Salts zintl

The Zintl-Klemm concept evolved from the seminal ideas of E. ZintI that explained the structural behavior of main-group (s-p) binary intermetaUics in terms of the presence of both ionic and covalent parts in their bonding description [31, 37]. Instead of using Hume-Rother/s idea of a valence electron concentration, ZintI proposed an electron transfer from the electropositive to the electronegative partner (ionic part) and related the anionic substructure to known isoelectronic elemental structures (covalent part), e.g., TK in NaTl is isoelectro-nic with C, Si and Ge, and consequenUy a diamond substructure is formed. ZintI hypothesized that the structures of this class of intermetallics would be salt-like [16b, 31 f, 37e]. 

More often, polyhedral clusters with strong metal-metal (or metalloid-metalloid) bonding are the major structural motifs of classic Zintl phases. These are nominally salts composed of reduced p- (i.e., post-transition) elements that are usually inter-bonded into closed shell polyanions plus active metal cations, originally the alkali... 

Each of the examples (5.1)—(5.4) illustrates the linking of molecules into supramolecular species that constitute new units of liquid, gaseous, or solid phases. Other provocative examples of supramolecular aggregation include the al-lotropic phases of sulfur (composed of a variety of linear and cyclic polymeric chains)9 and the remarkable Zintl salts (with anions composed of variable metal clusters).10... 

An elegant access to larger cluster units is to make use of preformed smaller clusters as they occur, e.g., in Zintl phases [8, 9]. The phase KSi (or K4Si4) contains homoatomic Si4 tetrahedra, and thus a salt-like formulation (Na )4[Si4]" with a formal electron transfer is appropriate (Fig. 1) [10]. Such homoatomic anionic building blocks occur in binary or ternary intermetallic phases A E and A Pn j with A being an alkali and alkaline-earth metal, and E = Si-Pb and Pn = P-Bi. These phases are generally available in good quantities, and - since a few members... 

In a similar manner the so-called Zintl salts composed of alkali metal cations and clusters of metals as anions (see Chapter 16) were known in liquid ammonia solution but proved to be impossible to isolate Upon removal of the solvent they reverted to alloys. Stabilization of the cations by complexation with macrocyclic ligands allowed the isolation and determination of the structures of these compounds. 

Ammonium amalgams are Zintl phase salts with mercury anions [iii-iv]. 

Zintl compounds to be mentioned in other chapters are compounds or phases that have anions such as Sng-, Sn2Bi2 or Pb1. The alkali metal salts can often be isolated crystalline by complexation of the cation with crown ethers or cryptands. Recent examples are Na jSn16 and Li2Ba4Si6, the latter having a SiJ0- ring.17 Saline hydrides were discussed in Chapter 2. 

At very negative potentials neither the tetraalkylammonium ions nor the metallic electrode are inert they combine to form reduced TAA-metals [7]. Tetraalkylammonium (TAA) metals are composed of quaternary ammonium ions, electrons, and a post-transistion metal such as Hg, Pb, Sn, Sb, Bi [5-18] or Pt [19] most of them have the composition R4N" MeJ [13] or R4N" Mc4 [20] and have been described as Zintl ion salts or Zintl phases [21,22]. They have been shown to be useful intermediates in the electrochemical reduction of certain substrates that are reducible with difficulty. On reduction of the quaternary ammonium salt, the initial layer of the metal compound is controlled by a two-dimensional nucleation, whereas the bulk phase is initiated by a three-dimensional nucleation and a growth controlled by the diffusion of R4N from the solution. In some cases (A-methylquinuclidinium (MQ" ) mercury) the catalytic efficiency of the initial layer is greater than that of the bulk phase [18], whereas in other cases (A, A-dimethylpyrrolidinium (DMP" ) lead) the opposite is found [16]. 

Ziegler, K.. 718 Ziegler-Natta catalysis, 718 Zinc. 582-587 Zinc blende, 96-97 Zintl salts, 527. 817 ZSM-5, 747-748... 

Ten or so years later, in the 1930s, Edward Zintl conducted a series of more systematic studies of these systems. He carried out potentiometric titrations of liquid ammonia solutions of alkali metals with various p metal salts, typically halides. Thus, the titration of a sodium solution with lead(II) iodide revealed that the green anionic species in solution are Pbg. Zintl and coworkers also discovered that... 

Zeise s salt, 240 zeolite, UA, 266, 270 Ziegler-Natta catalyst, 269 zinc blende structure, 104 Zintl compound, 112, 160, 202... 

Early synthetic routes to Zintl ions (see Section 13.7) involved reduction of Ge, Sn or Pb in solutions of Na in liquid NH3. The method has been developed with the addition of the macrocyclic ligand cryptand-222 (crypt-222) (see Section 10.8) which encapsulates the Na" " ion and allows the isolation of salts of the type [Na(crypt-222)]2[Sn5] (equation 8.35). Zintl ions produced in this way include [805] (Figure 8.3), [Pb5], [Pb2Sb2] , [Bi2Sn2] , [Geg]2-, [Geg]" - and [SngTl] -. 

A further development in the synthesis of Zintl ions has been to use the reactions of an excess of Sn or Pb in solutions of Li in liquid NH3. These reactions give [Li(NH3)4] salts of [Sng]" and [Pbg]", and we discuss these Zintl ions further in Section 13.7. 

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