Fig. 4: Lewis acidity and reactivity of halogen-substituted silylium ions.

a, Experimental and theoretical evaluation of the Lewis acidity based on Müller’s FBN scale and calculated fluoride ion affinities. b, The FBN acidity scale for different silylium ions in benzene. The ¹⁹F NMR chemical shift for Ar₃Si⁺ (Ar = pentamethylphenyl) is taken from ref. ²². c, Chloride abstraction from a chlorosilane by a halogen-substituted silylium ion provides access to hydrogen-substituted silylium ions. d, Decomposition pathways of the halosilylium carborates. ^a19F NMR chemical shifts of the silylnitrilium carborates [iPr₂XSi(FBN)]⁺[HCB₁₁H₅Br₆]⁻ determined from the ¹⁹F NMR spectrum (471 MHz, 298 K) in C₆D₆. ^bDeshielding of the ¹⁹F NMR chemical shift relative to free FBN (δ(¹⁹F) = −103.9 ppm). ^cDirectly computed gas-phase fluoride ion affinities at the LH20t-D4/def2-TZVPD//BP86-D4/def2-TZVP level. For a comprehensive list and a comparison with the anchor-point approach using OCF₂, see Extended Data Table 5.