Harry 6. Gray Arthur Amos Noyes Laboratory California Institute of Technology Pasadena. CA 91125
HOWMuch Inorganic Spectroscopy and Photochemistry?
All modern courses in inoreanic chemistrv include some discussion of the electronic structures of molecules. Treatment of this imoortant t o ~ i varies c widelv, however, from course to course A d book & hook. It is cle&ly essential for students to have a firm understanding of the ground state electronic structures of inorganic molecules, hut there is nowhere near universal aareement on the exact list of topics to he covered nor on the ;me to he spent on said topics in thevarious inorganic courses. And the confusion only increases when the ireatment of electronic excited states is thrown in for consideration. Surely a serious student of chemistry should know something about inorganic electronic spectroscopy and photochemistry. The question is, "How much?" In mv view. three levels of courses are required to treat adequa;ely the ground state electronic structires, the spertroscoov. and the ohotochemistw of inowanic - molecules. The presentation in the various inorganic courses should pay proper attention to the student's background in physical chemistry (p-chem). At the third (advanced) level the course should have a p-chem prerequisite. The first level treatment presumably comes in a general chemistry course. I believe this level should be confined to building t h e most basic foundation for discussion of ground state electronicstructuresand stereochemistry.The two key suhjecta that must be taught are line-and-dot (Lewis) representations of electronic structure and stereochemistry. Discussion of the latter topic is facilitated by introduction ot'the cunrept of valence shell electron-pair repulsion. At this most hasic level. orbital theorv is not needed (nor, it could he argued, is i t desired). The second level treatment should emohasize ground state electronic structural descriptions. Both the valence bond and molecular orbital theories of electronic structure should be introduced and applied to representative inorganic molecules. Transition metal romplexes should be included here. S p metry prinriplescnn be used effectively at this level, hut it is not likely that students can handle formal group theory. The d orhitals lieand field ran he denoted bv the .- - in ~ an ~ octahedral ~ labels t2# and eg, hut any m&e detail than that will probably onlv, serve as a source of confusion. Presentation of the ~ h o toelectron spectroscopy of inurganic molecules works well at this staee. offerine as it does some exoerimental iustification pretty pictures of molecuiar orbital energy levels. for all Electronic ahsorption and emission spectroscopy (ground A - .
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764 I Journal of Chemical Education
state a excited state) should not be dealt with in great detail, however. A few simple examples where electronic repulsion effects can be ignored (the textbook case of Ti(H20)e3+ and similar cases ofone-electron excitation) should suffice. To be ----~-~~ sure, there are ways to rationalize the more complicated spectra of Cr(H20)e3+, Co(NH3)63+, N i ( H ~ 0 ) 6 ~ etc., + , but these are not very satisfying. I t is probably better to wait for the third (advanced) level to get into the many-electron problems. In my view, at this intermediate level it is better to develop a deeper understanding of the properties of the electronic ground states of a broad selection of inorganic molecules than to attempt to teach either electronic spectroscopy or photochemistry. The first two levels of inorganic electronic structural instruction can be achieved with a minimal p-chem background (such as obtained in general chemistry). The advanced level course should be built on the previous levels as well as on a full year of p-chem. At this level group theory should be used: manv-electron mound state and excited state ~ m h l e mshould s , be treated; and an introduction to inorganic photochemistry should be oresented. Both the d-d and charge transfer sDectra of inorgaiic complexes should be studied in some detail. At least three principal geometries (octahedral, tetrahedral, square planar) should be included in the discussion. A few key oraauometallic cases should be treated, and some attention shiuld be given to metal cluster compo&ds. ~~~~
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Harry B. Gray is W.R. Kenan, Jr. Professor at the California Institute of Technology where he has been on the faculty since 1966. He received the PhD at Northwestern Universitv in 1960. following which he spent a y& as an NSF Postdoctoral Fellow at the University of Copenhagen. He held a faculty appointment at Columbia University 1961-66. Gray has published over 280 research papers and I I buoki. He is a membrr of the National Academy of Science, and a rn Felluw uf the AAAS. He has r~ceiwdthe ACS Awardr in Pure Chemistry and in Inorganic Chemistry, the Remsen Award, nnd the Richard Tolman Award. in addition to numerous other ~~~-~~~ awards, including the MCA ~ w & for d Excellence in Chemistry Teaching. Gray is a Foreign Member of the Royal Danish Academy of Sciences.
