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.^[8]

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.^[8] Density measurements indicate a contraction of the product compared to reactants, similarly indicating ionic bonding within the phase.^[10] 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 Si⁴⁻
₄ (pseudo P₄, or according to Wade's rules, 12 = 2n + 4 skeletal-electrons corresponding to a nido-form of a trigonal-bipyramid) and 1x Si⁴⁻
₉ (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.^[8]

Structure of [As₇]^3- subunit in the Zintl phase Cs₂NaAs₇. The trianion adopts the structure of P₄S₃. Concept: As⁻ ~ S.^[11]

Diamondoid framework of Tl⁻ ions in NaTl. NaTl consists of a polyanion (—Tl⁻—)_n with a covalent diamond structure. Na⁺ ions are located between the anions. Concept: Tl⁻ ~ C.^[12]

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.^[13] 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.^[14]

The crystal structure of InBi with and without the In–Bi bonds to highlight the Bi–Bi polyanionic structure.

Examples

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).^[18] 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.^[18]

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.^[7]

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.^[17] 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.^[19]

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.^[18]

In solution, individual Zintl ions can react with each other to form oligomers and polymers. In fact, anions with high nuclearity can be viewed as oxidative coupling products of monomers.^[20] 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.^[15]

Zintl ion clusters can be functionalized with a variety of ligands in a similar reaction to their oligomerization. As such, functionalization competes with those reactions and both can be observed to occur. Organic groups, for example phenyl, TMS, and bromomethane, form exo bonds to the electronegative main group atoms. These ligands can also stabilize high nuclearity clusters, in particular heteroatomic examples.^[15]

Similarly in solids, Zintl phases can incorporate hydrogen. Such Zintl phase hydrides can be either formed by direct synthesis of the elements or element hydrides in a hydrogen atmosphere or by a hydrogenation reaction of a pristine Zintl phase. Since hydrogen has a comparable electronegativity as the post-transition metal it is incorporated as part of the polyanionic spatial structure. There are two structural motifs present. A monatomic hydride can be formed occupying an interstitial site that is coordinated by cations exclusively (interstitial hydride) or it can bind covalently to the polyanion (polyanionic hydride).^[21]

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.^[15] 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.^[16] These reactions tend to be unpredictable and highly dependent on temperature, among other reaction conditions.

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.^[18] Some unique clusters of Ge occur in non-deltahedral shapes that cannot be described by Wade’s rules.  The rules also become more convoluted in intermetallic clusters with transition metals and consideration needs to be taken for the location of the additional electrons.

Zintl-Kelmm-Busmann concept

The Zintl-Klemm-Busmann concept describes how in an anionic cluster, the atoms arrange in typical geometries found for the element to the right of it on the periodic table.  So “the anionic lattice is isometric with elemental lattices having the same number of valence electrons.”^[8] In this formulation, the average charge on each atom of the cluster can be calculated by:

${\displaystyle {\frac {{\text{anion valence}}+{\text{cation valence}}}{n_{a}}}={\text{VEC}}}$

where n_a is number of anion atoms and VEC is the valence electron concentration per anion atom, then:

${\displaystyle 8-{\text{VEC}}={\text{number of bonds per anion atom}}}$.^[18]

The number of bonds per anion predicts structure based on isoelectronic neighbor. This rule is also referred to as the 8 - N rule and can also be written as:

${\displaystyle {\frac {n_{e}+b_{a}-b_{c}}{n_{a}}}=8}$.

Not all phases follow the Zintl-Klemm-Busmann concept, particularly when there is a high content of either the electronegative or electropositive element.  There are still other examples where this does not apply.^[8]

Electronic theory

Wade's rules are successful in describing the geometry of the anionic sublattice of Zintl phases and of Zintl ions but not the electronic structure. Other 'spherical shell models' with spherical harmonic wave functions for molecular orbitals—analogous to atomic orbitals—that describe the clusters as pseduo elements. The Jellium model uses a spherical potential from the nuclei to give orbitals with global nodal properties. Again, this formulates the cluster as a 'super atom' with an electron configuration comparable to a single atom. The model is best applied to spherically symmetric systems, and two examples for which it works well are the icosahedral Al₁₃^- and [Sn@Cu₁₂@Sn₂₀]^12- clusters.^[22][23] 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.^[16]