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Encyclopedia of Physical Science and Technology EN002c-73 May 21, 2001 13:59

312 Boron Hydrides

following assumptions of how the two electrons in each Calculation of the charge distribution on the boron cage
bond—two-center or three-center—are shared among the of B 10 H 14 is hampered by the large number of possible
atoms in the bond: bonding situations that have to be considered. Molecular
orbital calculations, however, show that the boron atoms
1. The very similar electronegativity of boron and farthest from the open end of the boron hydride bear the
hydrogen result in even sharing of the two electrons highest negative charges. The most negative boron atoms
in the boron-hydrogen terminal bond. are B 2 and B 4 (Fig. 16) and the next most are B 1 and B 3 .
2. The two electrons in a boron-boron single bond Electrophilic alkylation, halogenation, and deuteration
should be shared evenly between the two atoms in the all occur most readily at B 2 and B 4 and next at borons B 1
bond. and B 3 .
3. The two electrons in a boron-boron-boron
three-center bond should be shared evenly among the
three boron atoms, that is, 2/3 of an electron per atom. VI. REACTIONS OF BORON
4. Of the two electrons in a boron-hydrogen-boron three HYDRIDE ANIONS
center bond, one is assigned to the hydrogen in the
bridge and 1/2 of an electron is assigned to each The closo boron hydride anions have been studied con-
boron atom. siderably more than the open ions, nido and arachno. The
reactions of the open ions are quite varied, and we will
These assumptions can be applied to the 4120 styx solu- consider only a few of these before proceeding to the re-
tion of B 5 H 9 to give the total number of bonding electrons actions of the closo anions.
assigned to each boron atom. Then subtracting this num-
ber from the number of valence electrons in the free boron
A. Open Anions
atom gives an approximation of the charge on each atom as
shown in Fig. 15a. Averaging the charges of the four boron Some boron hydride anions will add protons to produce
atoms at the base of the square pyramid to account for hy- neutralboronhydridesandthusprovideamethodofprepa-
bridizing the possible bonding arrangements to maximize ration for the neutral compounds, for example,
symmetry gives the charges given in Fig. 15b.
− −
According to this reasoning, all electrophilic substitu- 4HCl + 4B 3 H −→ 3B 4 H 10 + 4Cl + 3H 2
8
tion such as alkylation, electrophilic halogenation, and HCL + B 5 H − −→ B 5 H 11 + Cl + H 2
−
12
electrophilic deuteration of B 5 H 9 should occur on the
boron at the apex of the square pyramid (B1), and this Some of these ions can complex transition metals, such
−
is what is observed. asthecopper(I)complexofB 3 H andtriphenylphosphine,
8
B 5 H 9 + CH 3 Br −→ 1–CH 3 B 5 H 8 + HBr L L
B 5 H 9 + DCl −→ 1–DB 5 H 8 + HCl Cu
H
B 5 H 9 + Br 2 −→ 1–BrB 5 H 8 + HBr
H B
H
H
B
H
H B
H
H
L = triphenylphosphine

B. Closo Anions
The properties of the closo ions, B 6 H ,B 7 H ,B 8 H ,
2−
2−
2−
6 7 8
B 9 H ,B 10 H ,B 11 H , and B 12 H , are roughly simi-
2−
2−
2−
2−
9
11
10
12
FIGURE 15 (a) The charges calculated for one form of the bond- lar. We will concentrate on B 10 H 2− and B 12 H , the most
2
ing of B 5 H 9 using the simple model. (b) The same charges av- 10 12
eraged so as to increase the molecular symmetry by making the familiar and best studied.
+
+
bonding to each of the boron atoms around the base of the square Small unipositive cations such as Na and K form
pyramid equivalent. soluble salts with the B 10 H 2− and B 12 H 2− anions that are
10 12