Alkalide. formation

The equilibrium (1) is very sensitive to several factors. The complexing ability of L, the electron affinity of M and the lattice energy of the resulting salt drive the alkalide formation while the lattice energies ofthe solid metal M(s) and the complexant L, the ionization energy of M, and the unfavourable entropy of formation ofthe well ordered crystalline product oppose it. Not only alkalides with the cation and anion of the same element but also mixed ones such as K+C(222)Na have also been obtained [24]. 

Novel anions stabilized by alkali-polyether cations The ability of a crown (such as 18-crown-6) or a cryptand (such as 2.2.2) to shield an alkali cation by complex formation has enabled the synthesis of a range of novel compounds containing an alkali metal cation coordinated to a crown or cryptand for which the anion is either a negatively charged alkali metal ion or a single electron (Dye Ellaboudy, 1984 Dye, 1984). Such unusual compounds are called alkalides and electrides , respectively. 

Dissolution of lithium metal with 12.1.11 in methylamine results in formation of the dark-blue electride (Li 2.1.1 ])+e rather than an alka-lide as deduced from optical transmission and EPR spectroscopy (133). Transparent films containing (M[2.2.2])+M (M = Na, K, Rb) or elec-trides have been prepared by direct vapor codeposition of the metal and the cryptand (134). Optical transmission and infrared spectral data suggest that with the metal in excess only alkalides are formed, whereas electrides are preponderant with the cryptand in excess (138 ). 

The chemistry of the alkali metals has in the past attracted little attention as the metals have a fairly restricted coordination chemistry. However, interesting and systematic study has blossomed over the past 25 years, largely prompted by two major developments the growing importance of lithium in organic synthesis and materials science, and the exploitation of macrocyclic ligands in the formation of complexed cations. Section 12.4 deals with the use of complexed cations in the generation of alkalides and electrides. 

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. 

Solutions of the alkali elements, in a range of polar solvents, have been shown to contain M , M ", e, and the unusual anion, M , in equilibrium. Early studies were hindered by solubility and decomposition problems. The addition of a macrocylic chelate to these solutions provided a means of increasing the solubility of the metal as well as controlling what species predominate in solution. A metal chelate mole ratio of 2 1 leads to formation of the alkalides [M -i- L] M while a ratio of 1 1 leads to the electrides (see 10.2.2.5), [M -t- L] e (where M and M = Li, Na, K, Rb, Cs (not always the same) and L = two crown ethers (combinations of 12-C-4, 15-C-5, 18-C-6) or one cryptand [2.2.2]). Furthermore, these unique compounds could be isolated as solids and fully characterized. A wider range of alkalides than electrides is known including those with other chelates such as ethylenediamne , and aza crown ethers . Some representative examples are [Na -t- crypt]Na"(the first to be structurally characterized) , and [Cs + 18-C-6)(15-C-5)]Na- . 

Many properties of alkalide crystals, powders, and films were measured. The original alkalide, Na (C222)Na , has been most thoroughly studied, in part because of the high stability of pure samples. In contrast to most alkalides, single crystals and vapor-deposited films of this sodide are stable in vacuo for many hours, even at room temperature. In addition to the crystal structure, we measured the thermodynamics of formation by an EMF method, optical... 

Formation of the alkalides is favored both by the formation of donor-acceptor bonds between the cation and the crown ethers and by the non-negligible electron affinity of the metal atoms. See Table 3.3. 

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