NaSn5: An Intermetallic Compound with Covalent α-Tin and Metallic β-Tin Structure Motifs

A novel and unusual three-dimensional network of tin atoms is present in NaSn₅, in which metallic layers analogous to those in β-Sn alternate with tetravalent units analogous to α-Sn. The compound shows the emergence of pentagonal-dodecahedral units from the metallic β-Sn modification (see structure on the right; all unlabeled spheres are Sn atoms). Quantum-mechanical investigations indicate the simultaneous presence of structural regions with localized and delocalized bonds.     

Tellurium-Rich Tellurides

The tellurides represent a class of compounds which exhibit a wide range of physical and chemical properties. In addition to the salt-like compounds, some tellurides show metallic properties while others have strong covalent bonds. Like tellurium itself, many of these are semiconductors, e.g., the II-VI compounds CdTe and MnTe, and in this respect are of special interest. Other tellurides, e.g., NbTe₄ and ZrTe₅, have low-dimensional electronic transport properties. Some ternary transition metal-main group metal tellurides appear in an amorphous state with spin glass properties. The tellurium-rich tellurides are characterized without exception by directed Te-Te bonds. It turns out that general structural chemical relationships exist between them which allow these different compounds to be treated as a single comprehensive class of substances. They represent a link between the Zintl phases on the one hand, and molecular compounds on the other.     

Zintl Phases: Transitions between Metallic and Ionic Bonding

Experimental studies on compounds of alkali and alkaline earth metals with semi- and metametals have considerably broadened the basis for a discussion of the transition from metallic to ionic bonding. Current interest is focused mainly upon the elucidation of the principles governing the structure of such compounds which are subject to a wide range of variation within this class of materials. A new definition of the term Zintl phase is proposed after consideration of available findings.     

What Do We Know about the Metal-Metal Bond?

Almost all main group and subgroup metals are able to form metal-metal bonds. The bond order ranges from weak interaction to a quadruple bond, and the degree of aggregation from a dinuclear entity to a three-dimensional network. In spite of numerous physicochemical studies, not all aspects of the metal-metal bond are understood. The ability of metal-metal linked polynuclear complexes to serve as a reservoir for missing or excess electrons enables them to react both with nucleophilic or reducing reagents and with electrophilic or oxidizing reagents. The intermediate position occupied by clusters between simple complexes and the bulk metal is of theoretical and practical significance.     

Zintl Defects in Intermetallic Clathrates

In many cases, idealized crystal structure models cannot rationalize the actual properties of intermetallic compounds. For a realistic approach in materials research, microstructures and defects need to be taken into account. In case of clathrate compounds, particularly the intrinsic framework vacancies (denoted as Zintl defects) demand consideration. Consequently, clathrate research produces evidence that modern-day structure chemistry involves the utilization of advanced X-ray diffraction methods combined with elaborated bulk phase analyses, the investigation of phase relations, and the study of mutual interrelations in the triangle chemical bonding–structure–properties. Herein, we review some fundamental contributions to the specific defect chemistry of intermetallic clathrates.     

Homoatomic Bonding of Main Group Elements

Clusters of main group elements are not rare. On the contrary, it is becoming difficult to avoid the discovery of new substances of this type. Clusters are the natural intermediate stages between an element and its isolated atoms or ions. In the form of polycations and polyanions they offer models for the stepwise oxidation and reduction of an element and represent a bridge between the elements. The great majority of homonuclear bonded structures are already present in the solid phases of simple systems. Mobilization of these clusters as molecules represents a great challenge.     

Boron-Phosphorus Compounds and Multiple Bonding

Boron-phosphorus compounds have not been as thoroughly studied as their boron-nitrogen counterparts. Until recently many classes of B-P compounds that had been well established in B-N chemistry were either unknown or poorly characterized. This statement is particularly true for compounds involving possible multiple bonding between boron and phosphorus. For example, detailed structural information on simple monomeric phosphino-boranes, R₂BPR′₂, did not become available until 1986 even though the isoelectronic SiC double bonded species, the silenes, had already been reported. However, new work has shown that it is possible to prepare and characterize several novel types of boron-phosphorus compounds with varying degrees of multiple B—P bonding. These include not only monomeric phosphinoboranes but also phosphanediylborates (borylphosphides), three- and four-membered rings (diphosphadi-boretanes), boron phosphorus analogues of borazine, B-P skeletal analogues of allyl cations and anions, butadiene and cage compounds. Structural, spectroscopic (mainly NMR) and theoretical studies reveal some important differences between B-P and B-N compounds which in many cases can be traced to the presence of a high inversion barrier at phosphorus that reduces the π interaction. This usually causes compounds such as R₂BPR′₂ to associate through σ bonding between B and P. Supporting evidence for this view comes from species that involve phosphorus and nitrogen in competitive π bonding with a boron p orbital in which the dative interaction between B and N is dominant and the phosphorus center remains pyramidal. Recently published work has shown that steric and electronic factors can be used to favor π bonding and give an approximately planar system. Furthermore, theoretical studies reveal that p? p π overlap in a planar B-P system is of similar efficiency to its B-N analogue. Good examples are seen in the phosphanediyl borates, the boron-phosphorus analogues of borazine and the π-allyl cations, whose molecular configurations and B—P bond lengths support strong boron—phosphorus π bonding.     

Clusters of Valence Electron Poor Metals—Structure,Bonding, and Properties

Metals in low oxidation states are capable of forming metal–metal bonds. An attempt has been made to classify the numerous phases and structures occurring in such metal-rich systems of valence electron poor metals in some sort of order from a rather general point of view. With this purpose in mind, clusters of these elements, their different types of interconnections, and their condensation via shared metal atoms, which finally leads to extended M? M bonded structures, are described. Interstitial atoms play an important role in stabilizing electron deficient clusters, and can actually lead to the loss of all M? M bonds. Surprising similarities emerge between apparently very different systems as the metal-rich oxides of alkali metals, the oxides, halides, and chalcogenides of d transition metals, and the halides and carbide halides of the lanthanoids.     

Structure and Bonding in Cyclic Sulfur-Nitrogen Compounds

The coordination number of sulfur is used in this review as a classification principle; cyclic sulfur-nitrogen compounds in which the sulfur is di-, tri-, and tetracoordinated are discussed. Compounds with sulfur (and nitrogen) of coordination number 2 are electron-rich combinations of elements whose π-electrons are extensively delocalized. A correlation between the coordination number and the bond length can be observed in certain compounds containing tri- and tetracoordinated sulfur.     

Intermetallic Compounds and the Use of Atomic Radii in Their Description

Metallic radii, which are obtained from atomic distances in the pure elements, are generally used for the calculation of distances in intermetallic compounds. However, the procedure for using such radii depends on the individual structural type: (a) For high coordination numbers and only slightly differing distances between atoms of the same kind and different atoms, all distances in a structure are proportional to the sum of radii, weighted according to the compositon. Such a “Vegard” relationship for ordered compounds is obeyed by intermetallic compounds with topological close packings, but strictly only if the various kinds of distances are correlated via symmetry relationships. For compounds with low coordination numbers the simple sum of radii holds for atoms participating in the shortest bond (e.g. in ionic crystals).-(b) The number of neighbors determines the size of each atom. It can be shown that the bond strength-bond length concept, developed for valence compounds, and often dealt with in the literature over the last ten years, is also applicable for alloys. On this basis a formalism is developed which uniformly describes the size of the atoms as a function of the coordination number for both the limiting cases of multiple bonds in molecules and for close packed atomic arrangements in alloys.     

La5Al3Ni2 – an Intermetallic Phase Observed upon Crystallization of La50Al25Ni25 Metallic Glass

The yet unknown intermetallic phase La₅Al₃Ni₂ was obtained by partially crystallizing amorphous La₅₀Al₂₅Ni₂₅ at 550 K (further heating above 600 K leads to irreversible disappearance of this phase), and its crystal structure was determined from X‐ray powder diffraction data. The crystal structure of the La₅Al₃Ni₂ phase constitutes a new structure type (Cmcm, a = 14.231Å, b = 6.914Å, c = 10.460Å, oC40) and is built from [Al₃Ni₂] chains surrounded by La atoms. In the ternary system La‐Al‐Ni La₅Al₃Ni₂ is located on the section La₅₀Al_50−nNi_n (0 ≤ n ≤ 50) with the binary compounds LaAl and LaNi as end members. Strikingly, also the crystal structures of the end members can be conceived as chain structures with Al and Ni chains surrounded by La, respectively.     

Reactivity and Controlled Redox Reactions of Salt-like Intermetallic Compounds in Imidazolium-Based Ionic Liquids

Substituted imidazolium ionic liquids (ILs) were investigated for their reactivity towards Na₁₂Ge₁₇ as a model system containing redox-sensitive Zintl cluster anions. The ILs proved widely inert for imidazolium cations with a 1,2,3-trisubstitution at least by alkyl groups, and for the anion bis(trifluoromethylsulfonyl)azanide (TFSI). A minute conversion of Na₁₂Ge₁₇ observed on long-time contact with such ILs was not caused by dissolution of the salt-like compound, and did thus not provide dissolved Ge clusters. Rather, a cation exchange led to the transfer of Na⁺ ions into solution. In contrast, by using benzophenone as an oxidizer, heterogeneous redox reactions of Na₁₂Ge₁₇ were initiated, transferring a considerable part of Na⁺ into solution. At optimized conditions, an X-ray amorphous product NaGe_6.25 was obtained, which was thermally convertible to the crystalline type-II clathrate Na_24–δGe₁₃₆ with almost completely Na-filled polyhedral cages, and α-Ge. The presented method thus provides unexpected access to Na_24–δGe₁₃₆ in bulk quantities.     

Thermally Stable Heterobinuclear Bivalent Group 14 Metal Complexes Ar2M−Sn[1,8-(NR)2C10H6] (M=Ge,Sn; Ar=2,6-(Me2N)2C6H3; R=CH2tBu)

The first thermally robust Ge ^II −Sn ^II compound 1 and the structurally characterized Sn^II-Sn^II analogue 2 , which maintain their structural integrity in solution, were obtained by treating MAr₂ (M=Ge, Sn; Ar=2,6-(Me₂N)₂C₆H₃) with Sn[1,8-(NR₂)₂C₁₀H₆] (R=CH₂tBu). On the basis of structural and spectroscopic data, the M−Sn bond is regarded as the interaction of a MAr₂ donor with an Sn[1,8-(NR₂)₂C₁₀H₆] acceptor.     

Smart Solution Chemistry to Sn‐Containing Intermetallic Compounds through a Self‐Disproportionation Process

Developing new methods to synthesize intermetallics is one of the most critical issues for the discovery and application of multifunctional metal materials; however, the synthesis of Sn‐containing intermetallics is challenging. In this work, we demonstrated for the first time that a self‐disproportionation‐induced in situ process produces cavernous Sn?Cu intermetallics (Cu₃Sn and Cu₆Sn₅). The successful synthesis is realized by introducing inorganic metal salts (SnCl₂ ? 2 H₂O) to NaOH aqueous solution to form an intermediate product of reductant (Na₂SnO₂) and by employing steam pressures that enhance the reduction ability. Distinct from the traditional in situ reduction, the current reduction process avoided the uncontrolled phase composition and excessive use of organic regents. An insight into the mechanism was revealed for the Sn?Cu case. Moreover, this method could be extended to other Sn‐containing materials (Sn?Co, Sn?Ni). All these intermetallics were attempted in the catalytic effect on thermal decompositions of ammonium perchlorate. It is demonstrated that Cu₃Sn showed an outstanding catalytic performance. The superior property might be primarily originated from the intrinsic chemical compositions and cavernous morphology as well. We supposed that this smart solution reduction methodology reported here would provide a new recognition for the reduction reaction, and its modified strategy may be applied to the synthesis of other metals, intermetallics as well as some unknown materials.     

Planar Complexes Containing Metal-Metal Bonds

Planar complexes, and particularly those of platinum, can form structures containing linear chains of heavy metal atoms with metal-metal distances as short as 3.1 Å. The bonding in these chains can be strengthened by partial oxidation, and the bond lengths can be reduced to 2.8 Å in this way. Model structures for the novel nonstoichiometric products, such as K₂[Pt(CN)₄]Cl_0·32 · 2.6 H₂O, are considered. The bonding is also discussed on the basis of a one-dimensional band model.     

Monovalent Group 13 Organometallic Compounds: Weak Association to Monomeric,Versatile Two-Electron Donors

A weakly associated hexamer is formed for [GaCp*] (Cp*=C₅Me₅) in the solid state (see picture). The recent X-ray crystal structure analyses of [GaCp*] as well as the monomeric In^I and Tl^I compounds [M(2,4,6-Trip₃C₆H₂)] (Trip=2,4,6-iPr₃C₆H₂) throw new light on the association and aggregation of monovalent Group 13 elements in the solid state. The synthesis of [Ni⁰{In[C(SiMe₃)₃]}₄], a complex with terminally bonded In^IR ligands, offers alternative σ-donor/π-acceptor ligands to organometallic chemists. The newest results in this area are likely to open up new and intriguing possibilities in the preparation of main group–transition metal clusters.