The literature about carbanions makes use of carbonyl or nitro compounds with charge on hetero atoms due to pi-delocalization of the electrons.  Carbanions with a beta-chlorine could stabilize the negative charge but are unstable and eliminate chloride ion with such ease that they are said to undergo concerted eliminations.  A beta-F acts as the beta-Cl with “glue” to keep it in place, and allows the study of carbanions stabilized by halogens instead of pi-delocalization.  Carbon-fluorine bonds also have different bond strengths depending on the environment and results in fluoride ions with different leaving abilities from -CH₂F, -CHF₂, -CF₂CF₃ or -CF₃. CF₃CHCl₂ and C₆H₅CHClCF₃ have similar gas phase acidities; however, their kinetic acidities measured by methanolic methoxide hydron-exchange kinetics differ by over 45,000.  The {CF₂CCl₂}^- is a localized carbanion with 45% of the charge on the negative carbon.  Although {C₆H₅CClCF₃}^- is pi-delocalized, Gaussian calculations have 37% of the charge on the benzylic carbon.  It is apparently not the amount of charge on the carbon, but whether the charge is in a localized or delocalized orbital.  There is also a difference for the exchange reactions of 9-phenylfluorene compared to C₆H₅CH(CF₃)₂ or C₆F₅H when carried out in methanolic sodium methoxide.