Chemical Education Today
Letters A Coordination Geometry Table With regard to the article “A Coordination Geometry Table of the d-Block Elements” (Moore, J. S.; Venkataraman, D.; Hirsch, K. A. J. Chem. Educ. 1997, 74, 915), it should be noted that the table is of limited value, since many important coordination compounds have been deliberately excluded. Thus, in the same manner that the composition of oceans is not representative of the composition of the earth’s crust, concluding that the table (Tables 2a and 2b) “is an accurate indicator of the occurrence of coordination number and geometry as a function of element and oxidation state” is concluding too much because of the many restrictions (such as CNs 2–6, OSs 0–VII and data from the Cambridge Structural Database [CDS]) used for its preparation. Consequently, people unfamiliar with inorganic chemistry should take the information presented in Tables 2a and 2b with many precautions to avoid drawing erroneous conclusions. Consider how easy it is for a novice student to conclude, for example, that: •
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The CNs higher than 6 and OS out of the interval 0 to VII are irrelevant in d-block elements so that all this can be neglected Mn(VII) compounds are nonexistent; alternatively, since KMnO4 is known, Mn(III) chemistry is irrelevant (from Table 2a) Cr(0) species are dominant in the chemistry of chromium, while Mn(0) and Ni(0) species seem to be nonexistent (from Table 2a) The CN of the metals in common vanadates, chromates, and permanganates must be higher than 6 (from Table 2b)
For example, Mn(0) and Ni(0) complexes contained in the CSD possess ligands that chelate and/or form π-complexes. As stated in the paper, such complexes were purposely omitted because it was felt that such ligands could bias coordination geometries. In regard to the statement that Mn(III) chemistry is “irrelevant”, it should be stressed that our Coordination Geometry Table reports the frequency of coordination geometries and oxidation states as observed in single crystal structures. Of course, we do not mean to imply that frequency (of occurrence in the CSD by our selection criteria) necessarily equates with relative importance. Table 2a shows that there are far more Mn(II) complexes in the CSD than Mn(III) complexes. This fact does not in any way suggest that Mn(II) complexes occupy a more important role in modern chemistry than Mn(III) complexes. We do agree with Arnáiz, however, in regard to certain limitations of our search, and the reader is cautioned of these limitations in the “Conclusions” section of the paper. As stated, “We have presented one rendition of a coordination geometry table of the d-block elements and their ions.” In analyzing an enormous body of data such as that contained in the CSD, a set of reasonable criteria must be applied systematically to simplify categorization and interpretation. Certain limitations are bound to arise as a result and are unavoidable. Nonetheless, we still feel that our results are by and large representative of qualitative claims made in the literature regarding coordinating propensities of transition metals. We maintain the belief that the quantitative representation put forth of a fundamental aspect of coordination chemistry will be of benefit to practitioners of many fields of chemistry. We thank Arnáiz for his comments and his interest in our paper. Jeffrey S. Moore, D. Venkataraman, Keith A. Hirsch Departments of Chemistry and Materials Science & Engineering and the Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana, IL 61801
Apart from the necessary caution mentioned above, I consider these tables are of undeniable utility for those possessing some knowledge of inorganic chemistry, especially those familiar with the information contained in CSD. Comparing the results from this study with their own personal conceptions and knowing what class of species is the focus of attention in the last decades will provide for many helpful discussions. Francisco J. Arnáiz Laboratorio de Química Inorgánica Universidad de Burgos 09001 Burgos, Spain
The authors reply: Prof. Arnáiz raises some good points in analyzing our manuscript (J. Chem. Educ. 1997, 74, 915–918), which we would like to address in terms of our chosen search criteria. As stated in our paper, all of the data were selected from the Cambridge Structural Database (CSD), which compiles crystal structures of metal complexes containing organic ligands. Therefore, complexes such as KMnO4 were necessarily omitted from the tables. Also, Mn(0), Ni(0), and high-coordination number vanadate, chromate, and permanganate complexes were omitted as a result of the chosen search criteria. 28
Cut-Out Molecular Models In our training year as school teachers, we are asked to select and discuss recent articles on the teaching of chemistry. We have found a special interest in the paper by R. J. Kashmar, “The Use of Cut-Out Molecular Models on the Overhead Projector to Illustrate Stoichiometry and Limiting Reactants” (J. Chem. Educ. 1997, 74, 791), in time to make some use of the models in our classes. We have introduced a few improvements on this strategy: a.
b.
The circles corresponding to each atom had different sizes, depending on the element (covalent radii were used). The circles for each molecule were arranged in accordance with the actual molecular geometry.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
Chemical Education Today
c.
d.
help if deans were to see what kind of increases we face in our supplies’ costs. I opened up several past catalogues from a wellknown chemical supply house and picked out a representative list of items that we all must buy from time to time—reagents, hardware, and glassware. The table below shows how the total costs have risen over the past six years (in the case of reagents, I list the “ACS-certified” grade). I’m not faulting the chemical suppliers here—their costs are going up too. Also, they often give substantial discounts on orders. But the percent increases, discounts included, would remain about the same. A kinetic analysis of the growth rate shows that these totals are increasing at an exponential rate with a k = 0.0699 yr᎑1— almost a perfect fit when we plot ln(cost) versus year. This reflects a doubling interval of 9.9 years. Are our budgets increasing at this rate?
The rearrangement of the circles for each reaction was performed with the projector off or with the position of the mirror changed, in order to avoid misconceptions regarding the reaction mechanism. When a molecule “appears” in the projection for the first time, we simulated the molecular vibrations (with references to rotations and translations). Ana Luisa Silva, Carla Fernandes, Olivier Wasterlain, Sandra Costa, Ana Maria Mendes Escola D.Dinis 3020 Coimbra, Portugal
Kinetics Lesson for College Deans
Fred Hadley Department of Chemistry Rockford College Rockford, IL 61108
How about a kinetics lesson for college deans? Like everyone else in academe, we must make our annual plea to the administration for increases in the departmental budget. It might Item Acetic acid Acetone Ammonia Aniline Benzoic acid t -Butanol Chloroform Copper sulfate pentahydrate Hydrochloric acid Hydrogen peroxide, 30% Iron(III) nitrate nonahydrate Methanol Methyl ethyl ketone Nickel chloride hexahydrate Nickel sulfate hexahydrate Potassium chloride Potassium iodide Sodium acetate trihydrate Sodium thiosulfate pentahydrate Toluene Zinc chloride 250-mL Pyrex erlenmeyer flask 250-mL Pyrex beakers Solution storage bottles, 240 mL General-purpose tubing 250-mL volumetric flasks pH electrode, plastic body, Ag/AgCl ref Capillary tubes, mp, 150 mm Total % Increase over previous catalog
Amount 2.5 L 1L 2.5 L 500 mL 500 g 1L 1L 500 g 2.5 L 500 m L 500 g 4L 1L 500 g 500 g 500 g 500 g 500 g 500 g 1L 500 g 1 pack of 12 1 pack of 12 24 12 feet 12 1 10 vials N/A N/A
1992 $41.25 22.00 25.00 34.00 53.20 33.00 25.00 41.00 28.00 46.00 35.00 31.00 35.00 76.00 75.00 16.35 98.00 18.65 15.40 13.85 50.85 33.54 27.14 36.00 25.00 205.00 90.00
1994 $47.10 26.80 30.75 40.70 63.70 39.55 30.55 50.15 32.20 59.50 41.90 41.55 41.90 90.95 89.70 20.90 115.45 22.25 18.45 19.10 60.85 37.11 30.07 42.00 25.00 205.00 94.00
1996 $48.70 36.20 32.20 43.60 65.30 42.90 42.00 55.40 35.60 64.40 53.40 44.90 42.90 106.40 113.80 28.50 124.50 31.50 28.00 31.80 82.90 41.30 33.50 45.75 27.00 234.00 98.00
1998 $61.51 40.36 41.45 50.61 74.67 49.75 50.51 63.42 41.66 71.26 61.11 49.33 49.75 124.18 130.15 32.64 141.80 36.07 32.09 36.90 94.85 44.85 36.30 79.77 30.54 271.72 110.00
50.00 $1,280.23 N/A
60.00 $1,477.18 15.4
60.00 $1,694.45 14.7
72.75 $1,948.62 15.0
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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