Potassium hexacyanochromate

Potassium hexacyanochromate (III) (BHjO) [ 13601-11 -1 ] M 418.5. Crystd from water. 

Potassium hexacyanochromate(III) is available by a literature method.A solution of [Cu(tren)(0H2)][C104]2 is prepared by mixing aqueous solutions of Cu(C104)2 6H20 (2.60 g, 7.00 mmol) in 20 mL of water and tren (1.02g, 6.98 mmol) in 10 mL of water. The dropwise addition of a hexacyanochromate(III) solution, prepared by dissolving 0.38 g of K3[Cr(CN)6] (1.16 mmol) in 15 mL of water, to the copper solution results in the immediate formation of a turquoise precipitate. Once the addition is complete, the precipitate is collected by filtration washed successively with cold water, ethanol, and ether and then air-dried to afford a blue powder of [ Cu(tren)(CN) 6Cr][C104]9 H2O Yield 1.50 g, 55%. 

Potassium hexacyanochromate(III) is a yellow solid, highly soluble in water, that crystallizes as large square platelets. It crystallizes in the orthorhombic system, with the space group Pcan. The unit parameters are a = 8.53, b = 10.60, c = 13.68 A. Because of its disorder behavior, it presents a complex crystallographic problem. The compound shows a j/cn band at 2131 cm. The molar extinction coefficients in aqueous solution of the two observable d d bands at 376 nm ( A2g —> 72) and 309 nm (4 2 T l ) are 93 and 62 L mol ... 

Use the information in Table 16.4 to write the formula for each of the following coordination compounds (a) potassium hexacyanochromate(III) ... 

Cs [Ni Cr (CN)5].2H20 is synthesized through soft chemistry engineering at room temperature. A solution of cesium chloride [CsCl] is dropped into a solution of potassium hexacyanochromate K3[Cr (CN)d to get the yellow precipitate Cs 2K[Cr (CN)5]. Then a solution of Ni(NO i)2.6H20 (KF jjjol in 300 ml) is added dropwise to a 100 ml solution... 

Cruser and Miller have described a method for the preparation of potassium hexacyanochromate(III) whereby chromium(VI) oxide is dissolved in hydrochloric acid, reduced with alcohol, evaporated to dryness, taken up with water, and poured into a hot solution of potassium cyanide. The resulting solution is digested several hours, filtered, and allowed to evaporate in air. The following method is a modification of that given by Christensen. It differs from that of Cruser and Miller in that a dichromate is reduced with sulfur dioxide and that chromium(III) acetate instead of chromium(III) chloride is poured into potassium cyanide solution. In following the directions outlined here, special care should be exercised to obtain pure chromium(III) acetate and to avoid decomposition of the relatively unstable potassium hexacyanocbromate(III). 

A hydrolysis solution containing the neutral and cationic cyanoaquo-chromium(III) complexes can be prepared by combining potassium hexa-cyanochromate and perchloric acid in a molar ratio of 1 5. In a typical experiment, 0.65 g (2.00 mmoles) of potassium hexacyanochromate is dissolved in 250 ml of 0.04 M perchloric acid. The solution is kept in the dark at room temperature until the pH of the solution rises to 2.2, which takes approximately 40 to 45 hr. A stream of nitrogen is continuously bubbled through the solution to sweep out the liberated molecular hydrogen cyanide. 

A solution containing only the neutral and cationic cyanoaquo complexes is obtained when more than a 3 1 ratio of perchloric acid is added to a solution of potassium hexacyanochromate(III). The chilled hydrolysis solution, prepared as described previously, can be first passed through a short anion-exchange column in the perchlorate cycle to remove any possible anionic complexes and then loaded onto the cation exchange column in the sodium cycle. Because the separation of the cationic species takes more time than the separation of the anionic complexes, due to the greater number of species present on the column, it is advantageous to use a water-jacketed column and to carry out the separations near 0°C. 

When the eluant concentration was increased in successive steps from 0.1 to 0.4 M sodium perchlorate a small concentration of an unknown species was eluted from the column which contained 0.6 % of the total chromium present in hydrolysis mixture prepared as described previously. Separations carried out at various initial concentrations of potassium hexacyanochrom-ate ranging from 10 to 10 showed that the amount of this species formed depends greatly on the initial concentration of hexacyanochromate used, which may be taken as evidence of its polymeric nature. The species is not produced in detectable amounts if very dilute solutions and long reaction times are used. The ultraviolet and visible spectral characteristics of the species are intermediate between those of the dicyano and tricyano complexes. Because it is eluted just before the cyanopentaaquo complex, we believe that the species probably has an overall charge of +2 and that it is most likely a dimer. 

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