Tetramethylthiourea and tetramethylselenourea form 1:1 BF3 adducts which are partially converted to ionic structures (donor)2BF2+.BF4− in solution. The degree of ionization is small except at low temperatures when BF3 is in excess; ionization is essentially complete at −90 °C when a sufficient excess of BF3 is present. Formation of B2F7− from BF4− and BF3 at low temperatures is responsible for this. Hence under these conditions the 1:1 adduct is replaced by (donor)2BF2+.B2F7−. The existence of ionic adducts is further confirmed by the identification of BF3Cl− and BF2Cl2− in the mixed-halogen system tetramethylthiourea–BF3–BCl3.Methyl group nonequivalence in the proton spectrum below −70 °C indicates restricted rotation about the central C—N bonds of coordinated tetramethylthiourea.