Rhodium , ammine complexes

The Mononuclear Carbonyls When exchanged into the zeolite as Rh(NH )jCll-z+, the rhodium ammine complex could decompose in an oxygen stream at temperatures ranging 150-350°C into a rhodium Ill-hydroxy-species with a partial reduction into diamagnetic Rh(I) and paramagnetic (less than 10%) Rhll species. 

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 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-... 

The complexes generated by photolysis or radiolysis of rhodium(lll) precursors usually have very short lifetimes. The ammine complexes derived from radiolysis of aqueous solutions of [RhCl(NH3)5] + are easily oxidized. The radiolysis of solutions of [Rh(bipy)3] + generates the corresponding rhodium(II) complex, which undergoes disproportionation to rhodium(III) and rhodium(I) complexes, the latter containing only two bipy ligands (equation 27). 

A wide range of rhodium(III) complexes contain halo, aqua, ammine diaminoalkane, or dicarboxylato ligands. Rhodium(III) complexes whose net ionic charge varies from -1-3 to -3 are known. With complexes that react slowly, it is possible to isolate most intermediate complexes in the interconversions of [Rh(H20)6] + and [RhCle] " (Scheme 10). The slow reaction rates of these complexes also allow geometrcal isomers to be isolated. Additionally, the cfr-[RhX2(LL)2]+ and [Rh(LL)3] + complexes containing bidentate ligands are chiral and may be resolved into their optical isomers. 

A second difference between the two is the behaviour when the catalysts are fired in air. Claus salt initially decomposes to rhodium metal but in the presence of air is converted to the oxide which sinters rapidly. Thus a worse dispersion of rhodium is observed when Claus salt is fired in air than when it is fired in nitrogen or hydrogen/nitrogen. In the case of rhodium chloride a superior overall rhodium dispersion is achieved and air firing is not so detrimental to dispersion as it is for the ammine complex. These observations can again be explained in terms of the decomposition chemistry of the precursor. Newkirk and McKee (ref. 51) have studied the decomposition of rhodium chloride, both unsupported and supported on alumina,... 

Rhodium(lll) complexes Collaborative studies between van Eldik, Ford and coworkers have led to thorough parameterization of pressure effects on photosolvolysis of the rhodium(III) halopentaammines Rh(NH3)sX + (Eq. 6.18) [39-45]. For these systems LF excitation is followed by rapid intersystem crossing (cDisc 1) to the lowest energy LF state E from which reactive (kp), radiative (k,) and non-radiative (k ) deactivation occur competitively (Fig. 6.10) [41, 46]. Rate constants for individual excited state processes were calculated from phosphorescence quantum yields fl>r, lifetimes r and quantum yields for halide ([Pg.198]

Rhodium(III) complexes typically contain ammine, 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 (111) 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 (111) complexes and even less on the higher oxidation states. 

The present series of syntheses is timely in that currently there is considerable interest in the thermal and the photochemical reactions of mono- and dinuclear rhodium(III) ammine and amine complexes.The dinuclear complexes have also proved to be suitable starting materials for the synthesis of new types of dinuclear rhodium(III) complexes with other bridging ligands. ... 

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