D. P. Graddon University of N e w South Wales Kensington, Australia 2033
A Simple Host Lattice Experiment
It has been known for many years that cobalt(I1) compounds of the type CoXZB2, where X = C1, Br, I, or NCS and B = NH,, pyridine, or some other base, may exist in two forms: pink and blue.' I n the blue forms the cobalt atom has tetrahedral stereochemistry and in the pink forms bridging by the halide or pseudohalide leads to octahedral stereochemistry.
cylinder and a stirring rod. The following aqueous solutions are necessary: cobalt nitrate 0.1 M, zinc nitrate 0.1 M, copper sulfate 0.5 M, potassium thiocyanate 0.5 M, and pyridine 10% (this must be freshly prepared).
Sometimes the monomeric, tetrahedral form is the Procedure more stable, sometimes the polymeric, octahedral form. (1) P u t about 200 ml water into a beaker, add 10 ml 0.1 M I n a few cases both forms can be isolated. cobalt nitrate, then 10 ml 10% pyridine, (use Hood!), finally 10 When X = NCS and B = pyridine the stable form ml 0.5 M potassium thiocyanate. A pink precipitate appears, usually quite slowly, but precipitation is almost complete after a t room temperature is the polymeric, octahedral about ten minutes. The pink compound has the formula Coform, though a t temperatures above about 120°C the (NCS)zpyd. If required, i t can be converted to the grey-pink, blue, monomeric, tetrahedral form is more stable. At polymeric form of Co(NCS)zpyZ by heating in an oven a t about room temperature the tetrahedral form reverts to the 100°C for an hour or two. octahedral form in a few hours. I n the experiment (2) To about 200 ml water add 10 mlO.l M zinc nitrate, then 10 ml 10% pyridine (use Hood!), finally 10 m10.5 M potassium here described the blue form is stabilized a t room thiocyanate. A white precipitate forms almost a t once, which temperature by incorporation in host crystals of the has the formula Zn(NCS)zpyz. corresponding zinc compound, Zn(NCS)zpy2, which (3) To about 200 ml water add 9.5 ml 0.1 M zinc nitrat3 and 0.5 mlO.l M cobalt nitrate, then 10 mllO% pyridine (use Hood!), exists only in the tetrahedral form.2 finally 10 ml 0.5 M potassium thiocyanate. A white precipitate The zinc compound can also be used to stabilize a Zn(NCS)zpyz forms almost immediately. As the crystals tetrahedral form of the copper compound, CU(NCS)~ of grow they become bright blue in color, due to incorporation of pyz. This, like the cobalt compound, is normally the otherwise unstable blue form of Co(NCS)zpyz. These blue polymeric, with octahedral stereochernistry and is crystals can be kept indefinitely. (4) To about 200 ml water add one drop 0.5 M copper sulpale blue-green in color. When incorporated in the phate, then 10 ml 10% pyridine (use Hood!) (the blue color of zinc compound a beautiful violet color is observed, the copper-pyridine complex ion will be observed), finally 10 ml presumably due to the otherwise unknown tetrahedral 0.5 M potassium thiocyanate. A small amount of green-blue form of the copper compound. precipitate will form very slowly. Tetrahedral compounds of zinc and cobalt are regular, (5) To about 200 ml water add 10 ml 0.1 M zinc nitrate and one drop 0.5 M copper sulfate, then 10 ml 10% pyridine (use but those of copper suffer distortion due to the JahnHood!), finally 10 ml 0.5 M potassium thiocyanate. A white Teller effect. I t is interesting to observe that the zinc precipitate of Zn(NCS)zpyz forms almost immediately. As the compound used here as a host will accommodate large crystals grow they turn a beautiful violet color, presumably due concentrations of the cobalt compound, but only small to incorporation of a tetrahedral form of the copper compound (Cu(NCS)zpyz. concentrations of the copper compound. It has not been found possible to stabilize a tetrahedral form of the SIDGWICK, N. V., "The Chemical Elements and their Comcorresponding nickel compound in this way. pounds," Oxford University Press, Oxford, 1950, 11, p. 1385; WELLS,A. F., l i S t r ~ ~ t ~Inorganic ral Chemistry," 3rd. Ed., OxD. P., ford University Press, Oxford, 1962, p. 922; GRADDON,
Experimental
The equipment required is very simple, consisting of a beaker of 250-500 ml capacity, a 10 ml measuring
"An Introduction t o Coordination Chemistry," 2nd. Ed., Pergamon Press, Oxford, 1968, p. 77. GRADDON, D. P., HENG,K. B., AND WATTON, E. C., Aust. J. Chem., 19, 1801 (1966).
Volume 47, Number 7 0, October 1970
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