Lauren R. Wilson Ohlo Wesleyan Un~vers~ty
Delaware,
ohlo43015
A Three-Dimensional Model for Introductory Coordination Chemistry
Increased emphasis upon coordination chemistry at the freshman level stresses the need for a simple three-dimensional model of the d orbitals. This model needs to illustrate the nonequivalence of orbitals resulting from interaction with ligands of different geometries. If the model is also space filling, it helps to convey to students the spherical character of atoms and ions. Several proposals for such a model have been set forth in THIS JOURNAL.' However, in the experience of the author, students can more quickly become comfortable with ligand field splittings if they can initially see the ligand geometry (octahedral, tetrahedral, etc.) independent of the d orbitals. Once students are satisfied that the ligands are in a particular geometry the ligand framework may be fitted over the collective set of d orbitals, and it is then easy to decide which d orbitals are nearest to the ligands and should therefore result in a degenerate set. The composite set of d orbitals can easily be constructed either by the method of Douglas1 or more conveniently by purchasing the orbital "lobes" as 78 mm X 44 mm orbitals from Plasteel C~rporation.~ The size orbital provides a space filling model with no steric hindrance. The model can be assembled either by soldering l/lrin. brazing rod sections to a common
484
/
Journal o f Chemicol Education
origin or by sharpening sections of '/8-in. wooden doweling, pushing these into a 11/4-in. hard surface expanded polystyrene sphere, also available from Plasteel, and then cementing the doweling in place. This framework is then used to support the orbital lobes. The latter method provides a more sturdy model. In this case, approximately 15 mm has been trimmed from the base of each plastic foam orbital in order to achieve the better space filling appearance shown in Figure 1. Six of the lobes are painted" single color and are used to represent the .,d, and d,. orbitals on the x, y, and z axes. The remaining 12 orbital lobes which represent the d,,, d,,, and d,, orbitals can be painted a different shade of the initial color DOUGLAS, B. E., J. CHEM. EDUC.,41,40 (1964). NICHOLSON, D. G., J. CHEM. EDUC.,42. 148 (1965). BAKER, W. L., J. CHEM. EDUC:,45, 135 (1968).~ Plateel Corooration. 26970 Princeton. Inkster. Micb. 48141; 10 orbital lobes ior $0.72. a Caution must be used in selecting the paints used on plastic foam materials. Most enamels and lacquers will dissolve styrofoam. A suitable enamel for use on these materials is Testor's Pla enamel, avai1.ilableat hobby stores, or concentrated color kits m e avaihble from Plasteel. Any latex base paint is compatible with styrofoam and may be painted over with conventional lacquers or enamels.
(e.g., dark and light red) in order to clearly differentiate the &, from the e, orbitals. A model so constructed (Fig. 1) has about a 7-in. diameter and can be seen easily by all students in classes as large as 100. For upper level courses the importance of the mathematical sign on $ should be stressed and the suggestions of Douglas' are useful. The geometrical arrangement of ligands is achieved by linking l'/a-in. expanded polystyrene spheres2 with chenille stems (pipe cleaners) available in most department stores. The octahedral and tetrahedral arrangement of ligands are shown in Figures 2a and 2b. Once the class is convinced of a particular ligand geometry, the chenille stems provide a flexibility to the entire framework which allows the ligands to be fitted around the set of d orbitals. In the octahedral case, Figure 1, it is convenient to loosen the bottom ligand to facilitate insertion of the d orbitals into the octahedral cage. Holes are drilled in the ligands to accommodate
Figure 1. Composite set of d orbitals in octahedral fleld.
the penetration of the cartesian axes so that the entire ligand framework is suspended on the z, y, and z axes. Once the ligands are in place, students easily see which orbitals are nearest the donor atoms. The creation of the square planar complex can be achieved by slowly withdrawing two transligands from the octahedral model and subsequently passing through the distorted octahedral or tetragonal configuration. The tetrahedral geometry is usually more difficult for students to visualize since the familiar cartesian coordinates are not obviously involved. However, when a tetrahedral arrangement of spheres, representing ligands (Fig. 2b) is slipped over the d orbital model students quickly see that in fact the ligands "nestle" orbitals which are indibetween the d,, d,, and CL cated by their color (see Fig. 3). An interesting variation can be achieved by painting the ligands with fluorescent paint and showing the model with the ligands in place under ultraviolet light.
Figure 2. Left, octahedral lipand f r a n a v o k tetrahedral ligand framework.
Right,
Figure 3. Composite set of d orbitals in tatrehedrol field.
Volume 48, Number
7,July 1971 / 485
