Exposed to the elements

I expect that the reaction of those who gain access to this encyclopedia will be one of surprise: not because such a collection of articles exists but because many of the articles and entries are interesting and informed. Indeed those with just a passing acquaintance with inorganic chemistry will be astonished that this subject, admittedly necessary, is also so alive and, in spite of the local lack of encouragement, exciting. Many of the articles reflect this excitement and convey to the reader much that is enthralling. The editor-in-chief, R. Bruce King, deserves much credit for enlisting the support of many distinguished contributors and for arranging the articles in a style that stimulates inquiry. However, such an encylopedia deserves, indeed demands, the attention of those concerned with electronic publishing. In its present form the cross-referencing is just about adequate, but it could really benefit from the remarkable facility and surprising productiveness with which CD-ROMs can correlate one subject with another.

I turned first to those articles more connected with my own interests. In "Iron transport: siderophores", B. F. Matzanke begins by quoting G. Wachterhauser: "For all plants and animals, and for virtually all microbes, life without iron is impossible. Why iron has gained such an essential role in the course of biological evolution remains open to speculation." But what else could have been used? What other element is present at such high abundance, has a variety of oxidation states open to it and is relatively catholic in its choice of ligand partners? Fortunately, the subsequent description of these iron-chelating reagents is informed and extensive.

Indeed most of the articles are very well-written, though there is often a disparity between the space allocated to equations and formulae and the space given to structures derived from X-ray diffraction studies, the latter being over-generous. However, in "Calcium-binding proteins" by N. C. J. Strynadka and M. N. G. James, all was forgiven: I have rarely read such a good account and one so well produced. Similarly that by R. E. Blankenship on "Photosynthesis" is detailed and authoritative.

There is a nice juxtaposition of information from X-ray structures and chemical information in "Iron: heme proteins, peroxidases and catalases" by A. M. English, and "Biomineralization" by R. B. Frankel and S. Mann is written with style and clarity.

"Nickel enzymes and cofactors" by R. Cammack and M. N. Hughes is good and timely but why was the article on "Nickel: models of protein active sites" (by M. J. Maroney) so long? I have always found the attention given to "models" of the inorganic cores of proteins to be excessive. There has been some excellent work in this area, as in the work of R. H. Holm, but much of that reported here, while serving to advance our knowledge of inorganic complexes, has helped us but little to understand the structures, to say nothing of the functions, of these key components of inorganic biochemistry. Of course, it could be that exhaustion of the reviewer was now at hand but, no, the article on "Nitric oxide in biology" by J. R. Lancaster Jr. turned out to be full of intriguing information, easily digestible. The Irving-Williams Series crops up in all sorts of places but it was the irrelevant heading of a page in "Iron: organometallic chemistry" by J. R. Green and W. A. Donaldson. "Iron Porphyrin Chemistry" by F. A. Walker was most comprehensive: I doubt if it will need revising for some time to come.

Often a disproportionate amount of space seems to be allocated to certain subjects. This can be annoying, but "Tungsten proteins" by M. W. W. Adams is fascinating. These proteins, perhaps unknown to many inorganic chemists or even biochemists, seem to have existed in some primitive organisms. The immediate question is: why tungsten? What can it do that, say, molybdenum cannot? This may of course be a misleading question: in the primitive environment perhaps tungsten was the only relevant element that could be accessed.

The most exciting aspect of zinc biochemistry concerns its interaction with DNA through (for example) "zinc fingers". Our knowledge that zinc is involved in biochemical systems owes its origins to the almost life-long work of Bert Vallee. It is thought, by some inorganic chemists, to be an element with rather uninteresting properties: colourless, diamagnetic and only moderately reactive. That, of course, could be the essence of its involvement in nearly every aspect of life: it supplies a charge at interesting locations in proteins and enzymes and binds with a wide range of ligands, where other elements would be waylaid by their affinity for a more attractive environment.

These propensities are described by D. P. Giedroc in an interesting article, "Zinc: DNA-binding proteins". Overall, the articles on inorganic biochemistry present a fair reflection of the excitement the subject has generated, and will no doubt continue to generate in the future. But I do have some criticisms. In an otherwise good description by A. Bock of "Selenium Proteins containing Selenocysteine", why were no details of the X-ray structure of glutathione peroxidase reported? Why did "Vanadium in biology" by R. Wever, which is mainly about bromoperoxidases, deserve eight pages, with little mention of the vanadium-containing nitrogenase?

In the short entry on "Vitamins", these com£ are described as "organic substances": but vitamin B12, a cobalt-containing complex, represented such a key discovery in coordination chemistry because it was the first natural organometallic compound. (I was frustrated to read the article on "Cobalt: B12 enzymes and coenzymes" by B. Krautler; it made me realise how the field has moved on since I worked in it.) The figures representing the structure of nitrogenase in "Nitrogenase and the iron-molybdenum cofactor" by P. W. Ludden left much to be desired.

I was surprised by "Nutritional aspects of metals and trace elements" by P. Saltman, which is far too brief and does not convey any of the subject's importance. There is a strange section, within "Alkaline earth metals: inorganic chemistry" by W. S. Rees, Jr., on beryllium biochemistry: has the latter really commanded much interest? However, the article on "Chromium: biological relevance" by J. B. Vincent is nicely toned and replete with an appropriate amount of scepticism.

But why were there only 11 lines on dinitrogen and dinitrogen complexes compared with four pages on "Dioxygen and Related Ligands" by R. R. Curry and K. D. Karlin? And while I know there is much debate and argument on electron transfer in biological systems, the article by B. Durham and F. S. Millett on "Iron: heme proteins and electron transport" presents a rather one-sided view.

When one considers the wealth of information provided on inorganic chemistry as a whole (as opposed to inorganic biochemistry), certain articles stand out. One of the editors, J. K. Burdett, excellently describes "Electronic structure of main group com£", and his article on "Superconductivity" is timely and extremely useful. There is a good article on "Magnetic oxides" by J. E. Greedan; an authoritative review of "Diffraction methods in inorganic chemistry" by J. A. K. Howard and L. Aslanov; and a useful account of "Defects in solids" by R. J. D. Tilley. The "NMR of fluxional processes" by E. W. Abel and K. G. Orell is very good.

Of the rest, "Hypervalent com£" by M. Lattman is interesting, surprisingly so; and "Intercalation chemistry" by A. J. Jacobson is informative. Similarly, "Chalcogenides: solid state chemistry" by P. M. Keane and H. F. Franzen is very good; and so is M. J. Almond's article on "Short-lived intermediates".

Also important are "Structure and property maps for inorganic solids" by J. K. Burdett (and J. R. Rodgers), "Sol-gel synthesis of solids" by J. Livage, "Solids: computer modelling" by C. R. A. Catlow, "Sonochemistry of metals and their com£" by K. S. Suslick, "Heterogeneous catalysis by metals" by F. H. Ribeiro and G. A. Somorjai and "Surfaces" by S. S. Perry and G. A. Somorjai.

Few aspects of recent inorganic chemistry have caught the imagination more that the fullerenes. This subject is well described in "Carbon: fullerenes" by J. D. Crane and H. W. Kroto.

Descriptive inorganic chemistry is hard to make interesting, as many chemistry graduates remember. However, the articles by B. Chaudret and S. Sabo-Etienne give a very good account of "Ruthenium; inorganic and coordination chemistry" with excellent cross-referencing. "Scandium, Yttrium and Lanthanides: organometallic chemistry", by R. D. Kohn, G. Kociok-Kohn and H. Schumann, is authoritative and well illustrated but surprisingly poorly referenced. "Oxidation catalysis by transition metal complexes" by J. P. Caradonna is valuable; and there are some very well produced figures in "Boron Hydrides" by J. T. Spencer. Though the account of "Actinides: inorganic and coordination chemistry" by G. L. Soloveichik was brief, too brief, that on "Actinides: organometallic chemistry" by D. L. Clark and A. P. Sattelberger was good.

R. B. Martin, in "Metal toxicity", addresses the limitations of the theory of hard and soft acids and bases. Though the theory is often used, or misused, here it gets just 14 lines to itself. Does that say something about its standing?

Of course there are a number of entries and articles that strike me as incomplete, misleading or obscure. For example, there is a paucity of data on elemental abundance and a simplistic, three-line statement about the Pauli Principle - strange, given that the principle underpins much of inorganic chemistry. "Periodic table: trends in the properties of the elements" by G. P. Wulfsberg is idiosyncratic. "Phosphides: solid state chemistry" by H. G. von Schnering and W. Honle, is very different in style from most of the articles, being full of tabular data. It was amusing to find two pages on "Polonium: organometallic chemistry" by R. A. Zingaro: no doubt it exists but is it really important?

There are some strange lapses in some of the minor entries. For example, there is a rather unhelpful seven-line article on "Overlap integral" with four, rather than the claimed three, figures in section; an item about Class a and Class b behaviour was careless and misleading. The "Chelate effect" surely deserved a longer article. Given that there is a developing interest in the importance of relativistic effects, we should have been given more than nine lines on them. And where is the section on nuclear chemistry? In "Silicon: organosilicon chemistry", the author, H. Sakuri, was, at best, equivocal on the old chestnut of d-orbital participation. "Molecular orbital theory", by P. T. Czech, was not very advanced, perhaps deliberately so; and that on "Mossbauer Spectroscopy" by T. C. Gibb would have benefited from the use of many more illustrative examples.

I was surprised that "Sulfur: organic polysulfanes" by R. Steufel and M. Kustos received so much space. At the beginning of the article on "Electron Spin Resonance" by R. Bramley, there is no mention of the true Hamiltonian but, fortunately, it makes an appearance at the end of the article.

The articles on "Hydride complexes of the transition metals" by R. H. Crabtree are brief and disappointing, as are "Technetium and Rhenium: inorganic and coordination chemistry" by the same author, and "Titanium: inorganic and coordination chemistry" by C. A. McAuliffe and N. Bricklebank. Why does "Transition metal carbonyls: infrared spectra" by J. A. Tinney, deserve 12 pages as opposed to titanium's nine? "The electronic structure of solids" by E. Canadell is difficult but by now I admit I was suffering from fatigue.

It would be surprising if, in such a series of comprehensive volumes, one had no criticisms. However, they should not detract from the interest stimulated by many of the articles and the new material imparted. I learnt much from the encyclopedia. All concerned in it deserve enormous credit for their efforts in bringing to fruition this magnificent enterprise.

All science libraries should have this encyclopedia, most teachers of chemistry would benefit from it, and every serious student of inorganic chemistry should have easy access to it.

H. A. O. Hill is professor of bioinorganic chemistry, University of Oxford.