One-Dimensional Zigzag Chains of Cs<sup>-</sup>:  The Structures and Properties of Li<sup>+</sup>(Cryptand[2.1.1])Cs<sup>-</sup> and Cs<sup>+</sup>(Cryptand[2.2.2])Cs<sup>-</sup>

The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li<sup>+</sup>(C211)Cs<sup>-</sup>, are reported. Li<sup>+</sup>(C211)Cs<sup>-</sup> is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali metal anions. The distance between adjacent Cs<sup>-</sup> anions, 6 Å, is shorter than the sum of the van der Waals radii, 7 Å. Optical, magic angle spinning NMR, two-probe alternating and direct current conductivity, and electron paramagnetic resonance measurements reveal unique physical properties that result from the overlap of adjacent Cs<sup>-</sup> wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside, Cs<sup>+</sup>(C222)Cs<sup>-</sup>, were also studied to compare the effects of the subtle geometric changes between the two 1D zigzag chain structures. Li<sup>+</sup>(C211)Cs<sup>-</sup> and Cs<sup>+</sup>(C222)Cs<sup>-</sup> are both low-band-gap semiconductors with anisotropic reflectivities and large paramagnetic <sup>133</sup>Cs NMR chemical shifts relative to Cs<sup>-</sup>(g). An electronic structure model consistent with the experimental data has sp<sup>2</sup>-hybridized Cs<sup>-</sup> within the chain and sp-hybridized chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs<sub>3</sub><sup>3-</sup>, support this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density between adjacent Cs<sup>-</sup> anions is visualized through an electron density difference map constructed by subtracting the density of three cesium atoms from the short Cs<sub>3</sub><sup>3-</sup> fragment.