Structures and stabilities of higher coordinate onium-boronium dications (X(+)BH(3)+ and X(+)BH(5)+; X = NH(3), PH(3), H(2)O, and H(2)S).

Structures of higher coordinate onium-boronium dications (X(+)BH(3)+ 1-4a and X(+)BH(5)+ 1-4d; X = NH(3), PH(3), H(2)O, and H(2)S) were calculated by using the ab initio method at the MP2/6-311+G** level. All of the dications 1-4a contain a four-coordinate boron atom with a three-center two-electron bond involving boron and two hydrogens. On the other hand, all the dications 1-4d contain a six-coordinate boron atom with two three-center two-electron bonds. The thermodynamics of the complexations of 1-4a and H(2) to form 1-4d were computed. Deprotonations of 1-4d were found to be substantially endothermic.