Color Classification of Coordination Compounds Laurence Poncinil and Franz L. Wimmer School of Natural Resources, University of the South Pacific, Suva, Fiji In his famous texthook A Method for the Identification of Pure Organic Compounds, published in 1904, S. P. Mulliken (1) drew attention to a dilemma. While color is often the most salient property of a chemical compound and changes in color can furnish identification tests, yet color tests are not held in the same esteem as the measurement of other physical constants. This heing due principally to subjective influences in determining color. He sought to minimize the defects in his own work by consistently adhering to a more definite color terminology than had previously been used for chemical purposes. Colorimetry is routinely used in industry (2) and the arts (3) and has been investigated in the field of organic chemistry (4-8). However, to our knowledge there has been no extension of colorimetric methods to metal complexes. Sometimes there is considerable difficulty in choosing an appropriate name for the color of a newly synthesized compound. The converse of this is trying to imagine the color of a compound reported in the l i t e r a t ~ r eThis . ~ is hardly surprising since it has been estimated that there are of the order of 10 million distinguishable color differences that are described by words (91, although the actual number of color names in use is far fewer than this. For example, the Maerz and Paul (10) color dictionary contains less than 4,000. The naming of a color, not unexpectedly, is easiest for a pure spectral color (i.e., red, yellow, etc.), yet even here there are various shades. For example, yellow can he descrihed as heing light, pale, bright, deep, canary, lemon, etc. The naming becomes somewhat more difficult at the boundary between two spectral colors. Thus, light a t 650nm will hecalled either yellowish green or greenish yellow depending on the observer (11). The assignment of a name is most difficult when the colors are nonspectral. Colors such as olive green and wine red come under this category. A further complication is that different observers have differing impressions of the color associated with a particular name. An observer assigns a name to the color of an ohject by comparing i t with his color memory. I t has been shown (12) that what a person imagines to he the color of a familiar ohject (say a canary) may be different from the actual color of that ohject.
The color of a substance, heing diagnostic, is an important property and thus should he accurately reported. T o illustrate the method? we have classified, using a fluorescent light ~ o u r c ea, ~number of complexes5 that are descrihed as yellow. This has been done using the Methuen (13) and Munsell (14) color system^,^ as summarized in the table. Wherever possible, the complexes were used in the form arrived a t after preparation. Only in the case of very intense colors was i t necessary to grind the crystals so that they could he fitted into the available color ranees. The effect of particle size was also investigated. This is meatabulated under "treatment". which is an ao~roximate .. sure of the degree of grinding employed. For "pure" colors (i.e., with a high degree of lightness, high Munsellvalue), the effect of grinding is only a reduction in the saturation (chromaL7 whereas for colors with a low degree of lightness (e.g., K2PdC14)a change in hue is also observed.
'Corresponding author, present address: 13 Lawson Avenue. Camden. NSW 2570, Australia. Some examples taken from the literature include orchid, rose. salmon, and buff. The method of color assessment was similar to that described in ref 9. Each test sample was prepared by placing a portion of the compound onto a microscopic slide to give an area (-2 cm2) of uniform appearance and thickness with a flat man surface. The bottom of the slide immediately beneath the sample wascovered with foil.Alternatively a shallow aluminum holder can be used. 'When comparing the sample with the standard colors, the angle of incidence of light (fluorescentsource)was 4S0 while viewing at an angle of 90' to the surface (see refs 9and 1I). Most compounds can be easily matched with this method. 5 P O t a ~ ~ ichromate ~m was obtained from Koch-Lioht LM. and K,PdCI, and K,RCI, from Jonnson Manhey Ltd The other compounds were prepare0 by methods fomd in referencesgiven n the table " We have included the Methuen system because it is found in most libraries, is easy to use, and resembles the DIN color system (DIN 6164). For those unfamiliar with color terminology, the excellent article by Wyszecki, G. W. In The World Book Encyclopedia; World Book: Chicago, 1985; Vol. 4. pp 660-667, should be consulted.
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Color Classlflcatlon of Some Yellow Coordlnatlon Compounds Reported Compounda
Color4
N~PYzC~ K,CrOl
pale Y-G Y(17)
Pt(bpy)C12 Pd(bpyICI2
Y(I@ light Y (19) bright Y (20) YG-E (17)
trans-[Rh(py),CI,]C1.5H20 K2PdCI,
K2RCls
hans[Rh(p~)~Br,]Br.8H~O
y(f7) Y-0 (21) 0-Y (23
(16)
Treatmentr P G, Ga P P P
Color Notation
Melhuen ( 13)
28.5A4 30A8 30A6
1A5.5 30A2 2A6
G2
408
Ga P P
485
3.5A8 4A8
M~nsell~,~
65GY 8.513 3.5GY 8.5111.5
3.5GY 916.5 1GY 915 1GY 911
8.5Y 918.5 4.5Y 5.517.5 3.5Y 8/55 3 Y 6113.5 2.5Y 8/14
Systematic Color Name ( 13) pastel green yellowish green yellowish green greenish yellow greenish white yellow olive brown greyish yellow vivid yellow deep yellow
'PY = pyridine. bpy = 2.2'-bipyrldyl. =Yellow. 0 = orange. G = green. GY = greenish yellow. YG = yellowish green. B = brown. 'P = assessed ar prepared. GI = coarsely ground. G2 = finely ground. Gs = very finely groun0. 'Revised Munsell renotation. see ref 15. The three Eymboio indicate the hue. value. and chroma, respectively.
Volume 64
Number 12 December 1987
1001
Systematic color names together with the reported colors are included in the table. I t is surprising that potassium chromate, which is traditionally regarded as being yellow, is actually yellowish green. This system apprnrs tu be of greatest utility for nonspectrnl colors. Thus the color of Kd'dCI, has been described as yellow-greenish brown (see table). since it is commonly obtained as intensely colored lumps, the color can only be ascertained by grinding (see tahle), and thus i t varies with the particle size. We propose, therefore, that colored compounds he classified hv reference to a standard color-order svstem incorvorating a color dictionary. As a consequence-of this, in the future, the colors of new comvounds could be incorvorated into the characterization proc&s and into computer-storage systems. Such an approach would enable other workers t o obtain an accurate impression of the color as well as giving rise to a standardization of color names.
1002
Journal of Chemical Education
Literature Clted 1. Mulliken,S. P.A Method for LheIdentificolionof Pure Organic Compounds; Wiley: ~ New York, 190%Vol. I, p 230. 2. dudd. D. B.; Wysrecki.G. W.Ccliuiii B~~iiiii~SsiiiiiiidI~du~fry, 2nded.: Wiley: New York, 1967. 2. %home, R. Lights ond Pigmenlr: Murray: Landon, 1980. 4. I.ewir, G.N.: Calvin. M. Chem.Rlu. 1939,26,278-326. 6. Macen1l.A. Quart. Reua (London) 1947. I(l1. 16-58. 6. Rumpi, P. Reu.Sci. 1919.87.17&1M. 7. Dewar. M.J.S. Chem. Sac. lLnndan1,Sper. Pub1 1956,4,61-87. 8. .ludd, D. B. In Encyclopedia of Industrial Chemical Anaiyris, Snail, F. D.; Hilton. C. L.;Edl.; Interscience: New York. ,966: Vol. 1. 9. Judd,D. R.; Kelly, K. L. J.Res, Net. Rur.SLond. 1939.23.856385. 10. Maen. A : Paul, M. R. A DiclionaryofCo1our;McCrew-Hill: NewYozk. 1960. 11. Bourma. P. J. Physical Aspects of Coiour: Philips: Eindhoven, ,947. 12. K l i n ~J., W.; Rigg. L. A. Woodworth and Schlosberg's Exp~rimenlalPsychology, 3rd en.; ~ d t~. i ~and~winston: h ~ N
