In the $\ce{BH4-}$ ion, what bonds are present between boron and hydrogen?

Are there three covalent bonds between boron and three hydrogen atoms (using the boron 2s, 2px, and 2py orbitals), with the fourth hydrogen being a hydride ion $\ce{H-}$ which forms a coordinate or dative bond to the boron 2pz orbital?


2 Answers 2


No, this is simply a bond between four hydrogen atoms ($\ce{H}$) and a boron anion ($\ce{B-}$). These bonds are identical to and indistinguishable from the other $\ce{B-H}$ bonds. They are all covalent bonds of length $1.21\:\mathrm{Å}$.

Your idea of $\ce{H-}$ completing $\ce{BH3}$'s $2\mathrm{p}_z$ orbital isn't quite right either. Atom's individually have atomic orbitals, but when they form bonds these atomic orbitals combine to form molecular orbitals. This is picture more accurately demonstrates the bonding between $\ce{B-}$ and the $\ce{H}$ atoms.

MO for BH4-

[Photo Source]


Borohydride ion
(source: mcat-review.org)

You are quite correct, the boron forms covalent bonding with three Hydrogens, and then a coordinate bond with H- ion (1s2)

Extra info: This ion is fairly water soluble and acts as a ligand, using bridging hydrogens as three-centre two-electron donor atoms, forming complexes like Al(BH4)3 and Be(BH4)2

Reference: Principles of Descriptive Inorganic Chemistry By Gary Wulfsberg

  • $\begingroup$ what is opinion about ringo's answer? $\endgroup$
    – Edifice
    Commented Nov 6, 2015 at 19:14
  • $\begingroup$ I looked up what ringo answered, and it appears that his answer is more accurate. I am basing my answer on basic middle school chemistry, whereas his is a little more advanced. I don't know a lot, as of now, I am only in high school after all :p $\endgroup$ Commented Nov 6, 2015 at 19:20

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