Science scientific politics make it impossible to predict who will receive the prize. Hoffmann described the kind of bridge he is trying to build between the conceptual pictures that chemists draw to understand what happens in organic reactions and the experimental data that are now available on organometallic complexes. His goal, he says, is to make organometallic complexes look less forbidding to theoretical chemists used to looking at organic molecules. He wants to get them to stop automatically flipping past the pages in journals presenting new organometallic structures. Hoffmann calls his bridge between organic thinking and inorganic (or at least metal-containing) molecules the isolobal analogy. In essence, the lobes of the frontier orbitals formed by metal atoms and their associated ligands resemble somewhat the lobes of frontier orbitals in organic molecular fragments. Thus, a magnesium pentacarbonyl group and a methyl group look much the same to a hydrogen atom or a Lewis acid that might react with them. The groups, therefore, are isolobal. The orbitals of isolobal groups are
not identical; in fact, they are substantially different when examined in detail, Hoffmann says. However, there are similarities in the number of these orbitals, their symmetry, extent in space, and energy. In a fair number of reactions they behave in a similar way, he says. Thus, it is useful to consider their similarities. In the past few years, Hoffmann and his colleagues at Cornell have built up an extensive "library" of isolobal pairs of organic and organometallic molecular building blocks. Just as Mn(CO)5 is isolobal with CH3, Cr(CO)ô, with one less bonding electron, is isolobal with CH3+. Similarly, Fe(CO)5 and Ru(CO)5 are isolobal with CH3~. The value of the isolobal analogy, Hoffmann says, is that rather complicated inorganic structures can begin to be seen analogous with better understood organic systems. Interesting series of molecules that stretch from purely organic to purely organometallic begin to emerge. Ring systems can be made, for example, ranging all the way from the purely organic cyclopropane through rings that contain one or two metal atoms to the purely inorganic
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triosmium carbonyl clusters. The geometry and ligand placement of these systems are roughly what would be predicted from considering them all as analogous with cyclopropane. Just how far the analogy can be applied usefully remains to be seen, Hoffmann says. Carbonyl ligands in organometallic compounds very eas ily form bridges between adjacent metal atoms, for example. And the increased stability these bridges af ford the complex means that for many organometallic complexes, the structure predicted by isolobal anal ogy is not the stablest one. Even in such cases, though, the structure predicted by the analogy may exist as a less stable isomer. And the conti nuity that the analogy begins to de scribe between organic and inorganic chemical reactions is exciting and useful to chemists of both disciplines, Hoffmann says. D
Molecular lanthanide hydrides isolated Chemists at the University of Chicago and the University of Alabama have, for the first time, synthesized and crystallographically characterized molecular lanthanide and yttrium hydride complexes. William J. Evans, head of the Chi cago group, notes that simple binary lanthanide hydrides (LnHx) have been known for many years. Intermetallic hydrides such as LaNi5Hx also are known and are being studied intensively as possible hydrogen storage materials. But until the recent work, the Group III transition met als—scandium, yttrium, and the lanthanides—comprised the last group of d- or /-orbital metals for which molecular organometallic hy drides had not been synthesized and crystallographically characterized. (Although earlier work at Chicago had given evidence of the existence of such molecular hydride complexes, they couldn't be isolated and char acterized.) Now, however, Evans and cowork ers have reported the synthesis and x-ray crystallographic characteriza tion of several such molecular lan thanide and yttrium hydride com plexes, including two remarkable trimetallic hydrides \J. Am. Chem. Soc, 104, 2008 (1982)]. The task wasn't trivial, Evans comments, noting that at least four other research groups in the U.S. and Europe have been involved in similar efforts. The hydrides were synthe-
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Science Trimetallic erbium complex has hydrogen at center of triangle
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C&EN April 12, 1982
sized by a standard method, hydrogenolysis of a metal alkyl: (C 5 H 4 R)2Ln[C(CH3)3](THF) + 2 H 2 - [(C 5 H 4 R) 2 LnH(THF)] 2 + 2 HC(CH 3 ) 3 with R = H or CH 3 and T H F = tetrahydrofuran. Both the precursor alkyls and the product hydrides are extremely air- and moisture-sensitive. The methylcyclopentadienyl precursors are also unstable thermally at room temperature, which makes it difficult to purify the hydride products. Crystallographic data also were difficult to obtain. X-ray-quality crystals of [(C 5 H5) 2 LnH(THF)] 2 could be obtained for Ln = Lu, Er, and Y. However, the lutetium derivative was unstable with respect to desolvation. And, although "good" x-ray data sets were obtained for the erbium and yttrium complexes, the data weren't quite good enough to reveal the complexes' structures. "Finally, using the methylcyclopentadienyl ligand, and looking at the fourth and fifth crystal systems investigated, we obtained structural data on [(CH 3 C 5 H 4 ) 2 LnH(THF)] 2 , with Ln = Er and Y. With the loweratomic-number yttrium complex, the hydride ligands could be located," Evans says. That's only half the lanthanide hydride story, Evans continues. The other half involves β -hydrogen elim ination. Although thermal decom position of rigorously purified (C5H5)2Er(£er£-C4H9)THF generates 2-methylpropene and the same dimeric organolanthanide hydride ob tained in the hydrogenolysis reaction, yields are lower and the technique is generally inferior. However, if the precursor isn't pu-
rified so rigorously, and contains lithium chloride (a by-product of the precursor synthesis), the reaction proceeds quite differently, forming trimetallic hydrides—for example, (C5H5)2Er[C(CH3)3](THF) ^
[[(C5H5)2ErH]3Cl][Li(THF)4]
That complex and an analogous lutetium complex (prepared by a somewhat different technique) are triangular arrangements of three (C5H5)2Ln units bridged by hydride and chloride ions, with a hydrogen atom in the center of the triangle. Since metal-metal bonded triangular
trimetallic transition metal complexes don't have enough room to accommodate a hydrogen atom in the center of the triangle, these lanthanide complexes are unique in polymetallic polyhydride chemistry, Evans says. University of Chicago graduate students Andrea L. Wayda and James H. Meadows worked with Evans in synthesizing the new compounds. Jerry L. Atwood and graduate student William E. Hunter of the University of Alabama carried out the x-ray structure determinations. The work was supported by the division of basic energy sciences of the Department of Energy and the National Science Foundation. D
Education Teach more scientifically , educators say The latter approach usually, works better, Reif says. Similarly, a scientific "argument" can be presented—taught—as a succession of, say, 15 steps leading logically from a premise to a conclusion. However, the same argument can be When it comes to teaching chemistry, presented hierarchically. "The same chemists could learn a lot from argument might consist of four major science educators. That theme was steps, grossly described, which sumechoed by several speakers—scien- marize the entire argument. Each of tists as well as science educators—at these steps then would be elaborated a symposium held by the Division of into three or four more detailed steps," Reif says, adding that experChemical Education. Not only "could," but "should," iments have shown that material says Frederick Reif, a physicist at the taught in such a hierarchical form is University of California, Berkeley, better remembered, more easily and also a member of that school's modified if some of the premises are Group in Science and Mathematics changed, and more easily corrected if errors are made. Education. Although scientists pursue their Reif recommends that science own disciplines analytically and sys- courses, especially the early ones, tematically, they usually don't bring shouldn't be limited to science "conthe same approach to tasks outside tent." They also should include, extheir disciplines—relying instead on plicitly and separately, the general "rules of thumb" and "common-sense knowledge essential to good probnotions" of questionable validity, Reif lem-solving: how to describe probsays. Usually, he adds, "We tend to be lems effectively, how to make good equally unscientific in our decisions in searching for solutions, teaching." how to assess the solutions arrived at, For example, Reif says, chemistry and how to organize large amounts of teachers should teach problem solv- knowledge efficiently. ing; it doesn't come naturally. TypiThere are two competing theories cally, novice students try to solve of learning, "One we believe in and problems by proceeding, in linear, one we use," according to John W. sequential fashion, to piece together Renner of the Norman, Okla., Science various mathematical formulas. In Education Center. One—Theory A, contrast, "experts" approach prob- Renner calls it—is based on the aslems using qualitative arguments and sumption that knowledge is directly seemingly vague language, then for- dependent on what is passed on by mulate plans that only later get re- those who already know. The only fined into mathematical language. purpose of science education is to
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impart "mastery of content." The teaching procedure can be summed up as "inform, verify, and practice." However, Renner notes, it's been said that "science isn't knowledge; science is the quest for knowledge." Which brings him to Theory B. That theory also aims to impart mastery or content, he says, but it assumes that learners can and will, with proper experience, create for themselves what is to be learned. Learners are given experience with a phenomenon and absorb its meaning from that experience. Only then are they exposed to the "language" used to label the phenomenon. Teacher guidance mustn't include informing the student of what the concept is that is to be learned, Renner says. It does include helping students collect thé data needed to "invent" the concept to which the investigation leads. Probably more than 90% of the "teaching-material market" adheres to Theory A, Renner says. However» he adds, controlled experiments in Oklahoma junior and senior high schools, using curricula based on each of the theories, indicate that procedures based on Theory Β lead to learning that is superior to that gained from Theory A procedures. Now the goal is to find out why. For good or ill, the textbook still forms the basis of most science courses. And, says science educator William G. Holliday of the University of Calgary, Alberta, science teachers can improve their students' learning by using research-based criteria to select their textbooks. One "objective" criterion is the readability of the text. Holliday rec ommends what he calls the "cloze method" for assessing readability. "First, select about 250 or more run ning words from a text sample. Sec ond, present the passage to the stu dents—leaving the initial and last sentences intact while deleting every fifth (or 20th) word in the remaining sentences. Replace deleted words with blanks. Third, ask students to 'close the gaps' by making contextual inferences from the remaining words." In addition, Holliday offers several "subjective" recommendations. For example, he says, look for a text that's clear and to the point, with a struc turing of ideas and a central focus. Select materials with questions that require students to reorganize and interpret ideas, not just memorize strings of words. Also, texts should contain diagrams and charts that highlight and clarify attributes of ideas and their interrelationships. D April 12, 1982 C&EN
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