Oxalic acid rhodium complex

The synthesis of the ds-tetraamminedichlororhodium(III) ion, using the reaction of tetraammine[oxalato]rhodium(III) perchlorate with boiling 6 M hydrochloric acid, was first reported in 1975. The oxalato complex is formed by the reaction of pentaamminechlororhodium(III) chloride with oxalate and oxalic acid in water at 120°. The synthesis has been repeated on numerous occasions in our laboratory, and we have found that only in a minority of cases is it possible to isolate [Rh(NH3)4(C204)]C104 H20 in sufficiently pure form to prepare the pure perchlorate by recrystallization. In a majority of cases the product that precipitates from the reaction mixture on addition of perchloric acid contains an appreciable quantity of an unidentified impurity that is not removed by recrystallization. The procedure given below permits the preparation of pure cis-[Rh(NH3)4Cl2]CH/2H20 from the latter crude, impure oxalato complex. 

Obtained by these methods rhodium trichloride is a red powder, insoluble in water and in acids. It is decomposed by concentrated aqueous potash, the resulting solution behaving like that of rhodium sesquioxides in the alkalies. It dissolves in concentrated solutions of potassium cyanide and of alkali oxalates, yielding complex cyanides and oxalates respectively. 

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