Chemical Properties of Dialkyl Halonium Ions (R₂Hal⁺) and Their Neutral Analogues, Methyl Carboranes, CH₃-(CHB₁₁Hal₁₁), Where Hal = F, Cl

Chloronium cations in their salts (C_nH_2n+1)₂Cl⁺{CHB₁₁Cl₁₁ ^-}, with n = 1 to 3 and exceptionally stable carborane anions, are stable at ambient and elevated temperatures. The temperature at which they decompose to carbocations with HCl elimination (below 150 °C) decreases with the increasing n from 1 to 3 because of increasing ionicity of C-Cl bonds in the C-Cl⁺-C bridge. At room temperature, the salts of cations with n ≥ 4 [starting from t-Bu₂Cl⁺ or (cyclo-C₅H₁₁)₂Cl⁺] are unstable and decompose. With decreasing chloronium ion stability, their ability to interact with chloroalkanes to form oligomeric cations increases. It was shown indirectly that unstable salt of fluoronium ions (CH₃)₂F⁺(CHB₁₁F₁₁ ^-) must exist at low temperatures. The proposed (CH₃)₂F⁺ cation is much more reactive than the corresponding chloronium, showing at room temperature chemical properties expected of (CH₃)₂Cl⁺ at elevated temperatures.