Boron(III), like other nonmetals in positive oxidation states, exists in aqueous solution not as a cation, but as a hydroxide at low pH and a hydroxyanion at high pH. Hence the boron-containing product of the reaction of the two B(III) compounds below with water is the same.
B(OH)3 + 3HCl  BCl3
  

 
B + 2HCl
   BF3BN + H2



 		B2O32B(OH)32B(OH)4–

B + MgO

Formation of the hydroxoanion at high pH is a Lewis acid-base reaction.
Lewis acids are electron pair acceptors.
Lewis bases are electron pair donors.
A Lewis acid-base reaction results in a bond formation. (HO)3B + :OH– → [(HO)3B–OH]–
Dr Sheila Woodgate
© The University of Auckland 2002