Ammines rhodium

Like the ammines, rhodium complexes of ligands like bipy and phen have a significant photochemistry. Therefore, on irradiation, solutions of cw-[Rh(L-L)2X(H20)]2+ (X = halogen) gradually convert to c/s-[Rh(L-L)2X(H20)]2+, though much more slowly than with the ammines [101]. 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Figure 2.44 Syntheses and reactions of rhodium(III) ammine complexes.

Though thermally stable, rhodium ammines are light sensitive and irradiation of such a complex at the frequency of a ligand-field absorption band causes substitution reactions to occur (Figure 2.47) [97]. The charge-transfer transitions occur at much higher energy, so that redox reactions do not compete. 

Figure 2.47 The limiting photosubstitution mechanism for rhodium(III) ammine complexes. (Reprinted from Coord. Chem. Rev., 94, 151, 1989, with kind permission from Elsevier Science S.A., P.O. Box 564, 1001 Lausanne, Switzerland.)...

Figure 2.48 Quantum yields for the photolysis of rhodium(IIl) ammine complexes.

The iridium(III) complexes are broadly similar to the rhodium(III) ammines a selection of synthesis is shown in Figure 2.84. 

No structural studies have been reported on these complexes, but detailed study of their vibrational spectra permits the assignments shown in Table 2.13. Like the rhodium analogues, iridium ammines are photoactive therefore, on excitation of ligand-field bands, solutions of [Ir(NH3)6]3+ or [Ir(NH3)5Cl]+ afford [Ir(NH3)5(H20)]3+. 

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. 

All three metals of this group give rise to ammino-derivatives the compositions of which, however, differ considerably. The ammino-derivatives of ruthenium mostly contain a nitroso-group as well as ammonia the ammino-derivatives of rhodium salts closely resemble the cobalt -ammines and the ammino-derivatives of palladium salts correspond to the ammino-derivatives of platinum salts. 

Ammonia unites readily with iridium salts, giving rise to complex ammino-derivatives. The first compounds described appear to be ammines analogous to those of palladium and platinum, to which they were compared by Berzelius 8 and Skoblikoff.4 A further series were described by Claus 5 wliich he represented like those of ammino-rhodium salts, as they bore a marked resemblance to these. After Jorgensen had established the constitution of the ammines of rhodium, cobalt, and chromium salts, Palmaer gave similar constitution to the iridium compounds. 

Chapter XII. Metal-Ammines of Ruthenium, Rhodium, and Palladium. ... 

The difference in acid strength of the cis and trans isomers of (NH3)5Cr(0H)Cr(NH3)4(H20)5+ may be explained as an effect of the ligand trans to water. A comparison with the effect of trans ligand on the water ligand acidity observed for mononuclear ammine and amine complexes of chromium(III) or rhodium(III) (346) leads to the sequence Hi OH) < OH NH3 < H20 for increasing acidity due to the trans ligand. 

The observation that IfH(en) > Kh(NH3) is qualitatively supported by the enthalpy and entropy changes associated with the first acid dissociation equilibrium (Table XXII). Increased hydrogen bond stabilization should contribute negatively to both AH°(Kal) and AS°(Kal), which is consistent with the observation for both chromium(III) and rhodium(III) that it is the ammine complexes which have the highest AH°(Kal) and AS°(Kal) values. The greater acid strength of the ethylenediamine systems is then due to a decrease of AH0, which is greater than the decrease of AS0. The AH°(Kai) and param-... 

Because of the lability of the carbonyl group of the acylamides and the stability of the rhodium-ammine bond, the use of RhCl3 with benzonitrile as solvent might be suggested. The use of RhCl3 in glycole in the presence of formaldehyde also appears to be quite practicable [70]. 

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