Abstract Tetrafluoroborate anion boronfluorine bond activation, yielding a boron trifluoride-nucleophilic heterocyclic carbene (NHC) adduct, is reported. Bis-[2-(2-(trifluoroborane)-3-methylimidazolin-2-yliden-1-yl)ethyl]ether ( 4) was obtained as a by-product from the reaction of di-silver-bis-[bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether] di-tetrafluoroborate ( 3) and ZrCl 4. The Ag I(NHC)(BF 4) complex was prepared from bis-(2-(3-methylimidazolium-1-yl)ethyl)ether diiodide ( 2) and silver(I) oxide; spectroscopic data suggests that it is a dinuclear species. Compounds 2– 4 were spectroscopically characterised, the structures of the NHC:BF 3 adduct and parent bis-imidazolium salt being confirmed by single crystal X-ray studies. The geometry of 4 is similar to that of previously reported NHC:BF 3 adducts. Heating the NHC:BF 3 adduct in dimethyl-d 6 sulfoxide (d 6-DMSO) in the presence of CsF leads to complete hydrogen/deuterium substitution of the imC 4,5 positions. It was found that the H/D substitution was readily effected at room temperature on the free carbene bis-(2-(3-methylimidazolin-2-yliden-1-yl)ethyl)ether. Prolonged heating was required to observe a similar H/D substitution in 3, preliminary to its decomposition, and (2-(3-methylimidazolium-1-yl)ethyl)ether tetrafluoroborate was found to be resistant to substitution.