Modeling Chemistry for Effective Chemical Education: An Interview

Chemistry for Everyone

Modeling Chemistry for Effective Chemical Education: An Interview with Ronald J. Gillespie Liberato Cardellini Dipartimento di Idraulica, Strade, Ambiente, e Chimica, Universit a Politecnica delle Marche, 60131 Ancona, Italy [email protected]

The Valence Shell Electron Pair Repulsion (VSEPR) model has provided a useful basis for understanding and rationalizing molecular geometry. Because of its simplicity, the VSEPR model has been accepted as a didactic tool, and now it is included in almost all general chemistry and inorganic chemistry textbooks. This theory has done much to simplify and improve the teaching of molecular geometry and chemical bonding at an elementary level, and it has also proved to be useful for discussing the structures of many new inorganic molecules. For this reason, and for others, Ronald J. Gillespie (Figure 1) has earned a welldeserved, international reputation in chemical education. Now Professor Emeritus of Chemistry at McMaster University, in Hamilton, Ontario, Canada, Gillespie has taught inorganic chemistry and general chemistry at McMaster University and abroad for over 30 years. Born in London, England, on August 21, 1924, Gillespie became fascinated with chemistry during high school and in 1942 was awarded a bursary to do a special two-year wartime degree in chemistry at University College London. After graduation, he was invited by Sir Christopher K. Ingold, the distinguished pioneer of modern physical organic chemistry, to stay at the university, and Gillespie became part of the team working on the mechanism of aromatic nitration. Gillespie's problem was to study the ionization of nitric acid and the oxides of nitrogen in 100% sulfuric acid and to look for evidence for the presence of the nitronium ion, NO2þ, by cryoscopy or freezing point depression measurements. On the basis of Gillespie's work, Ingold wrote a group of nine papers, six of them carrying Gillespie's name only, published in 1950 in the Journal of the Chemical Society (1-9). Ingold gave Gillespie considerable freedom and encouraged him to develop his own interests, so when the nitration problem was solved, he began to study the ionization of other solutes in anhydrous sulfuric acid, such as nitrocompounds, carboxylic acids, and other organic compounds. He also extended his research to the physical properties of sulfuric acid itself. During a conversation, Ingold suggested that Gillespie should prepare a few lectures on molecular properties and weeks later Gillespie was appointed Assistant Lecturer at University College, before receiving his Ph.D. in 1948, at the age of only 24, and without having made a single application. The VSEPR theory was introduced in 1957 in a publication in the Quarterly Reviews of the Chemical Society (10), coauthored with Sir Ronald S. Nyholm, an outstanding Australian inorganic chemist and memorable teacher, and Professor of Inorganic Chemistry at University College. 482

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Figure 1. Ronald J. Gillespie, photographed by Tom Boschler; used with permission.

Dissatisfied with the absence of personal research funds and with the difficulties in obtaining research equipment, Gillespie started to look elsewhere and in 1958 he was offered a position at McMaster University. Tempted by an offer of twice his British salary, and the promise of both a new NMR machine and a Raman spectrometer, he accepted the offer to cross the Atlantic. This new start gave a big boost to his research work, and he moved into an extensive study of superacid chemistry. He found that the addition of the Lewis acid SbF5 to HSO3F greatly enhanced its acidity: this superacid media was named “magic acid” by George A. Olah, recipient of the 1994 Nobel Prize for Chemistry, for its ability to protonate extremely weak bases. Because Gillespie had the only NMR spectrometer in Canada, the earliest spectroscopic measurements of Olah's carbocations took place in Gillespie's laboratory. Gillespie has been associated with McMaster University for more than 40 years. He laid the basis of modern inorganic chemistry in Canada. He is known for his work in fluorine chemistry and superacid chemistry. He has received many prizes and awards for his work, including the American Chemical Society Awards for Creative Work in Fluorine Chemistry and for Distinguished Service to Inorganic Chemistry. Gillespie is recognized also for his passion for teaching and for his leadership in chemical education. Interested readers can find many of his contributions in chemical education and his provocative opinions in this Journal; searching the JCE index, 27 papers bear Gillespie's name.

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Gillespie is fascinated by and interested in chemistry and in the history, culture, and languages of several countries where he worked during several sabbatical leaves. He also participated in a number of sports, a passion cultivated from the time he was a student. Enamoured by the mountains, he met his wife, Madge Garner, a schoolteacher, on a mountaineering holiday in France. Gillespie regenerated his mental energy by enjoying skiing and rock climbing in the most beautiful mountains in the world, and by sailing in the lakes of Canada. Retired in 1989, he continues to seek to gain a greater understanding of molecular structure and geometry and to work to popularize the VSEPR model and his more recent ligand close packing (LCP) model, which is an important supplement to the VSEPR model (11, 12). A whole special issue of the journal Coordination Chemistry Reviews was created on the occasion of his 75th birthday (13) to honor Ronald J. Gillespie for his long and very successful career, marked by his adherence to sound principles, both scientific and personal.

Pauling and vigorously promoted by him in several editions of his book The Nature of the Chemical Bond (14) and in many lectures and conferences. I had come to realize that the usual presentation was a circular argument: methane is tetrahedral so sp3 hybrid orbitals must be used to describe the bonding, followed shortly after by the statement that methane is tetrahedral because the bonding is sp3. Moreover, the “shapes” of atomic and hybrid orbitals are produced like a magician “out of a hat” with no explanation. This is necessary because first-year students do not have the necessary background knowledge of quantum mechanics to have any understanding of orbitals. About this time I came across a paper by Sidgwick and Powell (15) who showed that the geometry of a variety of molecules could be predicted by simply counting the number of electron pairs in the valence shell of the central atom. I expanded this idea by allowing for the differences between lone pairs and bonding pairs in terms of a few simple rules and by applying the model to a much larger number of molecules. This is what I later called the VSEPR model (16).

Choosing a Career

What is the physical basis of the VSEPR model?

Liberato Cardellini: How did you become a teacher and why did you choose an academic career? Ronald Gillespie: I was a researcher before I became a teacher. I graduated from University College London in 1942. I was invited by Ingold to stay on there for my Ph.D. in chemistry, which I obtained in 1949. I was then appointed an Assistant Lecturer in the Chemistry Department. I did not apply for the position but as I had greatly enjoyed the research I had been doing and had already had the opportunity to extend it beyond the scope of what was required for my thesis, I saw no reason to refuse the position or to seek an appointment elsewhere. Of course teaching came with the job, and fortunately I found that I enjoyed teaching. I enjoyed the opportunity to transmit my knowledge and understanding of chemistry in a clear, simple, and understandable way to the students in my classes.

The physical basis of the VSEPR model is the Pauli principle, which in its general form states that (17):

Why do you find chemistry so fascinating and creative? Chemistry is fascinating because it is the basis for understanding the material world: everything on Earth is made of atoms and molecules, and chemistry may be said to be the science of atoms and molecules. Atoms can be put together in endless ways to create new molecules and this makes chemistry the most creative of the sciences. It is this particular aspect of chemistry, the ability to make new substances that have not existed on Earth before, that makes it so fascinating for me. The Models How was the idea of the VSEPR model (the GillespieNyholm rules) born? I proposed the VSEPR model in 1957 in a paper with Nyholm entitled “Inorganic Chemistry” (10). I wrote the first part of this paper on the stereochemistry of the molecules of the main group elements, and he wrote the part on the molecules of the transition metal elements. I was unhappy with the conventional way of teaching molecular geometry in the 1950s in terms of hybrid orbitals, which was a method suggested by Linus

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Electrons with the same spin have a zero probability of being found simultaneously at the same point in space, a low probability of being found close together, and are most probably to be found as far apart as possible.

However the effect of the Pauli principle in keeping same spin electrons apart is much stronger than the effect of electrostatic repulsions. The Pauli principle is usually discussed in introductory courses in terms of the rule that two electrons cannot occupy an orbital unless they have opposite spins. But the Pauli principle is more general than this statement, which applies only to the orbital model. There is no explanation for it in wave mechanics but only in a more general form of quantum mechanics. As a consequence, the eight electrons in the valence shell of the central atom of a molecule with at least two ligands are most probably to be found as four opposite spin pairs with a tetrahedral arrangement (10, 18, 19). What is the relevance of the ligand close packing model? The ligand close packing model (11, 19, 20), which is perhaps less known than the VSEPR model, supplements the VSEPR model particularly for predominantly ionic models. LCP assumes that the geometry of such molecules is predominantly determined by the repulsions between the ligands when the ligands are much larger than the central atom, as is the case for BF3, BCl3, CF4, and CCl4 so that BX3 and CX4 molecules are, respectively, trigonal planar and tetrahedral. From the intramolecular distance between two identical close packed ligands, the ligand radii of a number of ligands have been determined and shown to be additive to give the distance between two different ligands. The LCP model provides an explanation, for example, for why the BF bonds in BF3 are shorter than in BF4-: three ligands pack more closely around a central atom than four. This model also provides an explanation for why the bond angles in a number of OX2 molecules, for example, are larger than 109.5° rather than less than 109.5° as predicted by the VSEPR model. In a molecule

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such as OCl2, for example, the large size of the Cl ligands prevents them from being squashed together more closely than the sum of their radii. Hence, the ClOCl bond angle is larger than 109.5°. Another example is the molecule (SiH3)2O. This molecule, which is predominantly ionic because of the large difference between the electronegativities of oxygen and silicon, has a very large bond angle of 140° because of the very large size of the SiH3 ligand (19, 21). How is the Laplacian of the electron charge distribution related to the Lewis model of electron pairs? The Laplacian of the electron density has maxima in those regions where the electron density is locally more concentrated than in the surrounding regions (22, 23). It has been found in many cases that these regions of locally increased electron density coincide with the expected positions of the lone pairs and bonding pairs, but this is a purely empirical observation. The regions where the electron density is locally most concentrated are not necessarily the same as the positions where there is a high probability of finding a localized electron pair. However, the electron localization function (ELF) (24, 25) has maxima in those regions in which a greater probability exists of finding an electron pair in the molecule. It agrees well with the qualitative predictions of the VSEPR model. The maxima in the Laplacian generally agree approximately with the maxima in the ELF, although sometimes they do not, as for example in ethene (21). How can the electron density distributions enhance our understanding of bonding and molecular geometry? Electron density measurements show that the electron density in a molecule is not very different from the sum of the atomic densities. A very large proportion of the total density is concentrated around each nucleus. Only a very small proportion is in the bonding regions. Moreover, the density in the bonding regions of a molecule is only slightly greater than the sum of the atomic densities. There is certainly no maximum in the density in the bonding region, as the diagrams in some introductory textbooks appear to show. The unequal distribution of the electron density between two atoms clearly shows the polarity of a bond. Moreover, the electron density around each atom enables the charges on each atom to be determined, which therefore gives more quantitative information on the polarity of the bonds (19, 26). The electron density of a molecule can be interpreted in more detail by the Laplacian and ELF functions. Teaching Chemistry The conventional approach to bonding (overlapping of orbitals and hybrid orbitals) makes chemistry too abstract for many students. Could you suggest a better approach? Can we explain bonding without orbitals? The VSEPR and LCP approach to bonding and molecular geometry is much simpler and more easily understood than the localized and hybrid orbitals approach, partly because it does not require a prior understanding of quantum mechanics provided that the Pauli principle is assumed. Students understand this model much more easily than the orbital model because the VSEPR and LCP approach only requires a previous knowledge of Lewis diagrams. I have been 484


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disappointed that localized and hybrid orbitals continue to be taught in introductory courses, and often in high school courses for which they are particularly inappropriate. The Sienko and Plane textbook (27) has changed the way that chemistry is taught. There is now a feeling among some teachers that the pendulum has swung too much to the physical chemistry side. What great ideas of chemistry do you suggest to teach? I agree that the pendulum has now swung too far in the direction of physical chemistry. Chemistry may be said to be the science of atoms and molecules and their reactions. The most important topics for an introductory course are:

• The atom, as consisting of electrons with surrounding electrons that cannot be precisely located but can only be described in terms of probabilities and energy levels. • The molecule, as an assembly of atoms held together by chemical bonds in a definite arrangement or molecular geometry. • Kinetic theory, which does not necessarily mean a detailed mathematical treatment but simply the idea of kinetic energy and its relationship to temperature: the higher the temperature the faster the atoms move and the more energy they have. • A qualitative treatment of entropy as a measure of disorder. • Reactions and reaction rates.

The preceding concepts are what I have previously called “the great ideas of chemistry” (28). These fundamental ideas can, and should, be introduced not as isolated topics but in the context of so-called “descriptive chemistry”, in other words inorganic and organic chemistry: descriptive chemistry is not a recognized or useful subdivision of chemistry. For example, heat, energy, and molecular geometry can be conveniently discussed as part of a discussion of hydrocarbons. Molecular geometry can also be introduced in the discussion of the period 3 nonmetals. Of course, all these topics can be treated at various levels. But a majority of the students in a first-year course will not get another chemistry course. What these students need is a simple treatment that enables them to get a basic understanding of chemistry such as is necessary for the average educated person who needs a basic understanding of the three main areas of chemistry: inorganic, organic, and physical chemistry. Detailed and mathematical treatments can be left to later courses. More specialized topics, such as colligative properties and electrochemistry, can well be left to later courses. Overall less material and less detail treated more slowly would enable students to gain a much better understanding of chemistry than they do at present. Problems involving little more than arithmetic that are found in large numbers at the end of the chapter in current textbooks really only test the students' ability in arithmetic and their ability to use formulas that they have memorized with little real understanding. This “busy work” should be mostly replaced by qualitative problems that require a real understanding of basic concepts. What are the advantages of using enthalpies, entropies, and free energies of atomization? The use of enthalpies, entropies, and free energies of atomization makes the calculation of the enthalpy, entropy, and free energy changes in a reaction very easy for a student to understand. The energy change for splitting all the reactant molecules into free atoms is simply the sum of all the energies of


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atomization of the reactant molecules, while the energy gained in the formation of the product molecules is simply the sum of the energies of atomization of the product molecules: the difference in these two quantities is the overall energy change for the reaction. Students find this quite easy to understand as it is the same line of thinking used in thinking about the law of conservation of mass and balancing equations; namely, split the reactant molecules into atoms and use up all the atoms to form the product molecules leaving none of them unused (29).

have to be content with less travel and the less energetic pastimes of playing chess and Scrabble, and reading.

Further Reflections

1. Gillespie, R. J.; Hughes, E. D.; Ingold, C. K. J. Chem. Soc. 1950, 2473-2492; DOI: 10.1039/JR9500002473. 2. Gillespie, R. J. J. Chem. Soc. 1950, 2493-2503; DOI: 10.1039/ JR9500002493. 3. Gillespie, R. J.; Graham, J.; Hughes, E. D.; Ingold, C. K.; Peeling, E. R. A. J. Chem. Soc. 1950, 2504-2515; DOI: 10.1039/ JR9500002504. 4. Gillespie, R. J. J. Chem. Soc. 1950, 2516-2531; DOI: 10.1039/ JR9500002516. 5. Gillespie, R. J.; Graham, J. J. Chem. Soc. 1950, 2532-2537; DOI: 10.1039/JR9500002532. 6. Gillespie, R. J. J. Chem. Soc. 1950, 2537-2542; DOI: 10.1039/ JR9500002537. 7. Gillespie, R. J. J. Chem. Soc. 1950, 2542-2551; DOI: 10.1039/ JR9500002542. 8. Gillespie, R. J.; Hughes, E. D.; Ingold, C. K.; Peeling, E. R. A. J. Chem. Soc. 1950, 2552-2558; DOI: 10.1039/JR9500002552. 9. Gillespie, R. J. J. Chem. Soc. 1950, 2997-3000; DOI: 10.1039/ JR9500002997. 10. Gillespie, R. J.; Nyholm, R. S. Q. Rev. Chem. Soc. 1957, 11, 339– 380. 11. Gillespie, R. J. Coord. Chem. Rev. 2000, 197, 51–69. 12. Gillespie, R. J.; Robinson, E. A. C. R. Chim. 2005, 8, 1631–1644. 13. Coord. Chem. Rev. 2000, 197, 1-487. 14. Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University: Ithaca, NY, 1960. 15. Sidgwick, N. V.; Powell, H. E. Proc. R. Soc.London, Ser. A 1940, 176, 153–180. 16. Gillespie, R. J. J. Chem. Educ. 1963, 40, 295–301. 17. Gillespie, R. J.; Matta, C. F. Chem. Educ. Res. Pract. Eur. 2001, 2, 73-90; http://www.uoi.gr/cerp/2001_May/pdf/05Gillespie.pdf (accessed Feb 2010). 18. Gillespie, R. J.; Robinson, E. A. Chem. Soc. Rev. 2005, 34, 396–407. 19. Gillespie, R. J.; Popelier, P. L. A. Chemical Bonding and Molecular Geometry; Oxford University Press: Oxford, 2001. 20. Gillespie, R. J.; Robinson, E. A. Adv. Mol. Struct. Res. 1998, 4, 1–41. 21. Gillespie, R. J. Coord. Chem. Rev. 2008, 252, 1315–1327. 22. Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University Press: Oxford, 1990. 23. Popelier, P. L. A. Atoms in Molecules: An Introduction, Pearson Education Ltd.: Edinburgh Gate, Harlow, 2000. 24. Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397– 5403. 25. Malcolm, N. O. J.; Gillespie, R. J.; Popelier, P. L. A. J. Chem. Soc., Dalton Trans. 2002, 3333–3345. 26. Gillespie, R. J. J. Chem. Educ. 2001, 78, 1688–1691. 27. Sienko, M. J.; Plane, R. A. Chemistry: Principles and Properties; McGraw-Hill: New York, 1966. 28. Gillespie, R. J. J. Chem. Educ. 1997, 74, 862–864. 29. Gillespie, R. J.; Spencer, J. N.; Moog, R. S. J. Chem. Educ. 1996, 73, 617-622; 622-627; 627-631; 631-636.

You graduated doing research with Sir Christopher Ingold, the father of modern organic chemistry. Are there some memories of him that you would like to share with us? I found Ingold to be rather a remote and reserved person, and like all his other students I was rather in awe of him. I saw him about once a month for a brief chat about the work I was doing. He seemed quite happy with the progress I had made on the project he had suggested, and he allowed me to extend it in various ways that I suggested. He even did a lot of the writing of some of the papers based on my Ph.D. work and did not put his name on these papers. I have much to thank him for. But I never got to know him personally: he never gave parties for his students or invited us to his home. Could you assess your work and findings in superacid and fluorine chemistry? My work on superacid chemistry included the development of many superacid systems such as HF-SbF5, HSO3F, HSO3FSbF5 and the quantitative measurement of their acidity (30-32). The use of these superacid media led to the discovery of many new inorganic species such as polyatomic cations of the nonmetals (I2þ, I3þ, Br3þ, Se42þ, S82þ, S102þ, Bi53þ, etc.), which have interesting and sometimes difficult to explain structures and geometry (33, 34). The importance of this work can perhaps be judged by the fact that George Olah was awarded a Nobel Prize for his discovery of carbocations in the superacid media that I had discovered and investigated in some detail. My work on fluorine chemistry was a natural outcome of my work on superacids and on the VSEPR model. I showed that the geometry of the fluorides of the noble gases could be readily predicted by the VSEPR model, including that of the C3v geometry of XeF6, which had incorrectly been predicted on the basis of molecular orbital theory to have regular octahedral Oh symmetry. This further stimulated my interest in noble gas chemistry and I went on, particularly in cooperation with G. J. Schrobilgen, to discover and determine the structures of a number of new noble gas fluorides, such as XeF3þ, XeOF3þ, KrFþ, Kr2F3þ, XeOF2, and XeO2Fþ (35, 36). You have published five books and almost 400 papers. Do you find the time to pursue other interests? In school and in university, I participated in track and field and then became interested in rock climbing and mountaineering in Britain and the Alps. When I moved to Canada, I took the opportunity to hike the local trails, to camp with my family in many of the provincial parks of Ontario, to sail on the Great Lakes, and to ski in Canada, the United States, and Europe for many years. My wife and I have traveled extensively, but now I


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Acknowledgment I would like to thank the reviewers for the suggestions they made to improve the interview. Literature Cited


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30. Gillespie, R. J. Sulphuric Acid as a Solvent System. In Inorganic Sulphur Chemistry, Nickless, J., Ed.; Elsevier Publishing Company: Amsterdam, 1968; pp 563-586. 31. Gillespie, R. J.; Peel, T. E.; Robinson, E. A. J. Am. Chem. Soc. 1971, 93, 5083–5087. 32. Gillespie, R. J.; Peel, T. E. J. Am. Chem. Soc. 1973, 95, 5173–5178.

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33. Gillespie, R. J.; Passmore, J. Acc. Chem. Res. 1971, 4, 413–419. 34. Gillespie, R. J.; Morton, M. J. Q. Rev. Chem. Soc. 1971, 25, 553– 570. 35. Gillespie, R. J.; Schrobilgen, G. J. Inorg. Chem. 1974, 13, 765–770. 36. Gillespie, R. J.; Schrobilgen, G. J. Inorg. Chem. 1974, 13, 2370– 2374.


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Modeling Chemistry for Effective Chemical Education: An Interview

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