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Dihydrogen Bonds. Principles Experiments and Applications. By VladimirI

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Dihydrogen Bonds
Principles, Experiments, and Applications. By Vladimir I. Bakhmutov.
Wiley-VCH, Weinheim 2008.
242 pp., hardcover
E 94.90.?ISBN
978-0-470-18096-9
Books on hydrogen bonding are always
welcome, because the phenomenon can
be studied with a number of different
experimental and computational methods,
with new results often adding a different
perspective to an already well-established
concept.
The
dihydrogen
bond
XHd+иииHd A is a recent addition to our
collection of hydrogen bonds, and the
author of the present book has made a
good attempt at reviewing the area. This
book will be useful to workers and
researchers in the field and is an appropriate addition to the secondary literature.
Generally, there is no reason to
presuppose that a hydrogen bond
cannot be formed to a hydridic hydrogen
atom, and in a phenomenological sense
there is nothing really exotic about a
dihydrogen bond. It remains a threecenter four-electron interaction, and it
will have certain electrostatic, chargetransfer, polarization, and dispersive
characteristics, as do all other hydrogen
bonds. Of course, these characteristics
depend on the nature of the atoms or
groups of atoms represented by X and A,
and this is where the chemistry comes in.
Throughout the book, the author has
tried to compare dihydrogen bonds
XHиииHA with normal hydrogen bonds.
This is a good idea because it has always
been a standard practice to compare a
new type of hydrogen bond with ?stan-
7794
dard? or ?normal? hydrogen bonds,
such as NHиииO, OHиииO, or say
OHиииF . These are the benchmarks
against which other interactions such
as CHиииO or OHиииp have been compared in the past. Now it is the turn of
the dihydrogen bond to be so assessed.
And what do we find? Generally, the
dihydrogen bonds reviewed by the
author pass the test for hydrogen bonding reasonably well, except (and understandably) at the very limit of the
phenomenon where the X and A
atoms become equivalent or identical.
Questions of nomenclature have
always been an interesting sideline in
studies of hydrogen bonding. The author
refers to ?nonconventional? hydrogen
bonds as being those that involve unconventional donors and acceptors. He goes
on to say that these ?nonconventional?
hydrogen bonds can be weak, medium,
or strong, just like conventional hydrogen bonds. But what exactly does one
mean by ?nonconventional?? Nearly a
decade ago, when Thomas Steiner and I
wrote a book entitled The Weak Hydrogen Bond in Structural Chemistry and
Biology, we decided against using the
word ?nonconventional? in the title and
preferred the word ?weak?. We argued
that what is nonconventional today
might well become conventional tomorrow, whereas words such as ?strong? and
?weak? might not change their meaning
so quickly. I still have no reason to
change my opinion. Indeed, there now
seems to be a better consensus in the
hydrogen-bond community about what
one means by a ?weak? or a ?strong?
hydrogen bond.
Also very interesting is the argument
developed by the author regarding the
limits of the dihydrogen bond. He states
that there is no clear border between
weak dihydrogen bonds and van der
Waals interactions, since the energies
that control directionalities are insignificant. This is a straightforward argument?in fact Steiner and I published a
paper in Chem. Comm. in 1998 where
we showed this to be true for weak
hydrogen bonds in general. What is
more striking in the present context is
that the author goes on to say that there
is also a continuum between strong
dihydrogen bonds and the covalent
H H bond. In other words, HиииH interactions in suitably polarized systems go
. 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
all the way from a covalent bond to a
van der Waals interaction. All this is
very heady?we need to be careful
about what we call a covalent bond,
what we call a hydrogen bond, and what
we call a van der Waals interaction. In
the same vein, I am not in full agreement
with discussing CHиииHC interactions
and dihydrogen bonds together. The
former can be repulsive, and sometimes
could even be destabilizing. The latter I
would associate more with a situation
that involves two dissimilar types of
hydrogen atoms, one of which is hydridic and bonded to a main-group metal
or a transition-metal atom. Various
things are called H H bonds, HиииH
interactions, and dihydrogen bonds by
the author in ways that may be negotiated by the specialist, but are apt to
confuse the student and novitiate.
The strength of the book lies in the
up-to-date information on dihydrogen
bonds given in Chapters 6 and 7. Chapter 6 deals with dihydrogen bonds
involving elements in Groups 1A to 4A
and xenon. Chapter 7 deals with transition-metal hydride complexes. I found
the sections on crystal engineering
(Chapter 9) to be particularly important.
Crystal engineering that uses a new type
of interaction has always proceeded
through the stages of data gathering,
data systematization and analysis, and
finally use of the structural data in
crystal design. With regard to the dihydrogen bond, we are clearly in the first
of these three stages. I would predict
that the coming years will see a consolidation of the second and third stages.
The book is easy to read. Particularly
helpful are the itemized lists of points in
the concluding sections of each chapter.
In these days of information overload,
such sections are very helpful. The book
is also timely. In the Desiraju?Steiner
book to which I referred earlier in this
review, and which appeared a decade ago,
we devoted eight pages to the dihydrogen
bond. Bakhmutov has filled 240 pages
with useful and accurate information
about this interaction, and this alone
justifies the appearance of this book.
Gautam R. Desiraju
School of Chemistry
University of Hyderabad (India)
DOI: 10.1002/anie.200885622
Angew. Chem. Int. Ed. 2008, 47, 7794 ? 7795
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