The crystal structures of two cesides show that cesium(1-) is the largest monotomic ion

Electrides: Ionic Salts with Electrons as the Anions

“…(iv) electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy, induding the study of electrons at low concentrations in alkalides (16,19); (v) differential scanning calorimetry (13); (vi) optical transmission (films), reflectance (single crystals) and fluorescence spectroscopy (films, powders, and single crystals) (20); (vii) powder and single crystal x-ray diffraction (9,10); and (viii) photoelectron and thermionic electron emission (21).…”

“…Experimental proof of the existence of alkali metal anions in amines and ethers came quickly (6), and in 1974 the first crystalline salt of an alkali metal anion was synthesized and characterized (7). This class ofcompounds, called alkalides, has expanded to more than 40 salts of Na-, 8), and the crystal structures of 16 alkalides have been determined (9,10). Interestingly, in spite of many attempts, lithides have not yet been prepared, nor has Libeen unequivocally identified in solution.…”

The alkali metals: 200 years of surprises

“…Theoretical calculations show that such dimers (and chains), although unstable to dissociation in the gas phase, can be stabilized by the image positive charges of the nearby complexed cations [28]. The cesides, Li + (C211)Cs − and Cs + (C222)Cs − form zig-zag linear chains of Cs − anions with shortened Cs-Cs distances [29]. The structure of Rb + (18C6)Rb − is remarkable in that it not only forms Rb − chains, but each Rb − also forms a contact pair with a complexed Rb + cation that is incompletely encapsulated in the 18-crown-6 ring [30].…”

The structures of alkalides and electrides. V. Structures of caesium bis(15-crown-5) kalide and rubidium bis(15-crown-5) rubidide

“…The b axis is vertical, the a axis is horizontal and the c axis is normal to the paper. and 3.5 A as the average (Huang et al, 1987)]. Since alkali-metal anions are highly polarizable and H atoms may penetrate into the outer electron density, the estimated sizes may vary from compound to compound.…”

“…The crystal structures of several natrides and caesides have been determined and reported (Dawes, Fussa-Rydel, Ward, Huang & Dye, 1989;Huang, Ward, Kuchenmeister & Dye, 1987). It is of interest to determine the structures of two other types of alkalides, kalides and rubidides, and to estimate the sizes of K-and Rb-.…”