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The teaching of inorganic chemistry - past, present and future.

The Teaching of Inorganic Chemistry - Past, Present and Future

Inorganic chemistry has evolved and matured during the last 20-30 years to become a recognizable and well-defined subject area in chemistry. Prior to this time analytical chemistry and inorganic chemistry were inter-related areas of chemistry. The general content and emphasis for inorganic chemistry were focused by the requirements to understand aqueous chemistry involved in analytical determinations - typically bulk analysis via titrimetric or colourimetric analysis.

The teaching of inorganic chemistry considered binary compounds of the elements, periodic trends, redox behaviour and stability of aqueous species. Bond energies, electronegativity, ionization potentials and electron affinities were some of the tools to account for this behaviour. Lewis electron-dot structures were the primary basis to account for the structural features, and the various structural examples were based primarily on spectroscopic data with limited and isolated examples from X-ray structures. For example, amphoteric behaviour for aluminum would be described involving species [Al.sup.3+], [Al(OH).sub.3] and [AlO.sub.2-].

The Transition of Inorganic Chemistry

Certain incidents and new instrumental techniques signaled the transition of inorganic chemistry to a distinct area of chemistry. A substantial number of organometallic compounds had been known since the 19th century, however it was the discovery of ferrocene and the characterization of Zeiss salt that led to the resurgence of interest in inorganic chemistry. The study of organometallics now represents a major sub-topic of inorganic chemistry. The variety of compound types, modes of bonding and overall reaction for organometallic compounds continue to stretch the limits for chemistry and force the evolution of new ideas.

The development of new instrumental methods also catalyzed the emergence of inorganic chemistry. Nuclear magnetic resonance (NMR), particularly for the 19F and 31P nuclei, gave new insight to the structural behaviour for compounds of these elements in solution. Multinuclear NMR continues to extend this information and solid-state NMR is starting to contribute related information for the solid state.

Rapid development of electronic computers also contributed to these changes. X-ray crystallography was limited by computer speed and size, and only manual data techniques were available. Starting in the mid 1960s, computers were first used to provide automated data collection, and rapid increases in computer size and speed permitted routine solution of complex structures previously not possible. These changes have resulted in an avalanche of detailed structural information for solids which has had a lasting influence on the nature of inorganic chemistry.

These same computer developments have also contributed to the increased importance of bonding concepts and the role of molecular orbital (MO) concepts in inorganic chemistry. Without the ability to complete detailed MO calculations, it is not possible to test various models for compound formation, nor to establish the relative merit of various ideas. Partly as a result of these changes, inorganic chemistry has emerged as a distinct discipline in chemistry.

Inorganic Chemistry Today

Inorganic chemistry still is based on periodic trends; compound formation and properties remain important and previous theoretical concepts are still valid. But the subject has its own purpose to extend the knowledge of the chemistry of the elements with no necessary intent to serve other masters. Initially, these changes resulted in a severe decline in analytical chemistry which is still recognizable in many chemistry departments. However, analytical chemistry has experienced its own renewal fueled by the developments required for trace elemental analysis and the wide range of analytical problems requiring these methods.

The study of the non-metal elements not only includes the traditional content, but also considers many areas previously unknown or partially defined, the boron hydrides, carboranes, and metal-carboranes are compounds with unique structures and requiring molecular orbital descriptions to account for the bonding. These bonding descriptions increase our understanding of how electrons stabilize the bonding in these molecules. Small metal clusters such as [Bi.sub.9.sup.5+] have been characterized for some of the heavier non-metal elements, and our knowledge of species structures both in solids and solution is much more clear. For example, we now recognize the amphoteric behaviour of aluminum as a sequence of ionizations of one hydrogen from a water molecule coordinated to the aluminum ion. The Al([OH.sub.2])[.sub.6.sup.3+] ion ionizes sequentially to form hydrated aluminum hydroxide, Al[(OH).sub.3][nH.sub.2.O], and eventually forms the Al[(OH).sub.4-] ion. These differences give a more rational approach to the chemical behaviour of the elements.

The study of the transition elements has experienced a remarkable expansion. The classical study of binary transition metal compounds involves mainly the higher oxidation states of these metals and the conventional coordination environments for these metal ions. The organometallic systems stabilize the lower oxidation states of these elements and result in unique bonding situations. The `synergic' bonding models routinely encountered in these compounds also require molecular orbital ideas to account for the molecular stability. Several types of compounds are now the basis for major industrial syntheses or serve as industrial catalysts. Some emphasis for current research is to extend these types of applications.

Models for metal coordination chemistry are now well developed. The `ionic' crystal field model of metal coordination is complemented by molecular orbital descriptions for these complexes and the combination of these models accounts for a wide variety of metal ligand interactions. These ideas now extend beyond classical coordination compounds to include a wide variety of new ligand types including coordination interactions in biochemical systems. Thus teaching current transition metal chemistry requires an integration of classical content with new bonding concepts and an increased range of ligand types.

What the Future Holds

Many developments of inorganic chemistry will continue for the future. However, there are several areas where improved understanding is required. Due largely to the wide variety of element behaviour, there is no systematic description of reaction pathways for inorganic reactions. The extensive mechanistic basis for reactions now available for organic reactions has no parallel in inorganic chemistry. There are isolated examples but we are some way from a unified approach to the problem. Synthesis of organometallic compounds is only partially systematic. Many reactions are lowyield conversions with several side reactions and concepts such as `isolobal' groups or frontier orbital interactions are mainly guides for possible synthetic pathways but with no assurance of success. When the mechanism for these reactions is better understood, such ideas will both account for current reaction and lead to new more reliable syntheses.

For electron transfer reactions we have models for single electron transfers in well defined, relatively slow reaction, but we still have no explanation for the large number of very rapid multiple electron redox reactions. Many reactions occur essentially at the speed of mixing and remain beyond our current experimental limits for investigation.

Solid-state chemistry will be of increasing importance, particularly in terms of the applications in electronic circuitry, and as ceramics and superconducting materials. Solid-state electronic chemical probes are emerging. `High' temperature superconductors, zeolite catalytic systems and new semiconductor phases are an indication of potential impact for solid-state chemistry.

Inorganic chemistry is overlapping more and more with the other branches of chemistry. The investigation of solid-state materials is requiring a considerable overlap with physical chemistry. The study of metals in biological systems has given rise to bio-inorganic chemistry, while the search for polymers with novel properties has led to inorganic polymer chemistry. Links have developed with organic chemistry, where metallo-organic reagents are proving useful in new or improved organic syntheses. While beyond the regular horizons of chemistry, we find such sub-disciplines as inorganic geochemistry.

In Conclusion

Events have a way of coming full circle. Current environmental concerns require more detailed understanding of chemical interactions of trace chemical compounds. It is not sufficient to know what element but also we must know the specific species. These demands will require renewed interaction between the analytical and inorganic chemist to find methods to identify specific species at low concentrations in natural waters and sea water.

With all those potential developments for inorganic chemistry, it is clear the teaching of inorganic chemistry must continue to provide the widest flexibility in chemical content and theoretical concepts so that we may continue to adapt and encompass the new discoveries in compound properties and chemical behaviour.
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Author:Whitla, W. Alex
Publication:Canadian Chemical News
Date:Mar 1, 1990
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