Alkali metal anions

A particularly imaginative application of this concept has led to the isolation of compounds which contain monatomic alkali metal anions. For example, Na was reacted with cryptand in the presence of EtNHi to give the first example of a sodide salt of... 

Even though alkali metal anions may be freely generated in solution, the isolation of solid salts containing such anions is not straightforward. Thus, the disproportionation observed when an alkali metal is dissolved in an amine or ether... 

Highly reactive nanoscale metal particles (3 to 15 nm in diameter) are formed by reducing metal salts to metals by solvated elecrons or alkali metal anions that may also find use in organometallic synthesis [35]. 

Alkali metal anions have also been generated as a result of cryptand stabilization of the corresponding cation. Cryptands were found to enhance the solubility of zerovalent alkali metals in various organic solvents.156-157 Initially, the solutions apparently contain the cryptate cation and solvated electrons together with free ligand. When more metal is dissolved, metal anions, M , are formed.158 Dye and co-workers have isolated gold-colored crystals of [Na+ c 2.2.2]Na 159160 and the crystal structure has been determined.161,162 Anion clusters such as Sb] , Pb2 and Sn," have been isolated as crystalline salts of the [2.2.2] cryptate counterion [2.2.2].162,163... 

Of particular significance in this respect has been the ability to prepare, characterize and study most intriguing species, the alkalides [2.79, 2.80] and the electrides [2.80, 2.81] containing an alkali metal anion and an electron, respectively, as counterion of the complexed cation. Thus, cryptates are able to stabilize species such as the sodide [Na+ c 9]Na- and the electride [K+ c 9]e-. They have also allowed the isolation of anionic clusters of the heavy post-transition metals, as in ([K+ c cryp-tand]2 Pb52-) [2.82]. 

Since X-ray crystallography cannot observe the lone electron directly (Box 2.1), it is questionable whether it is really situated at such a distance from the Cs+ cation. If true, this would represent a very extreme example of the naked anion effect (Section 3.8.2). An alternative explanation localises the electron on the Cs+ cation, which would also account for the observed low conductivity. However, convincing evidence for the separation of cation and electron comes from the nearly isostructural sodide (Na ) and kalide (K ) analogues of [Cs ([18] crown-6) 2] + -e-. In these, species the alkali metal anions are situated in the same localised cavities as their electride analogues. 

Diamagnetic States Ion Triples to Alkali Metal Anions... 

Alkalides are crystalline compounds that contain the alkali metal anions, M-(Na-, K-, Rb, or Cs ). The first alkalide compound Na+(C222)-Na was synthesized and characterized by Dye in 1974. 

Table 12.4.1. Radii of alkali metal anions from structure of alkalides and alkali metals ...

More than 40 alkalide compounds that contain the anions Na , K , Rb , or Cs have been synthesized, and their crystal structures have been determined. Table 12.4.1 lists the calculated radii of alkali metal anions from structures of alkalides and alkali metals. The values of rM (av) derived from alkalides and rM- from the alkali metals are in good agreement. 

While the trapped electron could be viewed as the simplest possible anion, there is a significant difference between alkalides and electrides. Whereas the large alkali metal anions are confined to the cavities, only the probability density of a trapped electron can be defined. The electronic wavefunction can extend into all regions of space, and electron density tends to seek out the void spaces provided by the cavities and by intercavity channels. 

Special cases of charge-transfer spectra are the so-called charge-transfer-to-solvent (CTTS) spectra [17, 68]. In this type of CT transitions, solute anions may act as electron-donors and the surrounding solvent shell plays the role of the electron-acceptor. A classical example of this kind of CTTS excitation is the UV/Vis absorption of the iodide ion in solution, which shows an extreme solvent sensitivity [68, 316]. Solvent-dependent CTTS absorptions have also been obtained for solutions of alkali metal anions in ether or amine solvents [317]. Quantum-mechanical molecular simulations of the CTTS spectra of halide ions in water are given in reference [468]. 

Alkalides and electrides allow homogenous reduction reactions for various transition metals, even for main group metals in aprotic solvents. These reducers consist of alkali metal anions or electrons trapped in crown ethers. 

Alkali metal NMR in conjunction withcryptand complexation finally led to the sensational discovery of alkali metal anions by Dye and coworkers. (14) The complexing power of the cryptands is such that the ionophore is capable of extracting the cation from alkali metal in solutions of THF, methylamine, and ethylamine. The electron left behind is conclusively proved to form the anion M (M = Na, Rb, Cs) leading to a separate resonance line at low temperature. The corresponding shielding is close to the theoretical value computed for the metal anion. A most striking feature of this resonance is its solvent independence. The absence of solvent-induced chemical shifts for Na ... 

Salmon GA, Seddon WA. (1974) Production of solvated electrons, ion-pairs and alkali metals anions in tetrahydrofura studied by pulse radiolysis. Chem Phys LettlA 366-368. 

This chapter also provides examples in which modem chemistry has developed in ways surprisingly different from previously held ideas. Examples include compounds in which carbon is bonded to more than four atoms, the synthesis of alkali metal anions, and the now fairly extensive chemistry of noble gas elements. The past two decades have also seen the remarkable development of the fiillerenes, previously unknown clusters of carbon atoms. Much of the information in this chapter is included for the sake of handy reference for more details, the interested reader should consult the references listed at the end of this chapter. The bonding and stractures of main group compounds (Chapters 3... 

Although the alkali metals are known primarily for their formation of unipositive ions, numerous examples of alkali metal anions (alkalides) have been reported since 1974. 

The endo-endo conformation of cryptands can be internally protonated to form proton cryptates. With the small cryptands, e.g. [1.1.1]- and [2.1.1]-cryptand (15a and 15b), the two internal protons are so efficiently shielded from H2O and OH that deprotonation only very slowly occurs even in strong base (8UA6044). Alkali cation cryptates are able to stabilize unusual species as their counterions. Dye and coworkers have isolated several alkali metal anions by this method. The sodium species (Na [2.2.2]cryptand Na ) was obtained as gold metallic crystals and gave a Na NMR with a broad Na -cryptate resonance and a narrow, upheld Na resonance. The other alkali metals show similar behavior and an electride salt (Na [2.2.2]cryptand e l has even been isolated (B-79MI52105). Crystalline anionic clusters of the heavy post-transition metals (such as Sb7 , Pbs , Sng ) were first obtained with alkali metal cryptates as the counterions (75JA6267). 

Alkalides and electrides are stoichiometric salts containing alkali metal cations complexed by crown ethers. Charge balance is provided by the alkali metal anions (alkalides) or trapped electrons (electrides). Rb and Rb NMR has been used to study a number of mbidium alkalides, electrides and related compounds (Kim et al. 

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