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Main Group Elements 7

E. Borates portion occupies n corners of an (n + 1)-cornered poly-
hedron]; arachnoboranes [having more clusters in which
Borates constitute a class of about four dozen minerals that
B atoms occupy n contiguous corners of an (n + 2)-
almost rival silicates in diversity and complexity. The ba-
cornered polyhedron]; hyphoboranes [having the most
sic building block of many borates is a boron surrounded
open clusters in which the boron atoms occupy n corners
by three oxygen atoms; however, boron may also be sur-
ofan(n + 3)-corneredpolyhedron];andconjunctoboranes
rounded by four oxygens in a tetrahedral configuration like
(having structures formed by linking two or more of the
that of the silicates. Borates may contain simple [BO 3 ] 3−
latter types of clusters together).
ions or complex rings and chains. The most important
borate is borax, Na 2 [B 4 O 5 (OH) 4 ] · 8H 2 O, which contains
the cyclic [B 4 O 5 (OH) 4 ] 2− anion in which boron atoms are G. Hydroborates (Borohydrides)
attached to either three or four oxygen atoms.
Tetrahydroborate (BH ) is the most familiar hydrobo-
−
4
O H 2 rate. It is made from NaH and B(OMe) 3 and is com-
HO O + + +
B B O B O H mercially available. Different cations (Na ,Li ,R 4 N ,
etc.) result in differing reducing capabilities toward or-
O
O
B ganic functional groups. A wide range of reactivity has
O
H O also been provided by substitution of different groups
onto the boron itself, giving trialkyl hydroborates,
2−
−
In addition to its importance as the compound from [R 3 BH ], carboxylatotrihydroborates, [H 3 BCO ] and
2
−
−
which orthoboric acid is made, borax is used to prepare [H 3 BCO 2 R ], and cyanotrihydroborate, [BH 3 CN ].
washing compounds and to soften water. Tetrahydroborate and its derivatives have shown important
reducing capabilities toward organic functional groups
and have assisted in the establishment of the nature of
F. Boranes
hydroborate bonding to metals.
The structures and reactions of boron hydrides have been
studied since being pioneered by Stock, who developed
H. Carboranes
high-vacuum techniques to handle them.
−
The synthesis and reaction chemistry of boranes and Carboranes are polyhedral boranes in which a BH unit
their derivatives have been extensively examined. Most has been formally replaced by an isoelectronic CH unit.
boranes belong to either the B n H n+4 series (stable) or the An entire class of carboranes can be prepared from an
B n H n+6 series(unstable).Theusualterminologyhasapre- alkyne and a borane by pyrolysis or by reaction in a silent
fix signifying the number of boron atoms followed by a nu- electric charge. Derivatives are known, including those
derived by reaction of lithium carboranes with a variety
meralshowingthenumberofhydrogenatoms;thus,B 6 H 10
is hexaborane-10. Diborane (B 2 H 6 ) serves as the starting of reactants (CO 2 , HCHO, I 2 , NOCl).
material for almost all boranes by either heating or addi- Sulfur- and phosphorus-containing boranes are also
tion (or both) of hydrogen to the higher boranes. Boron known, such as carbaphosphaborane (B 10 CPH 11 ) and the
2−
skeletons of the higher boranes can be considered to be thiaborane ion B 10 SH . An entire class of metallo-
10
fragments of octahedra or of icosahedra, with B 12 H 2− be- carboranes has been developed by M. F. Hawthorne and
12
ing the basic icosahedral structure. The boranes are partic- colleagues.
ularly interesting because they are electron-deficient com-
pounds; that is, there are insufficient valency electrons for
I. Biologically Active Boron Compounds
adjacent atoms to be held together by electron-pair bonds.
It is useful to invoke the concept of three-center bonds A relatively new area of boron chemistry concerns boron
(three atoms are bound together by two electrons) to ra- compounds having biological activity. B. F. Spielvogel,
tionalize the hydrogen-bridge bonding and boron–boron the pioneer in this field, developed the first boron ana-
bonds (open and closed, three-centered). W. N. Lipscomb logues of α-amino acids and related compounds. Exam-
has extensively developed the bonding ideas related to ples include aminecyanoboranes, amine(ethylcarbamoyl)
boron hydrides. boranes, and amine carboxyboranes and their esters.
Neutral boranes (B n H m ) and borane anions (B n H ) An example of a boron analogue of an α-amino acid
x−
m + −
have been classified into five series according to struc- is trimethylaminecarboxyborane, Me 3 NBH 2 COOH, the
ture and stoichiometry: closoboranes (consisting of com- protonated boron analogue of the dipolar amino acid
(+)
plete, closed, polyhedral clusters of n boron atoms); betaine [Me NCH 2 COO ]. Other important deriva-
−
3
tives may be represented by the following general
nidoboranes [having nonclosed clusters in which the B n