Organometallic Chemistry A Course Designed for Sophomore Chemistry Students Gary L. Miessler and Gary 0. Spessard St. Olaf College, Northfield, MN 55057 Organometallic chemistry is traditionally given little attention at the undergraduate level except as covered in inorganic chemistry courses taken primarily hy juniors and seniors. If mentioned a t allduring the first two academic years, its coverage is generally slight. General chemistry texts may ignore the subject completely, and only recently have a few organic texts begun to include small discussions of organometallic chemistry (vide infra). In spite of the paucity of coverage of organornetallic topics during the first two years of college, a course in general chemistry and a semester of organic do provide sufficient background for offering an indepth introduction to this very important field of chemistry. Such an introduction can therefore be made a t a much earlier point in the curriculum than has ordinarily been thought to he the case. During the past three years we have introduced a new course in organometallic chemistry a t St. O l d College. This course has been offered during St. Olaf s one-month January Interim session, when students devote full time to a single course; it has carried the prerequisite of a single semester of organic chemistry. The course has met daily for 90-min class periods,givinga total time in class of 30 hfor the term. While our comments in this paper will describe our experiences during St. Olafs Interim, the course can easily he adapted for schools having quarters (which have approximately 30 class hours) as well as one-month academic terms. A moderate expansion of the course material would also make it suitable for a full-semester course. The vast majority of students taking this course have been sophomores, sandwiching one month of organometallic chemistry between two full semesters of organic. A few students, approximately 30%. have taken the course as juniors or seniors, following oth& chemistry courses. Student response has been very favorahle, and the course has been both challenging and rewarding to teach. In developing this course we were not able to locate infor-
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Journal of Chemical Education
mation about anv other attemnts to teach a course in oreanometallic chemis&y at the sol;homore level. No textbook of an~rooriate . level was available: the few oreanometallic texts on the market seemed aimed primarily at seniors or graduate students. For the first two years, Fundamentals of TransitionMetal Organometallic Chemistry by C. M. Lukehart (I) was used, while R. L. Crabtree's recently published text, The Organometallic Chemistry of the Transition Metals (2), served for the third and most recent year we have offered the course. T o adapt this material to students having a onesemester background in organic chemistry, we found i t necessary to prepare rather extensive supplementary materials, including class handouts, transparencies, and lists of useful iournals and other reference works. In addition to the assigned readings from these texts, we have also assigned supplemental readings from the chemical literature a t several points in the term. Throughout the course we made dailvorohlem assimments, with most of the problems taken from the texts. &.
Use of Chemlcal Llterature We felt that it was quite important to expose students to the relevant chemical literature as part and parcel to the coverage of the important topics of organometallic chemistrv throueh lectures and readine in the text. We assiened --, --three papers, suggesting an approximate length of 3-5 pages for naoer -~~each. The first . . was on a "classical" oreanometallic compound, one whose discovery played a significant role in the development of organometallic chemistry. Examples of such compounds included ferrocene, Zeise's salt, Wilkinson's comoound IRhCll(PPhd1, and Collman's reagent [~az~e(cd)~ r ] .h second d p a G , assigned during the portion of the course devoted to reaction mechanisms, required students t o discuss a specific kind of organometallic reaction such as oxidative addition or ligand substitution. In this paper the students were asked-to describe the reaction
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Outllne and Schedule ol the Coursea Week 1: I:
I.
Chemical Background Review of atomic and molecular orbitals Wl-llgafhi orbital interactions The ls- and 18-electron rules Octahedral camolexes Square planar and tetrahedra complexes Metal aKy 9 and meta hydrldes Metal carbonyl complexes Bonding modes of CO [Predlctlng infrared frequencies on the basis of molecular symmetry] ~
118.
IV.
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Week 2: V.
VI.
Mecbnisms of llgand substitution reactlons Complexes of pl-bound ligands Bondlno and reactions of memllocenes and other Cyclopntsolenyl Complexes Complexes 01 mner Cyclic p ligands Alkene, alkyne, and p8-ally comp exes Complexes of other linear pi ligands [Carbene and carbyne complexes] Reactions of organometallic complexes Oxidative a ~ t l o n Reductive ellmination Insertion Elimination
Week 3:
VI1.
Nucleophlllc and elemophlllc addNon and absvacUon Davles' rules Homogeneous catalysis Examples of catalytic cycles of Industrial Importance Carbenes: structure. reactions, and their role In metathesis and polymeriratian Activation of small m~lecules Appilcation~to organlc synhsls
Week 4: [VII. IX.
'TOPICS
Applications to aganlc synthesis (continued) Chemically equivalent groups In main group and transition metal chemistry] The isoloba analogy lsolobal molecular fragments Metal clusters in brackets were incivded duing the first two y e a n of me course.
mechanism and its nossible variations and stereochemical aspects. In addition it was required that students cite examd e s from the chemical literature with a t least one reference from 1985 or later. In the third and final assignment students were reauested to select a short research DaDer from the recent orghometallic literature (since 1985) and to analyze and explain its contents. Students were given rather wide latitude in selecting a research paper for this writing assimment, with the only restriction that the article e m ~ h a size-an application of organometallic chemistry to either organic synthesis or catalysis. In all the writing assignments students were required to use the original chemical literature to support a t least part of their paper and to cite references and otherwise prepare their papers according to ACS format. Some students even elected to have their papers satisfv the advanced writine reauirement that is Dart of the geneid graduation requirements at St. Olaf. u p i n completion of their papers, students made oral presentations on them during aregular class period. We found these presentations to be useful supplements to the written part of their paper assignments in that they not only provided an opportunity to help students develop oral communication skills, of the class to hea; but &o they allowed other &bers firsthand about aspects of organometallic chemistry that were not explicitly covered in lectures or reading assignments.
of Course Organlzatlon Over the three years the outline in the table has evolved. Some sliaht modifications have been made as the course has developeb, especially in the transition from being taught by an inorganic chemist (GM) during the first two years to an organic chemist (GS) during the third year-these changes are also given in the table. An important initial objective in the course was to build upon students' background in chemical bonding in simple inorganic and organic molecules in order to showhow sigma and pi bonds could form between organic ligands and metal atoms. This backmound and a discussion of the 18-electron rule (3)were essential for considering metal-ligand bonding as various types of organometallic compounds were introduced in later parts of the course. As this introductory material was being discussed in class, students were preparing their first paper on a "classical" organometallic compound important in the historical development of this area of chemistry. Metal carbonyls were the first category of compounds that we discussed extensively. After describing the various bondingmodes of CO and the structural types of binary carbonyl complexes, we discussed mechanisms of ligand substitution reactions, including factors having important effects on reaction rates. The usefulness of infrared spectroscopy in analyzing metal carbonyl complexes was illustrated with several examoles includine. ... on a aualitative basis. the relationshio hetween molecular symmetry and the number of IR-active vibrational bands. Additional tvDes .. of comolexes discussed are listed in the table. Following a discussion of the different types of organic ligands and of their mode of bonding to metals, we surveyed the most common categories of reactions of oreanometallic compounds, as listed in the table. Special attention was given to the mechanism of carbonyl insertion (alkyl migration) reactions as an example of how experimental design can provide valuable information about possible reaction intermediates (4)-for most students this provided the most detailed examination of a reaction mechanism to date. The textbooks alone did not prove adequate a t providing an organized guide to the different types of reactions; conseauentlv. the textbooks extensivelv with - . we suodemented . our own materials. Since organometallic catalysts are of significant and erowina- im~ortance, we devoted several class . periods to catalytic cycles, especially cycles of importanre in oreanic svnthesis and in industrial processes. This portion of the course was supplemented by the second and third writine assienments, both of which were devoted to discussions ofieaction mechanisms and their applications. As our final topic we chose the isolobal analogy, using sections of Roald Hoffmann's Nobel lecture (5). We did not have time to consider all the details of this analogy but concentrated on parallels between fragments of octahedral transition metal complexes and fragments of tetrahedral carbon.
Resources At this point we believe i t is appropriate to comment upon the various resources available to instructors who wish to teach a course in organometallic chemistry. Besides the two texts we used, there are several hooks covering primarily transition metal oreanometallic chemistrv (6-11). These are. not surprisingly, pkched to the a d v w c e i "ndeigraduate o; first-year graduate student. Lukehart's text (I) gives a good coverage of structure and reaction types and features applications of organometallic chemistry to organic s.ynthesis and catalytic cycles. End-of-chapter problems are provided, hut no answers are included save for an occasional reference to the article in the literature from which the problem was Volume 68 Number 1 January 1991
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taken. The book does suffer from the fact that footnotes referring directly t o literature articles are not to he found. Only a topic-oriented bibliography a t the end of each chapter is included. Our most recently used text by R. H. Crabtree (2) was published in 1988. This book also offers a broad coverage of the field placing more emphasis on reaction chemistry and less on structure when compared to Lukehart's hook. Crabtree's text is footnoted and up-to-date through 1987. Problems are provided a t the end of each chapter with brief answers given in an appendix. The other books referred to above were made available to students as reference works in the library. All offer useful surveys of modem organometallic chemistry hut for various reasons were not as suitable as texts for our course as the books we chose. We used reviews and series t o supplement the text and lecture material in the course. ~ombrehensiueOrganometallic Chemistry was quite useful as a source for students in writing their papers. Also available were Aduances in Orpanometallic Chemistry and Dictionary of Organometallic . Comoounds. Clearly the most important resource in a study of organometallic chemistry is the original chemical literature. At the sophomore level i t may be too much to expect that students will make the best use of that source of information. We felt, however, that the writing assignments would at least give students a feeling for reading the literature that could he useful in more advanced chemistry courses. Moreover, the third paper assigned did require that the student read and write about an article from the recent literature. Available in our library were all the major journals dealing with organometallic chemistry, withthe exception of Journal of Organometallic Chemistry. Most of the articles in the recent literature covering synthetic, structural, and mechanistic aspects of organometallic chemistry were accessihle directly by students. In addition to research journals listed above, the Journal of Chemical Education has featured numerous articles relating to various aspects of organometallic chemistry over the last 20 years. Notably there was an article by Ellis (12) that was addressed to the teaching of organometallic chemistry t o undergraduates. The paper did not describe a course that was being taught or resource materials for such a course but rather outlined some simple concepts and models that could ... - be annlied to the teachine of oarallels between main group and transition metal organometallic chemistry. Other articles. besides the one alreadv mentioned on the 18-elec-~~~. tron rule (3), have reported on aspects of catalytic cycles such as the Monsanto acetic acid orocess (13) and asvmmetrihydrogenation used to produce L-dopa (14). Finallv. i t is interestine to note that included in the spate of recent (since 1985) organic chemistry texts are hooks that mention aspects of transition metal organometallic chemistry. The latest edition of Solomons's text (15) devotes an entire special topics section to the suhject covering such areas as the 18-electron rule, various organometallic reaction types (oxidative addition, reductive elimination, etc.), Wilkinson's catalyst, and the hydroformylation reaction. Ege's recent second edition (16) discusses Wilkinson's catalyst and nucleophilic aromatic substitution of benzene-chromium complexes. Morrison and Boyd (17) and Streitwieser and Heathcock (18) also devote special sections to aspects of organotransition metal chemistry in their latest editions. ~
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Journal of Chemical Education
Conclusions We believe that a course in oreanometallic chemistrv fits very well into a curriculum where students have already taken a year of freshman chemistry and a t least asemester of organic chemistry. From the freshman course students can make use of their knowledee of s i m ~ l ebonding theorv including MO theory of diatomic moiecules. If ciordin&on compounds and crystal field theory are covered during the first year of chemistry, use can be made of this knowledge when discussing the bonding of transition metals to organic ligands, especially if bonding theory has been brought to hear upon developing an understanding of aromatic and alkenyi systems i n a sibsequent course inorganic chemistry. Moreover, simple concepts of thermodynamics and kinetics, first mentioned in the freshman course and amolified in the following organic course, can be used in explaining the various reaction tvoes .. found in oreanometallic chemistrv. For example, ligand association and dissociation have great similaritv t o S ~ and 2 Sy1 reactions. whichare usuallvcovered in first-semester organic chemist&. Students discover that the formation of a Grignard reagent is really an example of an oxidative addition reaction, that hydrohoration involves the insertion of an alkeneinto aB-H bond, and that theoxymercuration reaction begins with the formation of a mercuryalkene complex followed by nucleophilic attack a t one of the two carbons of the original carhon-carbon double bond. A discussion of catalytic cycles such as hydroformylation, the WackerSmidt reaction. and homoeeneous hvdroeenation . .. leads students into an appreciation of industrial processes of which thev would orohablv be unaware in a traditional undergraduate chemistry program. Coverage of applications of oreanometallic chemistrv to asoects of oreanic svnthesis can serve as a useful preview to examination of functional group chemistrvthat often occurs in the second and perhaps alsoin more advanced courses in organic chemistry:F'inaily, there are several instances where organometallic chemistry is involved in biochemical reactions (2, 19). This often is an exciting aspect of the suhject for students who are interested in the study of chemistry primarily because of its relevance t o biology. Literature Clted
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1. Lukehart, C. H. Fundamentals of Trouifion Metal Orgo~melollic Chemistry; BrookdCole: Monterey, CA, 1985. 2. Crabtree, R. I. The Orgonomfoilic Chemistry of the Tmuition Metois; WileyInuracience: Nev York, 1988. 3. Mitehell.P.R.;Parkh,R. V.J. Chem.Educ. 1969.46.811-814. 4. F1ood.T. C.: Jenson,d E.:Statler, J.A. J.Am. Chem. Soe. 1981.103.441C-4414. 6 Hnffmann.R.Annem. Chom. Int.Ed.Enel. 1982.21.711-724. 6. ~ ~ m ~ r nA. ~ f~ d& .~ ~ t r n r n i f i o~ne f o i c h ~ ~ i ~Wilcy-lnteracience: fry; New York, 19%. 7. Atwood, J. D. Inorganic and Orgonometollic Reaction Merhanirms; Brooks/Cole: Monterey, CA, 1985. 8. Co1irnsn.J. P.; Hogedus. L. S.: Norton,J. R.; Finke. R. G.Rincipl~a~ndApplicoliona of Organofmuition Metal Chemistry; Univeraihl Science: Mill Valley. CA, 1987. 9. Dsvies. S. G. Orgonotranailion Metal Chemistry; Applications to Orgonie Synihe8ir: Pergamon: Oxford. 1982. Berlin, 10. Haidue. 1.: Zuckermsnn. J. J. Basic Or~onomstollicChemblry: de Gru*,: 1985. 11. Powell. P. Binriples of Organomfollic Chemistry, 2nd ed.; Chapman and Hall: ~~
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960-962. 17. Morrison. R. N.:Boyd. R. N. Organic Chemialry. 5th ed.; Allyn and Bamn: Bwton. 1987: p p 7 U 3 5 7 . 18. Streitwieser. A,; Hesthcock, C. H. Introduction Lo Orgonie Chemiairy. 4th ed.; Maemillan: New York, 1985: pp 1089-1096. 19. Hughes, M. N. The Inorganic Chemiatn, of Biologieol Proreaaea; Wiley: Chicheater, 1981.
