Dinitrogen Difluoride Chemistry. Improved Syntheses of cis - and trans -N 2 F 2 , Synthesis and Characterization of N 2 F + Sn 2 F 9 − , Ordered Crystal Structure of N 2 F + Sb 2 F 11 − , High-Level Electronic Structure Calculations of cis -N 2 F 2 , trans -N 2 F 2 , F 2 N═N, and N 2 F + , and Mechanism of the trans−cis Isomerization of N 2 F 2 †