THE RENAISSANCE OF INORGANIC CHEMISTRY' RONALD S. NYHOLW University College London, London, England
THOSE of US who were familiar with the state of inorganic chemistry in universities twenty to thirty years ago will recall that at that time it was widely regarded as a dull and uninteresting part of the undergraduate course. Usually, it was taught almost entirely in the early years of the course and then chiefly as a collection of largely unconnected facts. On the whole, students concluded that, apart from some relationships dependent upon the periodic table, there was no system in inorganic chemistry comparable with that to be found in organic chemistry, and none of the rigor and logic which characterized physical chemistry. It was widely believed that the opportunities for research in inorganic chemistry were few, and that in any case the problems were dull and uninspiring; as a result, relatively few people specialized in this subject. The effect of this neglect, incidentally, became apparent during the last war and in the post-war years, when chemists with a sound knowledge of inorganic chemistry were required for the development of atomic energy projects. So long as inorganic chemistry is regarded, as in years gone by, as consisting simply of the preparation and analysis of elements and compounds, its lack of appeal is only to be expected. This state is now past and for the purposes of our discussion we shall define inorganic chemistry today as the integrated study of the formation, composition, structure, and reactions of the chemical elements and their compounds, excepting most of those of carbon. Many will regard this as an all-embracing definition and may suggest that I have defined not inorganic chemistry alone, but chemistry itself. I offer no defense; indeed, I accept this criticism as largely correct for it emphasizes one of our major themes-that the earlier divisions of chemistry are disappearing and the suhject is once more becoming an integrated whole. The modern inorganic chemist has scant regard for the distinctions between inorganic, organic, and physical chemistry. Thus, he has no hesitation in attaching organic groups to a metal atom if the properties of the resulting compound make it more convenient for investigation; similarly, he is prepared to use any of the available techniques of physical chemistry as may be necessary to solve his problem. The factors primarily responsible for the modern forward-looking spirit in inorganic chemistry are two external developments, which give it, first, a new sense of purpose, and, second, new tools with which to achieve this purpose. The first of these developments 1 Based on an Inaugural Address given at University College London on March 1, 1956. The original address was published by the College and is wailable from H.K. Lewis & Co. Ltd., London.
is the growth of the theoretical techniques of quantum mechanics to an extent permitting widespread chemical application. This has already proceeded far euough to show the unity of inorganic chemistry even though quantitative experimental investigations are required t o reveal it fully. The second external development consists of those new optical elertrical and magnetio techniques of physical measurement by which structure can be investigated in the physical terms demanded by the electromagnetic nature of matter. For a full appreciation of the way in which these advances have affected the development of inorganic chemistry a brief survey of the history of the subject over the past century is essential. HISTORY
We might well start with 1828, the year in which Wohler's paper appeared dealing with the conversion of ammonium cyanate into urea. This paper was the first move toward the rejection of the vitalistic theory and the launching of organic chemistry as a distinct branch of the subjecL2 Ammonium cyanate, XH&CO, is an example of that class of elements and compounds which includes metals, minerals, and rocks, and which were regarded as non-lit~ing. Urea, however, was believed to be formed only in living organisms. Compounds of this latter type thus came to be called "organic" while those in the former category were known as "inorganic." Wohler's syntheses of urea shall-ed the interrelationship between inorganic and organic chemistry. Thus the essential unity of chemistry is illustrated in one reac. tiou [XH,] lN=C=O] +
Action of hest
> Transition occurs
Ammonium eyanate INORGANIC
hTH2 ()=@
'NH, Urea.
PHYSICAL
ORGANIC
For the next fifty or so years inorganic and organic chemistry progressed side by side. The main work carried out in inorganic chemistry was concerned with the preparation of new compounds and the development of methods of analysis. Vast numbers of new corn-. pounds were described and important work was carried out on the determination of atomic weights. The year 1887 may be accepted as the date of emergence of physical chemistry as yet another branch of the subject; in that year the Zcitschrijt fur physilcalischa Chemie was founded. Many research workers were. 9 Dr. D. MoKie has discussed the significance of this paper in, his excellent article, Wohler's "Synthetic" Urea, and the Rejec; tion of Vitalism: a Chemical Legend, in Nature, Lond., 153, 608.
(1944).
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now attracted to physical chemistry because it offered an exactness which was lacking in inorganic chemistry. In the meantime, armed with the postulate of the four-covalent, tetrahedral carbon atom developed simultaneously by van't Hoff and le Be1 in 1874, organic chemistry developed into a system in which structure could be determined. Denied the technique needed for such stereochemical investigation, inorganic chemistry lagged behind. Thus we find that by this time organic chemistry, because of its system, and physical chemistry, because of its exactness, were steadily attracting workers from the more empirical and much less integrated field of inorganic chemistry. I t has been said by others that facts give a science subjects its substance, but it is the integrating theory which provides its strength. There was no lack of substance in inorganic chemistry in the late nineteenth century, but it was sadly lacking in cohesion. I t is primarily owing to the development of this integrating theory that inorganic chemistry has before it such exciting prospects a t the present time. ALFRED WERNER
At the turn of the century the work of that great chemist, Alfred Werner, did much to put order into inorganic chemistry. Werner is specially remembered for his studies on the apparently anomalous inorganic addition compounds, many of which had been described during the preceding half-century. Werner put forward the first satisfying theory for the structure of these compounds. His theory involved the idea of a definite coordination number for a metal with ligands arranged about it according to definite geometrical patterns, of which the octahedral, the tetrahedral, and the square planar arrangements are the most important. This theory enabled much of the conflicting mass of data t o he tied together for the first time. Paradoxically, however, Werner answered so many questions that he left many people with the impression that in inorganic chemistry there were but few advances to be made. However, no satisfactory explanation of the nature of chemical combination, or of valency, had been advanced and our knowledge of the stereochemistry was still limited to two or three common shapes. VALENCE THEORY
After the first world war, however, ideas on valency were gradually beginning to take firm shape. I t is important to stress the need for developments in valency theory and stereochemistry, because without an advance in these, any hope for an understanding of much of the factual matter of inorganic chemistry itself could not be expected. The fundamental ideas of the ionic and covalent bonds were crystallized in 1916 in papers by W. Kossel in Germany and G. N. Lewis in America. Without attempting to mention the names of all who contributed one can say fairly that a great step forward occurred in 1927 with the publication of Sidgwick's outstanding monograph, The Electronic Theory of V a l e w . Shortly after this we saw the application of quantum mechanics to problems of chemical combination. Of special importance to inorganic chemistry was the work of Linus Pauling. By the time we reached the middle thirties the new valency theory was being applied to a wide variety of VOLUME 34, NO. 4, APRIL, 1957
compounds. Quantum mechanics not only provided an explanation of how atoms joined together, but it also led on to some understanding of the strength of bonds and offered an explanation of their orientation in space. Equally important was the fact that quantum mechanics gave a sense of purpose to many physical measurements which previously were of much more limited value in deciding the structure of chemical compounds. Furthermore, these physical measurements provided quantum mechanists with many of the parameters which they needed for the development of their theories. Thus, in a sense, quantum mechanics and physical methods of attack on structure are complementary. For example, the determination of a magnetic susceptibility is easy; measurements having been carried out since the days of Faraday in 1850. But until quantum mechanics provided a theory which related the magnetic susceptibility with the number of unpaired electrons, it was not possible to draw conclusions of any significance about molecular structure. It is my essential thesis that the impact of quantum mechanics and of modern physical methods of attack are the main reasons for the renaissance of inorganic chemistry, leading to the present period of rapid growth. Some have maintained that quantum mechanics has provided little that could not have been obtained by other methods of attack. This view is unacceptable; hut a t the same time it should be remembered that the discussions which arose from Pauling's enunciation of his theories led chemists to the study of a wide variety of phenomena aimed a t proving or disproving these theories. This, of itself, was a good thing for inorganic chemistry. The main effect of the new development was not evident, however, until after the second world war. It has been frequently pointed out that many of the early hopes of quantum mechanics have not been realized. Sir Harold Hartley quotes Lord Rutherford as saying in 1919, "What's the use of going hack to chemistry when Bohr will soon he able to calculate anything you can find Similar optimism prevailed in 1929 when the new quantum mechanics was applied to valency; but it soon became apparent that in real molecules great 'mathematical difficulties are involved in obtaining a rigorous solution. As a result, various methods of approximation, some of which are unrealistic, have to be adopted. The most fruitful results have accrued from quantum mechanics when the theoretical workers are allied with the more experimental investigators. Inorganic chemistry, in particular, benefits most effectively from this kind of cooperation. TRENDS OF MODERN INORGANIC RESEARCH
Before discussing the present position and the likely direction of progress in research it is useful to have in mind the following table as a genealogy of the usual course of an inorganic chemical investigation. Once the preparation and the chemical analysis of a compound has been achieved one may study its properties from two different angles. On the one hand we are concerned with the structure of the compound in its widest sense, as embracing a11 bond properties and the a
HARTLEY, H., J . Ckem. Soe., 1947, 1282. 167
PREPARATION
A
Nature of bond Valency; bond tmes; bond strengths; auantrtl in-
St,ereoohemistry Elucidation and carrel.+ tion with nature of bond; nuantalin-
COMPOSITIOA
iTechniaues of anitlvsis)
I
r
.
React~on Thermo~indtics products dynsmics Nature of Energetics of Mechanism and steric reactions of chemical course of rereection action
shape of the molecule-the latter a function of the type of bonds present. On the other hand the chemical behavior of the substance leads us to investigations of its chemical reactions. The first interest is, naturally, reaction products and the steric course of the reaction. But we are also interested in the energy changes which occur in reaching equilibrium and in the rate at which the reaction proceeds. Indeed, this table illustrates the fusion of the various branches of chemistry. The preparation commonly involves the attachment of organic groups after which we proceed to study the product by classical inorganic methods and with the whole armory of physical chemistry. The latter rests on the three pillars of the kinetic theory, thermodynamics, and quantum mechanics as reflected in the table. Looking a t the current scene in the light of the table, one is struck a t once by the fact that the large amount of preparative work now in progress has a new sense of purpose which was absent from much of the older work. The driving force behind most of this modern preparative work arises from the predictions which follow from our studies of the structure and reactivity of known compounds. This preparative work includes, on the one hand, the chemistry of new elements (e.g., the actinides) and, on the other, the stabilization of formerly uncommon or unknown valency states of well-known elements-using the appropriate ligands. Today we know much more than formerly about the selection or synthesis of ligands which will stabilize unusual valency states; as a result univalent and zerovalent compounds of the transition elements are now quite common. Similarly the use of fluorine has led to the isolation of high valency states such as NiIV which were previously considered very doubtful. Ligands, other than the customary halide ions, ammonia and water, are being used extensively. Knowing something of why carbon monoxide stabilizes the zerovalent state, we are now able to use many other ligands, such as the phosphorous halides, t o achieve this purpose. A new chapter has been opened with the use of various unsaturated ligands, such as ethylene, oyclopentadienyl, and benzene, for the formation of metal complexes. The intriguing sandwich-like structure of the cyclopentadienyl compounds is of special interest for theories of valency. The study of the shape of molecules is today of major interest t o the inorganic chemist. Although we recognize the X-ray technique as of supreme importance in arriving a t the arrangement of atoms, the time and difficulty involved in cmying out a complete structure determination leaves much scope for the
application of other techniques. In particular, by making use of appropriate ligands one can prepare complexes whose solubility in organic solvents enables one to study their properties with a wide variety of physical techniques. Now that we are able fairly readily to prepare compounds in which the metal is not at the center of the usual octahedrom or tetrahedrom, the stereochemistry of less common coordination numbers has become of major interest. We may expect from now on to see much more work on shapes such as the square pyramid and seven- and eight coordinated complexes of the heavier metals. Allied with such purely stereochemical problems are the studies of valency theory and the elucidation of bond type and bond order. A brief reference only may be made to thermodynamics, where much work is in progress. In particular, the equilibria in solution between metal complexes and their constitutive metal ions and ligands are being extensively studied. The stabilit,y of a complex is, of course, of intrinsic interest to the chemist,, but other reasons for the great interest in this field are the importance of stability constants in analytical chemistry, in biochemistry, and in the more applied processes involving metal-ion separation and metal-ion inactivation. One may confidently predict that t,here will be major advances in the borderline field of inorganic chemistry and biology from now on. The role of metal complexes of iron in human and animal life processes, and of magnesium in plants, has long been recognized, hut more recent,ly many othermetals, such as copper and cobalt, have been shown to be very important in processes involving enzymes and vitamins. It is only in recent times, when our knowledge of the metal-ligand bond has reached a reasonable stage of development, that speculations as to structure and mechanism have proved very fruitful. Indeed, i t can fairly he said that much of the research needed in the biological field is in fact fundamental research in inorganic chemistry. This hrief outline of some fields of inorganic research omits many of major importance including the study of the chemistry of the lighter elements (boron, nitrogen, and sulfur, for example) and the chemistry of the solid state. In the latter field inorganic chemistry overlaps physics itself. The wide application of physical techniques for studying inorganic problems has brought, with it the necessity of acquiring much expensive apparatus. Modern inorganic chemistry uses as its tools all the usual branches of spectroscopy-visible, ultraviolet, and infrared-as well as the more recently developed fields like microwave and nuclear quadrupole spectra. Paramagnetic and nuclear magnetic resonance measurements are finding an ever increasing application in inorganic chemistry. Other techniques include the use of radioactive and heavy isotopes, while magnetic and electrical properties also provide valuable information concerning structure. INORGANIC CHEMISTRY IN THE CURRICULUM
The rapid strides made in research in this field, and the consequent systemization of the subject have profound implications for undergraduate courses in inJOURNAL OF CHEMICAL EDUCATION
organic chemistry. I have already referred to the undergraduate's lack of interest in inorganic chemistry in earlier years, and suggested the reason for this. But there is another factor. Most inorganic chemistry in universities has, until recently, been taught mainly in the first two undergraduate years, whereas during those years students have not been taught sufficient physical chemistry to provide an explanation of the experimental data. To illustrate this, we may consider the chemistry of the metal carbonyls. If this topic is dealt with relatively early in the course, i t is not possible to discuss with students the way in which stretching frequencies, double bonding, orbital hybridization, magnetic and electric dipole moment provide an understanding of the general chemistry of these compounds and of their structure. The value of magnetic nioments, in particular, in discussing problems of valency and bond type in the lanthanide and actinide series needs no emphasis. This does not mean, of course, that one studies the whole of physical chemistry before tackling the basic chemistry of the periodic table. We do not wish to train students in chemistry who can calculate with the molecular obital theory of valency without knowing the solubility or color of silver chloride! I do plead, however, for a better arrangement and interweaving of inorganic and physical chemistry in undergraduate courses. It is suggested that the center of gravity of the inorganic chemistry course he placed later in the curriculum than i t is in many universities today. Topics such as metal carbonyls, actinides, the chemistry of the solid state, inorganic reaction mechanisms, and nuclear and isotopic chemistry should be treated later in the course; but, before, or while, discussing the compounds of the periodic table earlier in the course students should have been given enough physical theory t o enable observed structures and properties to he explained. Thus, an introductory qualitative treatment of modern valency theory, stereochemistry, crystal chemistry, and introductory thermodynamics should come before the main treatment of the periodic table and not after it. A further example of this is the study of complex or coordination compounds. It should be emphasized to students from the outset that "complex chemistry" is an approach to the whole inorganic chemistry and not a topic to be dealt with alone or a separate course of lectures. I t is when we look at practical courses, however, that we have more cause for concern. Most students sum up their inorganic practical courses as consisting of scores of unknown salts to be qualitatively analyzed, a succession of uninteresting analyses, and a few preparations if time permits. The aims of the practical course should be to familiarize students with the appearance of chemical substances, their methods of analysis, their physical and chemical properties, and with the basic techniques of inorganic chemistry. How is it then that qualitative analysis has come t o occupy such a disproportionately large amount of time for inorganic practical work? The answer lies, I believe, in the historical development of inorganic chemistry. One hundred years ago the subject was closely allied with metallurgy and one of the main tasks of the chemist was the
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qualitative identification of elements in rocks and minerals and their quantitative determination. Having been originally an important part of the inorganic practical course, qualitative analysis of a particular number of metal ions has acquired a hallowed position ever since. Thus, in the "classical" separation tables cobalt has a position of honor whereas titanium is omitted, despite the fact that a t the present time compounds of the latter are more widely used in industry. There is no doubt whatever that the subject can also be a suitable means of teaching much inorganic chemistry -simple laboratory techniques, the colors and solubilities of compounds, and some important principles of physical chemistry. But let us stop a t this point and recognize that by forcing students to analyze more difficult mixtures, bearing little or no relation to commonly occurring chemical systems, we achieve little beyond enabling them to acquire skill in the automatic use of-and often a disgust for-separation tables. The distinctly separate objectives of a qualitative analysis as a vehicle for the transport of simple ideas of chemistry on the one hand, and as a utilitarian advanced analytical course in identification on the other, have become hopelessly mixed up at the expense of the students' interest in inorganic chemistry. The important subject of quantitative analysis occupies its rightful place in the undergraduate course, but there needs to be a wide recognition that the main aim should be to teach the principles of the various unit operations. Substances for analysis should be chosen for the technique which they teach, rather than as illustrations of types of compounds. I n the inorganic field analytical methods are standard, except in rare cases. For the investigator of inorganic compounds, however, a new compound or a mixture of substances usually results in a minor investigation t o find a reliable method of analysis. Often he will find his particular problem has not been investigated before. Hence, wide experience of the analysis of different standard substances is of less value than good technique at separating elements in a quantitative manner and estimating each in turn. I believe that we should find more time to enable students to acquire these techniques of inorganic chemistry, as for instance, the handling of substances in the absence of air or of moisture, the manipulation of gaseous substances, and reactions involving low or high temperatures. Finally, I am convinced that, in keeping with the new sense of purpose in inorganic chemistry, the maximum opportunity should be provided for the undergraduate to prepare a compound, to establish its purity by analysis, and to investigate as many of its properties by chemical and physical techniques as he is able to do. This means that we effectivelyillustrate the wholeness of modern chemistry, and I believe that we can thereby develop a genuine enthusiasm for this subject a t the undergraduate level. It is essential for us to stimulate students at the undergraduate level, otherwise we shall not he able to attract research students into this exciting field in which there are so many opportunities and, relatively, so few trained people available to explore them.
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