Zintl phase

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. 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. 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. 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. Exceptions There are examples of a new class of compounds that, on the basis of their chemical formulae, would appear to be Zintl phases, e.g., 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. Many examples similarly exist for heteroatomic clusters where the polyanion is composed of greater than one main group element. Some examples are listed below. Additionally, it is notable that fewer large cluster examples exist. 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. Reactivity As highly reduced species in solution, Zintl ions offer many and often unexpected, reaction possibilities, and their discrete nature positions them as potentially important starting materials in inorganic synthesis. After oxidation, the clusters may sometimes persist as radicals that can be used as precursors in other reactions. Zintl ions can oxidize without the presence of specific oxidizing agents through solvent molecules or impurities, for example in the presence of cryptand, which is often used to aid crystallization. The Zintl ion itself can also act as a ligand in transition metal complexes. This reactivity is usually seen in clusters composed of greater than 9 atoms, and it is more common for group 15 clusters. A change in geometry often accompanies complexation; however zero electrons are contributed from the metal to the complex, so the electron count with respect to Wade's rules does not change. In some cases the transition metal will cap the face of the cluster. Another mode of reaction is the formation of endohedral complexes where the metal is encapsulated inside the cluster. These types of complexes lend themselves to comparison with the solid state structure of the corresponding Zintl phase. These reactions tend to be unpredictable and highly dependent on temperature, among other reaction conditions. Examples • Group 14 anions functionalized with organic groups: [Ge9Mes]3−, [Ge9(CHCHCH2NH2)2]2−, [(CH2CH)Ge9Ge9(CHCH2)]4−, [Ge9(CHCHCHCH)Ge9]6−, [(CH2CH)Ge9(CH)4Ge9(CHCH2)]4−; • Silated anions: Ge9Hyp3Tl, [Ge9Hyp3]−; • Intermetalloid deltahedral clusters: [Co@Sn9]4−, [Ni@Pb10]2−, [Au@Pb12]3−, [Mn@Pb12]3−, [Rh3@Sn24]5−; • Exo coordinated transition metal complexes: [(ŋ2-Sn9)Hg(ŋ2-Sn9)]6−, [Ge5Ni2(CO)3]2−, [Sn8TiCp]3−, [(tol)NbSn6Nb(tol)]2−; • [Ni5Sb17]4− (Ni4Sb4 ring inside Sb13 bowl). == Electronic structure and bonding ==

Wade's rules The geometry and bonding of a Zintl ion cannot be easily described by classical two electron two center bonding theories; however the geometries Zintl ions can be well described by Wade’s rules of boranes. Wade’s rules offer an alternative model for the relationship between geometry and electron count in delocalized electron deficient systems. The rules were developed to predict the geometries of boranes from the number of electrons and can be applied to these polyanions by replacing the BH unit with a lone pair. DFT or ab initio molecular orbital calculations similarly treat the clusters with atomic, and correspondingly label them S, P, D etc. These closed shell configurations have prompted some investigation of 3D aromaticity. This concept was first suggested for fullerenes and corresponds to a 2(N+1)2 rule in the spherical shell model. An indicator of this phenomenon is a negative Nucleus Independent Chemical Shift (NICS) values of the center of the cluster or of certain additional high symmetry points. == Use in catalysis and materials science ==

Some Zintl ions show the ability to activate small molecules. One example from Dehnen and coworkers is the capture of O2 by the intermetallic cluster [Bi9{Ru(cod)}2]3−. Another ruthenium intertermetallic cluster, [Ru@Sn9]6−, was used as a precursor to selectively disperse the CO2 hydrogenation catalyst Ru-SnOx onto CeO2, resulting in nearly 100% CO selectivity for methanation. In materials science, Ge94− has been used as a source of Ge in lithium ion batteries, where is can be deposited in a microporous layer of alpha-Ge. The discrete nature of Zintl ions opens the possibility for the bottom up synthesis of nanostructured semiconductors and the surface modification of solids. ==See also==

• Video of preparation of K4Ge9 (subscription required)