Boron and Hydrogen
Name: Paula B.
Why does BH3 exist when the boron only has 6 electrons surrounding the nucleus?
I know that it is trigonal planar and that it readily form B2H6 (which also puzzles me with the
number of electrons surrounding each B nuclei).
The bonding in boranes (boron-hydrogen) compounds is non-traditional. The atomic number of
boron is 5 so the electrons are 1s2, 2s2, 2p1. To form BH3 (a fact) one invokes electron
"promotion" to an electron configuration of 1s2, 2s1, 2px1, 2py1 and an empty 2pz0, followed
by rehybridization to give 3 equivalent B-H bonds. The same thing is done in the case of
carbon, and it sounds a little "hokey", but it is allowed by the rules of quantum mechanics.
It is true that BH3 forms a dimer B2H6 (fact). It is also true
that the molecular structure is the two boron atoms bridged by two hydrogen atoms and four
terminal hydrogens (two on each boron atom) [fact]. These weird behaving bonds are called
"electron deficient" bonds for the reasons that puzzle you. The data are not in question.
The chemist then has to "pick up the pieces" and formulate a bonding scheme that "explains"
the experimental data. If you consult a text on borane chemistry you will find it full of
strange chemistry as measured by the usual "octet" rules, but
that is the data, and we have to try to make some semblance of sense out of it.
From your question, I cannot quite tell what you would expect instead of BH3. No stable hydride?
BH5 to finish a Lewis shell of 8 outer electrons?
Boron makes BH3 for the same reason Be makes BeH2 an Li makes LiH: it can only make one bond
for each outer electron it starts with.
The first 2 of boron's 5 electrons are used up being inner-shell electrons, leaving only 3 to
make covalent or ionic bonds.
The metals in Boron's column of the periodic table (Boron, Aluminum, Gallium, Indium, Thallium)
are generally trivalent, so BH3 is very traditionally understandable.
Aluminum should make AlH3, too. That's not quite stable, but in combination: AlH3 + LiH =
LiAlH4 Lithium Aluminum Hydride, a stable compound.
Boron by itself has 5 electrons: H=1, He=2, Li=3, Be=4, B=5.
Of Boron's five electrons, the first two are paired with each other and sunk into a lower-
energy shell close in by the nucleus. They are not available to make bonds with other atoms.
They make up the inert "helium-like core" of the boron atom or the Boron
+3 ion, around which the remaining three electrons can play chemical bonding games.
When boron has its 3 hydrogens (or three chlorines in BCl3), then it is charge-neutral. It has
made as many "equal-partners" bonds as it can.
Then there are two electrons in the inner shell, and six electrons using the valence (outer)
But by quantum-mechanical rules, there is orbital room in the valence shell for two more
electrons . If there are to be any electrons there, both must be donated from another atom.
This happens, but it is a lot weaker than an equal-partners covalent
bond. It is called a coordination-bond. The resulting compound is called an adduct. Imagine
two molecules sort of half-glued together, one with an exposed lone pair of electrons, like
NH3, the other like BH3 with a missing pair, a Lewis vacancy.
BH3:NH3 is an adduct. Actually, BCl3:NH3 sticks together better.
. Cl H .
. \ / .
. Cl-B :N-H .
. / \ .
. Cl H .
The electrons in the coordination bond mostly stay with the donating atom, and spend less than
50% of their time on the boron side of the bond. This action give boron a completed Lewis
shell of 8 electrons, but weakly so.
BCL3, and to lesser extent BH3, are called "Lewis acids", because if the donor is one of the
dangling pairs of the oxygen in H2O, then one of the hydrogens is partially freed up, making
the water solution acidic: BCl3 + H2O <--> BCL3OH- + H+ .
BH5 is not a stable compound because the H2 molecule does not have enough electron-donating
power to make a coordination-bond with BH3.
(B2H6, though, is very un-traditional in structure.
. H H H .
. \ * / .
. B B .
. / * \ .
. H H H .
Those two middle H's actually have "bridge-bonds", a single bond bridging three instead of two
atoms, even though each bridge-bond only contains two electrons. )
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Update: June 2012