Page 168 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GPA Final Pages
Encyclopedia of Physical Science and Technology EN007D-343 July 10, 2001 20:13

826 Inorganic Exotic Molecules

boron-containing species. The latter species is so common ethane molecule in which the two carbons are replaced
that it is easy to forget its uniqueness as the archetype of by borons. Now, clearly, this is too simplistic since two
species with three-center/two-electron bonds, of electron- electrons are missing. Nonetheless, tetraborane(10) can be
deficient as opposed to electron-precise species, of boron logically understood as the corresponding butane deriva-
compounds, and the general cluster and cage chemistry tive. Now let us consider the more current structure of di-
that arises from these structural and electronic features. borane(6), that of ethylene, where the two carbon–carbon
We will not consider boron compounds or cluster species bonds have been replaced by two B H B units. Accord-
in any of their traditional glory. These are studies for other ingly, tetraborane(10) is related to diborane(6) much as
sections and other chapters, and their authors have also had butadiene, C 4 H 6 , is related to ethylene. There are other
to keep many of the highly interesting stories left unsaid. C 4 H 6 isomers, all of which are considerably higher in en-
Rather, we recall the simple equilibrium between dibo- ergy. Recall that there is no isolable B 3 hydride. However,
rane(6) and the formally simpler borane(3), BH 3 : the correct structure for B 4 H 10 contains two fused B 3 rings
and the most logical structure, most simply B 2 H 5 B 2 H 5 ,
B 2 H 6 ← 2BH 3 . (8)
→
is not even an experimentalist’s exotica—it remains the
This is so “simple” that we often speak of diborane and fancy of theorists.
borane without the designations of “(6)” and “(3).” We
say “formally” because BH 3 lacks “off-the-shelf chem-
D. Simple Clusters
istry”: it spontaneously dimerizes. The simple one- and
two-carbon hydrides CH 4 and C 2 H 6 are both easily iso- We could talk about strain energies and delocalization
lated and separated, as are one- and two-nitrogen hydrides energies, differences between σ and π bonds, as a way
NH 3 and N 2 H 4 and one- and two-oxygen hydrides H 2 O of organizing these boron and carbon hydrides. We will
and H 2 O 2 : methane, ethane, ammonia, hydrazine, water, not. Instead we close this section with an apologeti-
and hydrogen peroxide have very ancient and venerable cally brief mention of simple boron clusters, also con-
names reflecting very ancient and venerable chemistry. taining four borons, with the generic formula B 4 X 4 .We
BH 3 is more like CH 2 , with which it is isoelectronic. do so because their special, symmetric, tetrahedral shape
presages the plethora of triangulated, deltahedral boron
clusters, their isoelectronic and isolobal analogues, and
B. The BH 3 /CH 2 Comparison: Triboron Species
also of the likewise nonmetal tetrahedron, P 4 . That P 4 is
Even here the chemistry of borane(3) shows some sur- an ancient species, known since the interface of alchemy
prises and introduces us to some exotic compounds, only and chemistry, has a special symmetric shape, and is
some of which have been isolated by experimentalists. related to more current species adds to its exotic char-
To the extent that BH 3 and CH 2 are conceptually re- acter. It would appear that both boron and phosphorus
lated, we would expect to find an entire homologous series chemistry offer chemists numerous “exotica” to add to
(BH 3 ) n analogous to cycloalkanes (CH 2 ) n . Not so; while their amusement and aggravation. The next section dis-
the n = 3 carbon case of cyclopropane is well known, the cusses P 4 and related polyphosphorus and polynitrogen
corresponding triborane(9) qualifies as ephemeral, fragile, species.
and exotic, and indeed, there is no isolable binary boron
hydride with but three borons. The corresponding anion
made by formal deprotonation of the latter, [B 3 H 8 ] ,is VII. POLYPHOSPHORUS AND
−
stable enough to be isolable as salts, whereas protonated POLYNITROGEN SPECIES, THEIR
+
cyclopropane, [C 3 H 7 ] , is only known in gaseous or other ANALOGUES AND DERIVATIVES
nonnucleophilic media.
In the introduction we defined exotic as “beautiful, ex-
ceptional, weird, paradoxical, and counterintuitive.” We
C. Tetraboranes
also indicated that such species could represent fragile
The n = 4 case results in the even less well known and rare as well as simple and well-characterized classic
tetraborane(12)—four borons want but 10 hydrogens. inorganic compounds. In this section we will look at two of
Thatthestoichiometriesoftwooftheearliestknownboron the simplest inorganic classes of compounds: polyphos-
hydrides, B 2 H 6 and B 4 H 10 , are so familiar from hydrocar- phorus and polynitrogen species. We will dwell on the
bon chemistry, and the formulas of other boron hydrides following elementary question: Why is it that polyphos-
even more distinct, no doubt added to considering even phorus allotropes of the element are well known (white
these boron compounds as exotic. The simplest structure phosphorus; black phosphorus in its orthorhombic, rhom-
for diborane(6), long known to be wrong, is that of an bohedral, and cubic forms; red, amorphous phosphorus),