Among B, Al, Ga and In, Boron atom (B) is smallest in size and a large amount of energy is needed to remove 3 electrons. When one electron is removed from the p-orbital,He -like, fulfilled s-orbital is left. This is highly stable. So, the second ionisation enthalpy is quite high. Again when one electron is removed a half-filled orbital is left. So, third ionisation enthalpy is also quite high. B=1s22s22p1 [B(H2O)6]3+⇒B3+→ Highly unstable (due to very high ionisation energy) Since, the total energy needed to make B3+ is the total of all ionisation enthalpies an enormous amount of energy is required to form it.