Na₆ZnSn₂, Na_4.24K_1.76(1)ZnSn₂, and Na₂₀Zn₈Sn₁₁: Three Intermetallic Structures Containing the Linear {Sn−Zn−Sn}⁶⁻ Unit

The novel intermetallic compounds Na₆ZnSn₂ (1), Na_4.24K_1.76(1)ZnSn₂ (2), and Na₂₀Zn₈Sn₁₁ (3) were obtained from direct fusion of the pure elements, and their structures were determined by single crystal X-ray diffraction. All three compounds adopt new structure types and contain linear anionic {Sn−Zn−Sn}⁶⁻ units with rather short Zn−Sn contacts (2.55−2.58 Å), separated by alkali metal counterions. Compound 3 comprises layers of interconnected heteroatomic {Zn₇Sn₅} icosahedra as an additional unique structural motif. The bonding situation in this 16 valence-electron anion is analyzed by quantum chemical methods. The results of NBO, AIM, and ELF calculations (Gaussian03 on HF/3−21G level) reveal covalent bonding between Sn and Zn. The relationship to isovalent CO₂ is discussed. Band structure calculations on the density functional theory level (LMTO) show that 1 can be understood as a Zintl phase containing a {Sn−Zn−Sn}⁶⁻ anion; however, Na−Sn contacts must also be considered. Magnetic susceptibility measurements show a temperature-independent, weak diamagnetism for Na₆ZnSn₂ (1).