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

The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li⁺(C211)Cs^-, are reported. Li⁺(C211)Cs^- is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali metal anions. The distance between adjacent Cs^- 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^- wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside, Cs⁺(C222)Cs^-, were also studied to compare the effects of the subtle geometric changes between the two 1D zigzag chain structures. Li⁺(C211)Cs^- and Cs⁺(C222)Cs^- are both low-band-gap semiconductors with anisotropic reflectivities and large paramagnetic ¹³³Cs NMR chemical shifts relative to Cs^-(g). An electronic structure model consistent with the experimental data has sp²-hybridized Cs^- within the chain and sp-hybridized chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs₃^3-, support this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density between adjacent Cs^- anions is visualized through an electron density difference map constructed by subtracting the density of three cesium atoms from the short Cs₃^3- fragment.