I_h point symmetry.

The Hg-substituted type-I clathrates A₈Hg_3+xGe_43–x (A = K, Rb) were obtained by solid-state reactions from the corresponding Zintl phases A₄Ge₉ and Hg or HgO. The crystal structures of the compounds were determined by single-crystal and powder X-ray diffraction methods. They crystallize in the space group Pmn, with a = 10.849(1) Å and x = 0.19(5) for K₈Hg_3+xGe_43–x (1) and a = 10.875(1) Å and x = 0.03(7) for Rb₈Hg_3+xGe_43–x (2). Specific atomic positions (6c site) of the Ge framework are partially substituted by Hg atoms, whereas the alkali-metal atoms reside at the center of the cavities. Both compounds exceed the electron-counting rule for Zintl phases. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The mercury-substituted type-I clathrates A₈Hg₄Sn₄₂,with A = K, Rb or Cs, were obtained by fusion of the pure elements at high temperatures. The crystal structures of the compounds were refined from single-crystal X-ray diffraction data. They crystallize in the space group Pmn (No. 223), Z = 1 with a = 12.1255(4) Å for K₈Hg₄Sn₄₂ (1), a = 12.1838(4) Å for Rb₈Hg₄Sn₄₂ (2) and a = 12.2130(4) Å for Cs₈Hg₄Sn₄₂ (3). The 3D framework of four-bonded atoms defines two types of polyhedral cages of different size that are fully occupied by the alkali-metal atoms. All three compounds are considered as formally charge-balanced Zintl phases without any homogeneity range. Differential thermal analysis (DTA) indicates that the stability of the clathrates significantly depends on the size of the encaged cations. The thermal stability of the title compounds and the binary phases A₈Sn₄₄ (A = K, Rb, Cs) is discussed. Temperature-dependent magnetic measurements for compound 3 show also the expected diamagnetic behaviour. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)