Ammonium Hexachloroiridate IV

SuBuiTTED RT George B. Kauffuan and Larry A. Teter Checked by J. W. HoGARTHf and F. P. DwYBRft 

Iridium is an extremely inert metal which is not attacked by acids, aqua regia, or fused alkalies. It reacts readily with chlorine, however, especially in the presence of alkali 

Ammonium hexachloroiridate(IV) can be prepared by the metathesis of sodium hexachloroiridate(IV) with ammonium chloride or by the addition of ammonium chloride to either a solution of iridium(IV) hydroxide in hydrochloric acid or of iridium(IV) chloride. Although many workers have prepared sodium hexachloroiridate(IV) by chlorinating mixtures of sodium chloride and iridium, few exact data are available concerning optimum temperatures, reaction times, and yields. 

Among the hexachloroiridates, the compounds of tripositive iridium are more stable than those of the tetrapositive metal. Wohler and Balz report that in a stream of chlorine, sodium hexachloroiridate(IV) be ns to decompose into hexachloroiridate(III) at 570°. At 750° the product is primarily sodium hexachloroiridate(III), which in turn decomposes into metallic iridium above 900°. The chlorine content of the residue was found to be dependent upon the temperature. That the reaction proceeds stepwise has been shown in a more recent study by Puche. Typical results of chlorination studies of mi.xtures of sodium chloride (4.00 g.) and iridium (2.00 g.) are as follows  

Product Yield, %, for furnace temp., °C. 15°/reaction time hours  

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Insoluble iridium dioxide from the sodium peroxide fusion is dissolved in aqua regia, oxidized with nitric acid, and precipitated with ammonium chloride as impure ammonium hexachloroiridate(IV), To purify this salt, it is necessary to redissolve the compound and precipitate out the... 

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Ammonium hexachloroiridate(IV) forms strongly refracting octahedra which vary from red-black to almost black depending on crystal size. Above 200° it decomposes to metallic iridium, nitrogen, ammonium chloride, and hydrogen chloride. 

Aqueous solutions of ammonium hexachloroiridate(IV) are readily reduced to hexachloroiridate(III) by a variety... 

In the present synthesis, the more readily available ammonium hexachloroiridate(IV) is substituted for iridium-(IV) chloride. An alternative synthesis involving preliminary reduction of ammonium hexachloroiridate(IV) to hexachloroiridate(III) with ammonium oxalate has been found to give substantially the same total yield of both isomers, but the amount of cis isomer obtained was about twice that of the trans isomer. ... 

Since 6-coordinated iridium has an octahedral configuration, tetrachloro (dipyridine) iridates (III) occur in two isomeric forms—cis (orange) and trans (red). The sparingly soluble pyridinium salts are readily isolated and hence are convenient starting materials for the synthesis of other members of the series. Because of their widely differing solubilities in water, the cis and trans pyridinium salts are easily separated. The procedure given below, which is modified from Delepine s original preparation, employs readily available ammonium hexachloroiridate(IV), which is converted to hexachloroiridate(III) by reduction with ammonium oxalate. 

A solution of 0.711 g. (0.005 mol) of ammonium oxalate 1-hydrate in 20 ml. of boiUng water is added in 1-ml. portions with continuous stirring to 4.42 g. (0.010 mol) of ammonium hexachloroiridate(IV)f contained in a 100-ml. round-bottomed flask Caution. Effervescence). The dark reddish brown solution is warmed on the steam bath for 10 minutes to ensure complete reduction. J After the solution has cooled to room temperature, 8.06 ml. (7.91 g. 0.100 mol) of pyridine (density, 0.892) dissolved in 3.33 ml. 

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