Introduction to Modern Inorganic Chemistry Chapter 1

Introduction

Author: Eze-Odikwa Tochukwu Jed

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1.1 Inorganic Chemistry and the discovery of the elements

Chemistry is one of the oldest and most wide-ranging of the sciences, and hence of human knowledge and endeavor. It had already grown sufficiently by the end of the 19th century to be conveniently divided into the three classical branches of inorganic, organic and physical chemistry. Inorganic chemistry covers the properties and reactions of all the chemical elements apart from carbon—now exceeding 110.

A major theme of inorganic chemistry over the last two millennia has been the discovery and characterization of the elements themselves. This continues to the present day in the synthesis of ultra-high atomic number elements by high energy bombardment (Section 16.12).

The discovery of the elements is summarized in Table 1.1. If the pattern of discovery is plotted against time, a curve is obtained which mirrors the pattern of development in many sciences. A long slow period of completely empirical advance in the ancient world was followed by a phase mainly of preservation and rediscovery through the Arab alchemists and in India and China. For the century up to AD 1750, some of the basic ideas of what we now call chemistry were developed from more deliberate investigations. From AD 1750 up to the first half of the 20th century, there was a sharply accelerating pattern of discovery as theory and technique advanced in parallel. Within this period we see individual spurts reflecting specific advances, like the 18th-century studies of gases, the early 19th-century use of electrolysis to isolate the very active metals, or the recognition of the Rare Gas Group which gave five new elements in five years. Eventually the pace slowed, in the decade to 1940, because there were ‘no new worlds to conquer’ and all the elements up to uranium had been identified. This was not the end of the story, as it turned out that further post[1]uranium elements could be synthesized. This phase is now slowing down, reflecting the decreasing intrinsic stability of the nuclei. Whether this is finally the end of the story of the elements is not yet clear {compare Section 16.12). The overall pattern, found in many other developing fields, is of empirical discovery, acceleration fuelled by the interaction of greater understanding and improved methods, then maturity when the pace of change slows. Often, new accelerations start up from the mature phase, as unexpected observations or new ideas trigger off further developments.

1.2 Development

While we have followed the tale of the elements, the growth of inorganic chemistry as a whole followed a similar pattern. Inorganic chemistry was the first of the chemical sciences to flower in the course of the Scientific Revolution, and most of the work leading to the formulation of the atomic theory was carried out on inorganic systems, especially the gases and simple compounds like the nitrates, carbonates or sulfates. A critical advance in technique was the development of ever more accurate measures of the quantity of material—both by weighing and by measurement of gases. Once it could be established that a particular substance had the same composition when prepared by different routes (for example, an oxide prepared from the metal and air, from heating the carbonate, from precipitating the hydroxide from solution and igniting) the way was open to following changes quantitatively, to formulating generalizations like ‘the Law of Constant Composition’, and ultimately to Dalton’s atomic theory. It is worth remarking that even the most sophisticated modern experiment depends ultimately on accurate measurement of weight changes.

Table 1.1

In the first half of the 19th century, not only had more than half of the elements been isolated but a great many of their simpler compounds had been studied. It is remarkable that explosive nitrogen tri-chloride or highly corrosive hydrogen fluoride were under study around 1800. By contrast, only a few simple organic compounds were known by 1820 and little progress was being made in organic chemistry as much of the effort was directed to extremely complex materials like milk or blood. By the middle of the 19th century came the period of spectacular advance in organic chemistry, followed around 1900 by a great upsurge of interest in physical chemistry. These advances meant nearly a century of comparative neglect of inorganic chemistry.

Of course, very important advances were made, including the formulation of the Periodic Table, the discovery and exploitation of radiochemistry, and the classical work on non-aqueous solvents and on the complex chemistry of the transition elements, but it was not until the 1930s that the modern upsurge of interest in inorganic chemistry got under way. Among the seeds of this renaissance were the work of Stock and his school on volatile hydrides of boron and silicon, of Werner and others on the chemistry of transition metal complexes, of Kraus and Walden on non-aqueous solvents, and the work of a number of groups on radioactive decay processes. At the same time, the theories which play an important part in modern inorganic chemistry were being formulated and applied to chemical problems. The discovery of the fundamental particles and the structure of the atom culminated in the development of wave mechanics, which is the basis of all modern approaches to valency and bonding. This theory is outlined in Chapter 2, and its application to molecular structure is given in Chapters 3 and 4. A little later, the effect of an atom’s environment on the energy of its d electrons was brought into the treatment of transition metal compounds in the crystal field theory which is discussed in Chapter 13.

1.3 Recent Advances

All these developments prepared the ground for the expansion of inorganic chemistry, starting in the 1950s, which was stimulated both by developments on the academic side in experiment and theory, and by the demand for new materials and for knowledge of many elements hitherto scarcely studied.

The advent of atomic energy focused attention on heavy transition elements and lanthanides (for example, the chemically very similar Zr and Hf have quite different neutron absorption properties). Similarly, the growth of electronics, followed by computers, led to growth in the chemistry of lesser-known Main Group elements involved in semiconductors, such as Ge, Ga, In and Se. A further significant change in the latter half of the 20th century was the very rapid growth in the number of working scientists and technologists, allowing simultaneous growth throughout chemistry. In earlier times, fields expanded only at the price of relative neglect in other areas.

Starting in the 1950s, transition element chemistry grew from 10% of Honors courses to become the dominant area of interest for several decades, largely as a result of the strong mutual stimuli of experimental and theoretical advances. More recent growth has encompassed fields such as low oxidation state chemistry, organometallic compounds, metal cluster chemistry, dendrites and other macromolecules, and multiple metal-metal bonding (compare Chapter 16). A decade or so later came a similar expansion in Main Group chemistry, building from the more traditional compounds into a substantial range of new species, including rare gas compounds, compounds containing chains, rings or clusters of like atoms, and unusual oxidation states stabilized by specially designed ligands (compare Section 17.9 and Chapter 18). All these developments were led by advances in preparative chemistry and in efficient methods for separating and rapidly characterizing new compounds (see Chapter 7).

Far from slackening, the pace has further increased. Through the turn of the millennium, we live with a continuing headlong expansion in inorganic chemistry, fueled by new methods, new theories, new fields of interest like metals in biological systems, the search for new materials, new catalysts, more output for less pollution, and many other driving forces. Even unstable elements like technetium are finding uses in medicine, and highly radioactive isotopes, such as americium-241, find a variety of applications, including household smoke detectors. Such interests have generated research in every corner of the Periodic Table, and there are now no elements, apart from the very unstable heavy ones, where there is not a very substantial body of knowledge available.

This rapid growth of inorganic chemistry continues to make it a very lively and exciting subject in which to work and teach but it does lead to problems from the student’s point of view. Textbooks tend to be out of date by the time they are published and the treatment of each subject changes as new discoveries are made.

Particularly striking examples arise when the advance arouses widespread interest outside the specialist field. An illustration was provided by the announcement, late in 1986, of a superconductor whose critical temperature was around 40 K. This followed a long period when the highest critical temperature found had risen only very slowly from about 5 K to around 23 K. The new superconductors were oxide phases involving copper and elements like the lanthanides and the alkaline earth metals. Excitement was enormous, as higher temperature superconductors have tremendous potential for all electrical devices. There was a very rapid exploration of the chemistry. A superconducting phase of major interest is YBa₂Cu₃0₇__x, where x is around 0.1, and the pattern of exploration is exactly what any inorganic chemist in the last hundred years would have followed—basically study of complex oxides of related elements, guided by the Periodic Table (see Section 16.1 for a full review including other recently reported classes of superconducting materials).

Another example was the discovery, leading to the 1997 Nobel Prize, of new allotropic forms of carbon—the polyhedral carbon species commonly known as fullerenes, discussed in greater depth in Section 19.3. Coming after at least a century when the only established allotropes of carbon were diamond and graphite, this discovery initiated a wave of studies of these materials, producing new inorganic and organic derivatives, novel materials with interesting physical and electronic properties and many physical and theoretical studies. Such examples, where the driving forces range from the excitement of new types of molecule through to expectations of major practical applications, are representative of many advances. Some novel and unexpected discovery triggers a period of intense interest, where rapid and widespread exploration occurs, heavily based on the pattern of previous knowledge. To a substantial extent inorganic developments are rooted in the relationships of the Periodic Table and established systematic chemistry. The incidence and progress of the more novel new discoveries and growing points are unpredictable, and this should give pause to those who would attempt too closely to guide the development of science into areas deemed to be more ‘relevant’ to the problems of the day.

A textbook must attempt to reflect both the steadily growing core of basic material, and the areas of current interest and excitement. An introductory text can only sample, while an advanced text will make a valiant attempt to cover all areas of current interest.

Theory. In the area of theory, the inorganic chemistry student is presented with a number of approaches at different levels of sophistication. Because chemical entities and their interactions are relatively complex, chemistry is much less ‘theory-led’ than is physics. The power and sophistication of chemical computation is steadily increasing, but it is still the case that only quite simple inorganic chemical observations can be described exactly by theory. Much insight comes from approximate methods, and currently we are faced with a range of theoretical approaches varying in power and degree of approximation. While there have been arguments about which of several alternatives is the best, in most cases we are content to use overlapping and even apparently conflicting theories, depending on the specific application. For example, many species may be described in electrostatic terms—as charged ions, dipoles, etc.—and the energy changes calculated by electrostatics. The same species may equally well be described in terms of covalent bonding, with a theoretical approach which has its base in quantum mechanics. Often, neither approach gives an exact answer because approximations are needed to bring the calculation within the compass of even the most powerful computer. In general, that approach is chosen which gives the most convenient answer to a specific problem, and different methods may be used to tackle different parts of the same problem. It follows that there is no one answer to a question like ‘is this compound ionic or covalent?’, but rather an understanding that either description is more or less useful, and more or less of an approximation.

Difficulties can arise where a relatively simple approach allows the rationalization and systematization of a particular body of data. Because the approach is fairly simple, it is usually not complete—that is, there will be exceptions and anomalies. If the model is reasonably wide-ranging, it is worth retaining and using it even after cases appear which are not covered. On occasion, two different partial models will be used even though they overlap and are not fully compatible. Such situations are quite common, and usually do not greatly disturb the scientist working in the field. They can be confusing to the student on first acquaintance, as there is the feeling that only one can be ‘right’, and we tend to use fairly high-powered words like ‘law’, ‘theory’ or ‘principle’ to describe them. If they are seen as partial descriptions or models, to be used as convenient, many of these problems disappear for the chemist (however much they disturb the philosopher of science!).

In applying wave mechanics to chemistry, the two common approaches have been the valence bond and molecular orbital methods. Each is a different approximation to the wave equation for a system, and they converge to the same answer for very simple systems. For polyatomic molecules, approximation is essential and the theories are used side by side. Older preferences were for the valence bond approach which is closer to the classic picture of a molecule as linked together by discrete electron pair bonds. For example, the partial double-bonding in a species such as the nitrate ion was described in terms of ‘resonance’ between contributing forms, each of which was described in terms of single and double bonds.

This way of thinking is now favored rather less than the molecular orbital approach, which discusses such species in terms of delocalized bonds extending over all the molecule (see Section 4.4). In other areas, such as properties of excited states and of species with extended multiple bonding, the molecular orbital theory is more satisfactory. In this text, the structures of molecules and ions are described largely in terms of the molecular orbital theory.

Similarly, in transition metal chemistry, ligand field theory, which deals with compounds which have valency electrons in the d orbitals, subsumes the electrostatic crystal field theory and also wave-mechanical aspects giving a molecular orbital treatment of transition metal compounds with multi-centered bonds. Again the treatments are at a number of levels of generality and approximation, and are often used in tandem.

All these changes are reflected in the succeeding pages, but the reader will realize that the rapid rate of growth means that any text is somewhat out of date by the time it is published and the review literature should be consulted for recent advances.

Branches of Chemistry

Since the chemistry of one element, carbon, is so enormously ramified — probably similar in extent to the total chemistry of the remaining 109 put together — it has traditionally made up the separate field of organic chemistry. The bridging discipline covering the organic chemistry of the inorganic elements, organoelement or organometallic chemistry, is an extensive field to which we shall make substantial reference. In addition, part of the chemistry of carbon, covering the element and simpler compounds like its oxides and oxyions or the carbides, is traditionally covered in inorganic chemistry. No attempt is made to draw rigid boundaries.

The detailed study of energy changes, reaction mechanisms, much of bonding theory, the chemistry of polymers, chemistry which occurs at surfaces and interfaces, the behavior of metallic systems—all these fall into physical chemistry. Again, there are no rigid demarcations, and much of the most exciting work is done at the points of overlap. Other long standing subdivisions include analytical chemistry and theoretical chemistry.

As might be expected, the huge expansion of chemistry in the last few decades has led to further subdivision within inorganic chemistry—such as phosphorus chemistry or transition metal chemistry—as well as the defining of new fields with substantial overlap with inorganic chemistry. The latter include materials science, catalysis, inorganic biochemistry, computational chemistry and many more.

The roots of Inorganic Chemistry

The origins of inorganic chemistry are ancient. Observation followed by what we now describe as empirical experiments led to the slow development of new materials from the early stages in human history. Thus beads of glass and of ceramics are found in ancient Egyptian burials and pottery was made by the earliest civilizations. Considerable control was achieved: black or red pottery was made by reducing or increasing the proportion of air, and colors and glazes had developed to a high degree of sophistication by 500 BC.

The discovery of metal extraction and processing was a most important theme and led to substantial mastery of the technologies by ancient craftsmen. Gold is usually found naturally as the free metal and, as shown by grave goods, has always been highly valued. Modern analysis of ancient artefacts shows that it was understood that the addition of small proportions of silver or copper gave a harder, more wear-resisting metal, and gave desirable variations in color. The metal contents of gold coins held to highly consistent standards which correlated through many countries in a chain of related weights and gold contents — for example, from Macedonia to India in the 5th century BC. Copper has been known for about 7000 years and has been extracted from sulfide ores for around 5500 years. Small metal items like beads and bracelets are found in graves for several centuries before larger products like tools or weapons, suggesting a period where manufacture was difficult and not well understood. Analysis on ancient kilns shows that temperatures up to 1200°C were achieved in Bronze age copper smelting. Once production methods had evolved to the larger scale, it was rapidly found that alloying copper with tin to give bronze, or with zinc to give brass, produced metals of superior properties.

Tin, which is fairly inert to air, was probably prepared in a pure form many centuries before the more reactive zinc. Iron requires much more sophisticated treatment, and can be extracted only at the high temperatures achieved with air blown through a bed of charcoal. The carbon is also necessary to remove the oxygen from the ore. Even so, early temperatures did not reach the melting point, so casting was not possible and the metal was shaped by hammering. As iron weapons are much superior to bronze, it is thought that the initial discovery was kept secret for a millennium and knowledge of iron only became widespread after the destruction of the Hittite empire about 1200BC (recent archaeology suggests this picture is oversimplified). Compared with iron, isolation of the other elements found up to the 18th century is relatively easy, so iron-working represents a peak in technological achievement which lasted for something like 4000 years