In chemistry, a Zintl phase is a product of a reaction between a group 1 (alkali metal) or group 2 (alkaline earth metal) and main group metal or metalloid (from groups 13, 14, 15, or 16). It is characterized by intermediate metallic–ionic bonding. Zintl phases are a subgroup of brittle, high-melting intermetallic compounds that are diamagnetic or exhibit temperature-independent paramagnetism and are poor conductors or semiconductors.Sevov, S.C., Zintl Phases in Intermetallic Compounds, Principles and Practice: Progress, Westbrook, J.H.; *Freisher, R.L.: Eds.; John Wiley & Sons. Ltd., Chichester, England, 2002, pp. 113–132 Slavi Chapter
This type of solid is named after German chemist Eduard Zintl who investigated them in the 1930s. The term "Zintl phases" was first used by Laves in 1941.Laves F (1941) Naturwissenschaften 29:244 (Eduard Zintls Arbeiten über die Chemie und Struktur von Legierungen) In his early studies, Zintl noted that there was an atomic volume contraction upon the formation of these products and realized that this could indicate cation formation. He suggested that the structures of these phases were ionic, with complete electron transfer from the more electropositive metal to the more electronegative main group element. The structure of the anion within the phase is then considered on the basis of the resulting electronic state. These ideas are further developed in the Zintl–Klemm–Busmann concept, where the polyanion structure should be similar to that of the isovalent element. Further, the anionic sublattice can be isolated as polyanions (Zintl ions) in solution and are the basis of a rich subfield of main group inorganic chemistry.
History A "Zintl phase" was first observed in 1891 by M. Joannis, who noted an unexpected green colored solution after dissolving lead and sodium in liquid ammonia, indicating the formation of a new product.M. Joannis, Hebd. Seances Acad. Sci. 1891, 113, 795. It was not until many years later, in 1930, that the stoichiometry of the new product was identified as Na4Pb9 by titrations performed by Zintl et al.; and it was not until 1970 that the structure was confirmed by crystallization with ethylenediamine (en) by Kummer.
In the intervening years and in the years since, many other reaction mixtures of metals were explored to provide a great number of examples of this type of system. There are hundreds of such compounds composed of group 14 elements and group 15 elements, plus dozens of others beyond those groups, all spanning a variety of different geometries.zintl ions 14 15 review Corbett has contributed improvements to the crystallization of Zintl ions by demonstrating the use of chelating ligands, such as cryptands, as cation sequestering agents.
More recently, Zintl phase and ion reactivity in more complex systems, with organic ligands or transition metals, have been investigated, as well as their use in practical applications, such as for catalytic purposes or in materials science.
Zintl phases Zintl phases are intermetallic compounds that have a pronounced ionic bonding character. They are made up of a polyanionic substructure and group 1 or 2 counter ions, and their structure can be understood by a formal electron transfer from the electropositive element to the more electronegative element in their composition. Thus, the valence electron concentration (VEC) of the anionic element is increased, and it formally moves to the right in its row of the periodic table. Generally the anion does not reach an octet, so to reach that closed shell configuration, bonds are formed. The structure can be explained by the 8 − n rule (replacing the number of valence electrons, n, by VEC), making it comparable to an isovalent element. The formed polyanionic substructures can be chains (one-dimensional), rings, and other two-or three-dimensional networks or molecule-like entities.
The Zintl line is a hypothetical boundary drawn between groups 13 and 14. It separates the columns based on the tendency for group 13 elements to form metals when reacted with electropositive group 1 or 2 elements and for group 14 and above to form ionic solids. The 'typical salts' formed in these reactions become more metallic as the main group element becomes heavier.
Synthesis Zintl phases can be prepared in regular solid state reactions, usually performed under an inert atmosphere or in a molten salt solution. Typical solid state methods include direct reduction of corresponding oxides in solution phase reactions in liquid ammonia or mercury. The product can be purified in some cases via zone refining, though often careful annealing will result in large single crystals of a desired phase.
Characterization Many of the usual methods are useful for determining physical and structural properties of Zintl phases. Some Zintl phases can be decomposed into a Zintl ion—the polyanion that composes the anionic substructure of the phase—and counter ion, which can be studied as described below. The heat of formation of these phases can be evaluated. Often their magnitude is comparable to those of salt formation, providing evidence for the ionic character of these phases. Density measurements indicate a contraction of the product compared to reactants, similarly indicating ionic bonding within the phase.W Biftz and F. Weibke, Z. Anorg. Allg. Chem. 223, 321 (3935) X-ray spectroscopy gives additional information about the oxidation state of the elements, and correspondingly the nature of their bonding. Conductivity and magnetization measurements can also be taken. Finally, the structure of a Zintl phase or ion is most reliably confirmed via X-ray crystallography.
Examples An illustrative example: There are two types of Zintl ions in K12Si17; 2x (pseudo P4, or according to Wade's rules, 12 = 2n + 4 skeletal-electrons corresponding to a nido-form of a trigonal-bipyramid) and 1x (according to Wade's rules, 22 = 2n + 4 skeletal-electrons corresponding to a nido-form of a bicapped square antiprism)
Examples from Müller's 1973 review paper with known structures are listed in the table below. +Cation/Anion groupIIIIVVVIVIILiLi3Al Li9Al4 LiGa LiIn LiTlLi22Si5 Li7Si2 Li22Ge5
Li9Ge4 Li22Sn5
Li2Sn5Li3P LiP Li3P7 LiP4 LiP5 LiP7 LiP15
Li3As LiAs Li3Sb Li3Bi LiBiLi2S Li2Se Li2TeLiCl LiBr LiINaNaGa4 NaIn Na2Tl (the polyanion is tetrahedral (Tl4)8− Concept Tl2- ~ P)
NaTl (See Figure)NaSi (the polyanion is tetrahedral (Si4)4− Concept Si− ~ P) NaxSi136 (x ≤ 11) Na8Si NaGe Na15Pb4 Na13Pb5
Na9Pb4 NaPbNa3P NaP Na3P7 Na3P11 NaP7 NaP15
Na3As Na3Sb NaSb Na3Bi NaBiNa2S Na2Se Na2Se2
Na2TeNaCl NaBr NaIKKIn4K12Si17 K8Si46 K8Ge46 K8Sn46
KPb KPb2K3P KP K4P6 K3P7 K3P11 KP15
K3As K3Sb KBi2K2S K2S2 K2Se
K2Se2 K2TeKCl KBr KIRbRbIn4RbSi RbGe RbSn RbPbRb4P6 Rb3P7 Rb3P11 RbP7 RbP11 RbP15
Rb3As Rb3Bi RbBi2Rb2SRbCl RbBr RbICsCsSi CsGe CsSn CsPbCs2P4 Cs4P6 Cs3P7 Cs3P11 CsP7 CsP11 CsP15
Cs3Sb Cs3Bi CsBi2
Cs2NaAs7 (See Figure)CsCl CsBr CsIMgMg17Al12 Mg5Ga2 Mg2Ga
MgGa MgGa2 Mg2Ga5 Mg3In
Mg5In2 Mg2In
MgIn MgIn5 Mg2Tl MgTlMg2Si Mg2Ge Mg2Sn
Mg2PbMg3P2 Mg3As2 Mg3Sb2
Mg3Bi2MgS MgSe MgTe MgTe2MgCl2 MgBr2 MgI2CaCaAl2 CaAl4 CaGa CaGa2 CaGa4 CaIn CaIn2 CaTl CaTl3CaSi CaSi2 Ca7Ge
Ca2Ge CaGe2 Ca3Sn Ca2Sn
CaSn CaSn3 Ca3Pb Ca2Pb
Ca5Pb3CaS CaSe CaTeCaCl2 CaBr2 CaI2SrSrAl2 SrGa2 SrGa4 SrIn2 SrTl SrTl2 SrTl3Sr5Si3 SrSi Sr4Si7
SrSi2 Sr3Ge4
SrGe2 SrSn SrPb3Sr3P2 Sr3P14 SrBi3SrS SrSe SrTeSrCl2 SrBr2 SrI2BaBaAl4 BaGa2 BaGa4 BaIn2 BaIn4 BaTl2Ba5Si3 BaSi Ba3Si4
BaSi2 Ba2Ge
BaGe BaGe2 BaSn Ba5Pb3
BaPb BaPb3Ba3P2 BaP3 BaBi3BaS BaS3 BaSe BaTeBaCl2 BaBr2 BaI2
Exceptions There are examples of a new class of compounds that, on the basis of their chemical formulae, would appear to be Zintl phases, for example, K8In11, which is metallic and paramagnetic. Molecular orbital calculations have shown that the anion is (In11)7− and that the extra electron is distributed over the cations and, possibly, the anion antibonding orbitals. Another exception is the metallic InBi. InBi fulfills the Zintl phase requisite of element-element bonds but not the requisite of the polyanionic structure fitting a normal valence compound, i.e., the Bi–Bi polyanionic structure does not correspond to a normal valence structure such as the diamond Tl− in NaTl.
Zintl ions Zintl phases that contain molecule-like polyanions will often separate into its constituent anions and cations in liquid ammonia, ethylenediamene, crown ethers, or cryptand solutions. Therefore, they are referred to as Zintl ions. The term 'clusters' is also used to emphasize them as groups with homonuclear bonding. The structures can be described by Wade's rules and occupy an area of transition between localized covalent bonds and delocalized skeletal bonding. Beyond the "aesthetic simplicity and beauty of their structures" and distinctive electronic properties, Zintl ions are also of interest in synthesis because of their unique and unpredictable behavior in solution.
The largest subcategory of Zintl ions is homoatomic clusters of group 14 or 15 elements. Some examples are listed below.
Many examples similarly exist for heteroatomic clusters where the polyanion is composed of greater than one main group element. Some examples are listed below. Zintl ions are also capable of reacting with ligands and transition metals, and further 'heteroatomic examples are discussed below (intermetalloid clusters). In some solvents, atoms exchange can occur between heteroatomic clusters. Additionally, it is notable that fewer large cluster examples exist.
Examples
Homoatomic clusters [Si4]4− [Si5]2− [Si9]2− [Si9]4− [Ge4]4− [Ge5]2− [Ge9]3− [Sn4]4− [Sn5]2− [Pb4]4− [Pb9]4− [P4]2− [P7]3− [P11]3− [As6]4− [As7]3− [Sb8]8− [Sb11]3− [Bi4]2− [Bi7]3− [Bi11]3−
Heteroatomic clusters AsP3 [Ge2Sn2]4− [Sn2Bi2]2− [Sn3Bi3]5− [Pb2Sb2]2− Sn5Sb3 [InBi3]2− Bi14Ge4 [GaBi3]2− [In4Bi5]3− [TlSn8]3− [TlSn9]3− [Sb@In8Sb12]3− [Sb@In8Sb12]5−
Synthesis Zintl ions are typically prepared through one of two methods. The first is a direct reduction route performed at low temperature. In this method, dry ammonia is condensed over a mixture of the two (or more) metals under inert atmosphere. The reaction initially produces solvated electrons in ammonia that reduce the more electronegative element over the course of the reaction. This reaction can be monitored by a color change from blue (solvated electrons) to the color of the Zintl phase. The second is method, performed at higher temperatures, is to dissolve a Zintl phase in liquid ammonia or other polar aprotic solvent like ethylenediamine (on rare occasions DMF or pyridine is used). Some Zintl ions, such as Si and Ge based ions, can only be prepared via this indirect method because they cannot be reduced at low temperatures.
Characterization The structure of Zintl ions can be confirmed through X-ray crystallography. Corbett has also improved the crystallization of Zintl ions by demonstrating the use of chelating ligands such as cryptands, as cation sequestering agents.
Many of the main group elements have NMR active nuclei, thus NMR experiments are also valuable for gaining structural and electronic information; they can reveal information about the flexibility of clusters. For example, differently charged species can be present in solution because the polyanions are highly reduced and may be oxidized by solvent molecules. NMR experiments have shown a low barrier to change and thus similar energies for different states. NMR is also useful for gaining information about the coupling between individual atoms of the polyanion and with the counter-ion, a coordinated transition metal, or ligand. Nucleus independent chemical shifts can also be an indicator for 3D aromaticity, which causes magnetic shielding at special points.
Additionally, EPR can be used to measure paramagnetic in relevant clusters, of which there are a number of examples of the [E9]3− type, among others.
Reactivity