Urea boron trihalide adducts. IV. Ioniccovalent equilibria in the tetramethylthioureaBF3 and tetramethylselenoureaBF3 systems and the influence of B2F7 formation

Tetramethylthiourea and tetramethylselenourea form 1:1 BF 3 adducts which are partially converted to ionic structures (donor) 2 BF 2 + .BF 4 − in solution. The degree of ionization is small except at low temperatures when BF 3 is in excess; ionization is essentially complete at −90 °C when a sufficient excess of BF 3 is present. Formation of B 2 F 7 − from BF 4 − and BF 3 at low temperatures is responsible for this. Hence under these conditions the 1:1 adduct is replaced by (donor) 2 BF 2 + .B 2 F 7 − . The existence of ionic adducts is further confirmed by the identification of BF 3 Cl − and BF 2 Cl 2 − in the mixed-halogen system tetramethylthiourea–BF 3 –BCl 3 .Methyl group nonequivalence in the proton spectrum below −70 °C indicates restricted rotation about the central C—N bonds of coordinated tetramethylthiourea.