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Book Review Hypercarbon Chemistry. By G. A. Olah G. K. Surya Prakash R. E. Williams L. D. Field and K

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BOOK R E V I E W S
Hypercarbon Chemistry. By G . A . O[ah. G. K . Surya Prakash, R . E. Williams. L. D. Field, and K . Wade. John
Wiley, New York 1987. 311 pp., bound, $ 69.95.--ISBN
0-47 1-06473-4
Despite the wide occurrence of multicentered two-electron bonds in compounds of elements which lie to the left
of carbon in the Periodic System, hypercoordinated carbon
(hypercarbon) is often regarded only as an extreme special
case. The topic is usually completely ignored in organic
chemistry textbooks. What organic textbook is there which
even deals with carbaboranes? This monograph gives, for
the first time, a coherent account of the many different aspects of the chemistry of compounds containing hypercoordinated carbon atoms.
The first chapter (37 pp., 78 references) introduces the
definitions which are essential to the topic, and discusses
some typical structures containing hypercoordinated carbon. The different types of localized and delocalized multicentered bonds are described in terms of M O energy diagrams and the isolobal concept. The chapters which follow
introduce different types of compounds with hypercoordinated carbon, covering a remarkably wide range. The first
to be discussed are a representative selection of bridged
and associated alkyl compounds of main group and subgroup metals, which generally involve five- and six-coordinated carbon atoms (32 pp., 96 references).
The next two chapters discuss the importance of hypercoordinated carbon in the extensive area of cluster chemistry. The chapter on carbaboranes and metallacarbaboranes
(29 pp., 52 references) includes, in addition to a discussion
of structure and bonding, a short account of methods of
synthesizing carbaboranes and of their reactions. The following chapter (22 pp., 104 references) reviews the possible types of bonding and coordination of hypercoordinated carbon in mixed metal-carbon cluster compounds
and metal carbides. In addition to being of special value to
readers who are not very familiar with these more “inorganic” topics, these two chapters form a useful combination with the following chapter on hypercoordinated carbocations (42 pp., 160 references).
Methods for identifying carbocations with hypercoordinated carbon, and experimental results concerning their
structures, are summarized here in a similar form to that in
the book “Superacids” by Olah et al., which appeared
from the same publishers in 1985, with some updating of
the material. However, in the context of “Hypercarbon
Chemistry” this chapter has a different significance. It is
made clear to the reader that such carbocations are only
one aspect, though an important one, of the broader chemistry of compounds with hypercoordinated carbon atoms.
The 2-norbornyl cation, which unfortunately is too often
discussed only as an isolated case, is here considered logically along with other hypercoordinated carbocations such
as bicyclobutonium ions, bicyclo[2.1. llhexyl cations, trishomocyclopropenium cations, the Coates cation and the
Hogeveen dication. The latest revival of the old controversy about the “nonclassical”, i.e. hypercoordinated,
structure of the 2-norbornyl cation will be seen as an
anachronism after reading this book. The next chapter
(20 pp., 45 references) expands on the above topic by dealing with structural analogies and correlations between I3C
and ”B NMR chemical shifts, and again brings out very
590
clearly the common features of hypercoordinated carbocations and polyboranes.
The last and longest chapter (64 pp., 192 references) is
devoted to short-lived intermediates and transition states
involving hypercoordinated carbon atoms. Protolytic reactions of hydrocarbons in superacid media, and reactions
between other electrophiles, metal compounds with coordinative unsaturation, carbenes etc. and C-H and C-C Gbonds are discussed as examples; these provide a contribution to basic understanding of the mechanisms of many organic reactions. This chapter clearly shows that traditional
organic chemistry textbooks, in which the chemistry does
not begin until after describing the saturated hydrocarbons, are outdated.
There are some probIems, which quickly become apparent to the reader, with regard to the different ways of representing bonds to hypercoordinated atoms in structural
formulas, using either solid lines or broken lines. For example, one finds two formulas for the p-hydridoosmium
cluster compound [ O S ~ H ( C O ) ~ ~ C both
H ~ ] ,of which differ
from that in the original paper by Shapley et al. The
method introduced by Ofuh in 1972 of representing a
three-centered two-electron bond by broken lines in the
middle of a triangle formed by the three coordinated
atoms is not always clear, and can easily lead to misunderstanding, as there is no atom at the intersection of the broken lines. The structural formula for the 2-bicyclo[2.2.l]heptyl cation (2-norbornyl cation) on p. 167 is
only marginally different from that for the 2-bicyclo[2.l.l]hexyl cation (p. 170). The different ways of writing the same structure are not always formally equivalent.
The p-hydrido bridged cycloalkyl cation [l,6-Me2CloH,7]a
is represented in the introduction with hyper- (two-) coordinated hydrogen and hyper- (five-) coordinated carbon.
According to this the cation should have a structure with a
closed three-center bond, giving significant transannular
C-C interaction. At a later point the formula for the same
cation is written with only the y-hydrogen hypercoordinated. This corresponds to an open three-center bond, a
form which was in fact preferred in the original paper by
Sorensen et al.
The layout and execution of the formulas are not generally up to the normal standard. The diagrams are in some
cases too large and in others too small. Some formulas are
incorrect. Many of the entries in the subject index (22 pp.)
are of little use. For example, how many readers are likely
to look up phrases such as “Acute Mn-C-Mn bond angles”,
“Mathematical explanation”, or “Highly stereoregular”?
The book brings together under a unified point of view
many items of knowledge from different areas. It not only
deals with an aspect of organic chemistry which has hitherto been seriously neglected, but also cuts across the
traditional boundaries between inorganic and organic
chemistry. It is these qualities which give the monograph
its particular value, despite the criticisms of detail mentioned above. In their treatment of hypercarbon chemistry
the authors have succeeded in reawakening the reader,
who often may have all too easily become a specialist in a
narrow field, to the fundamental unity of the many different areas of chemistry.
Hans-Ullrich Siehl [NB 874 IE]
Institut fur Organische Chemie
der Universitat Tiibingen (FRG)
Angew. Chem. In,. Ed. Engl. 27 (1988) No. 4
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