# What is the formal charge on each atom in the tetrahydridoborate ion?

Mar 26, 2018

Well, we normally represent sodium borohydride as $N {a}^{+} B {H}_{4}^{-}$...
...and since this is clearly NEUTRAL...we split the salt up in to $N {a}^{+}$ and $B {H}_{4}^{-}$ ions....
...for the tetrahedral borohydride ion....there are $4 \times B - H$ bonds....each hydrogen atom is associated with ONE electron, and is therefore neutral. On the other hand, the boron atom get 4 electrons from the $B - H$ bonds, and there are two inner core electrons....six electrons in total. Since for boron, $Z = 5$, given 5 nucular charges, and SIX electronic charges....the boron atom has a formal negative charge.
Borohydrides ACT as hydride transfer reagents in their reaction chemistry....i.e. $B {H}_{4}^{-} \equiv B {H}_{3} + {H}^{-}$...they tend to be a bit milder reagents than aluminum hydrides...salts of $A l {H}_{4}^{-}$...