As part of the Project Firefly ionospheric modification campaigns conducted during the early 1960's, sulfur hexafluoride (SF6) was used to study the creation and consequences of artificially-induced electron depletion regions via the attachment process (SF6+e→SF6). Since those early experiments, a great many advanceś have occurred in theoretical, laboratory, and diagnostic techniques related to negative ion plasmas. This study examines the full range of negative ion chemistry in the upper ionosphere by using current reaction rate data to investigate the many chemical paths SF6 type injections might take in an F region environment. Particular attention is given to the conditions required to create heavy negative ions that persist long enough to affect the dynamical properties of the F region. The ambipolar diffusion characteristics of a three component plasma (O+, e-, SF6-) are described, and estimates of the incoherent scatter spectra obtained from such a plasma are presented. Model calculations using a first order chemical code are defined and tested to investigate the actual types of negative ion plasmas capable of being created under nighttime conditions. A versatile model for diffusion in an exponential atmosphere ws used to simulate the evolution of 1026SF6 molecules released at 222 km during a 1962 Firefly experiment. When examined in conjunction with the chemical model calculatins, the results suggest that the ionospheric perturbations recorded at the time probably resulted more from molecular and atomic ion neutralizations involving SF6-, SF5+, O-, O+, and &egr;-, rather than simple electron attachments, as had been expected. A similar use of SF6 diffusion scenarios for high-altitude releases (h=350-500 km) indicates that large-scale, long-lived negative ion plasmas could be produced by modest rocket or Shuttle-borne payloads to study dyndamical, chemical, and instability properties of space plasmas.