Rhodium , oxalato complexes

Rhodium(III) complexes typically contain anunine, halo, or aqua ligands, or the important bidentate ligands 1,2-diaminoethane (en), oxalato, or pentane-2,4-dionato (acac) and are invariably octahedral. Their wide variety is in part a reflection of the slow reactions, which take place at the low-spin d centers, which allow many intermediates, and geometrical or chiral isomers, to be isolated. It is fairly difficult to oxidize rhodium(III) complexes, but they may be reduced to rhodium(l) species in the presence of suitable ligands. However, there is little current work being carried out on the classical rhodium(III) complexes and even less on the higher oxidation states. 

Bidentate oxygen ligands form numerous rhodium(III) complexes. Several tris(jS-diketonato) complexes have been prepared from rhodium(III) nitrate (equation 35). The products are extraordinarily stable. They can be resolved into their optical isomers, and survive nitration and formylation reactions (equations 36 and 37). The tris(oxalato)rhodate(III) ion has also been resolved, but the enantiomers undergo slow racemization. Reaction of this complex with refluxing chloric(VII) acid leads to m-[Rh(ox)2(H20)2], which can be converted into cis- or frani -[RhX2(ox)2] complexes. 

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. 

After the resolution of 1-2-chloro-ammino-diethylenediamino-cobaltie chloride many analogous resolutions of optically active compounds of octahedral symmetry were carried out, and active isomers of substances containing central cobalt, chromium, platinum, rhodium, iron atoms are known. The asymmetry is not confined to ammines alone, but is found in salts of complex type for example, potassium tri-oxalato-chromium, [Cr(Ca04)3]K3, exists in two optically active forms. These forms were separated by Werner2 by means of the base strychnine. More than forty series of compounds possessing octahedral symmetry have been proved to exist in optically active forms, so that the spatial configuration for co-ordination number six is firmly established. 

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