Ammine ligand loss,

A steady stream of nitrogen is passed through the solution, which is lowered into a silicone oil bath preheated to 100°. After the solution is heated for 2 hr, it is removed from the oil bath and allowed to cool to room temperature. It is then chilled in an ice bath. Care should be taken not to heat the solution for prolonged periods or at temperatures above 100°, since this may result in further loss of ammine ligands. Slow addition of diethyl ether (40 mL) with stirring precipitates the complex. 

Details of the photoaquation mechanism of [Rh(NH3)5Br] have been probed by the use of N labeling, at the ammonia ligand trans to the bromide. Photoaquation results mainly in the loss of this axial ammine ligand, though about 10% involves replacement of cis ammines. Photoisomerization of [Rh(NH3)4(OH2)Br] has also been studied. " The quantum yield for the cis trans isomerization of this type of complex increases twofold when the H2O... 

In an early study of the solid state thermal deprotonation of coordinated ammine ligands, Tsin-Shen and Tronev S showed that ReCl3 4NH3 (a compound of uncertain structure) was transformed into a diamido-complex by loss of HCl, Eq. 12.16 ... 

At high initial [Fe2+] in the absence of added substrates, the stoichiometric ratio [Fe3+]oo/[L(H20)Rh00H2+]0 approaches 2.0. At lower concentrations of Fe2+, the overall reaction produces less Fe2+ because some of the newly formed L(H20)Rh02+ decomposes, presumably by loss of NH3 from the ammine complex and intramolecular ligand oxidation in the macrocyclic compounds, as observed for similar complexes of high-valent nickel, cobalt, and iron (58,118-120). Competition experiments were carried out at sufficiently high [Fe2+] to ensure that no L(H20)Rh02 + was lost in self-decay. 

The Rh(III)- and lr(lll)-ammines undergo efficient photosubstitution in marked contrast to the Co(III) analogues as mentioned above. However, the efficient photosubstitution found for the second and third row metal systems does appear to obey the essential rationalizations outlined in the models for LF substitutional reactivity. Two sets of complexes seem to behave quite nicely in this regard C4v, Rh(NH3)s X2+ 58) and DAh trans-M(en)2Xl (M = Ir, Rh X = Cl, Br, I).59,6°) In both sets of complexes the lowest excited state is likely one which features population of the d22(a ) orbital. Consistent with this fact, the photosubstitution chemistry can be viewed as resulting from loss of a ligand on the z-axis. For the Rh(III)-amines a dissociative mechanism is suggested for Cl photoaquation by the quantum yield data in Table 8.60) The... 

The first quantitative photochemical study of a Rh111 amine was reported by Moggi,8 who noted that both 254 nm (LMCT) and 365 nm (ligand field) excitation of [Rh(NH3)5Cl]2+ caused chloride labilization (equation 131). Other early reports include Basolo s study of the photoinduced stereo-retentive halide aquation from [M(en)2X2]+ (M = Rh, Ir X = Cl, Br, I), and Broomhead s observation of chloride aquation from [RhCl2(phen)2]+.726 While halide labilization dominates upon photolysis of [Rh(NH3)5Cl]2+, both bromo and ammine loss occur upon photolysis of the bromo analog (equation 132)685,707 and ammine is labilized from the iodo analog (equation 133).70 Biacetyl sensitization of the bromo complex quenches the biacetyl phosphorescence, but not the fluorescence,707 consistent with a photoreactive triplet state. 

Aquation of [Co(RNHa)5H20] +ions (R = Me, Et, Pr", Bu , or Bu ) is reported to proceed with loss of one amine ligand followed by spontaneous reduction to the cobalt(ii) state. The instability of the [Co(RNH2)4(H20)2] ion differs from that found for the analogous ammine complexes, since reduction to cobalt(n) only occurs for the [Co(NH3)ra(H20)e- ] ions when <3. ... 

Activation and reaction volumes (Table 8.4) and pressure dependences of quantum yields for isomerization and substitution reactions of cis- and frani-[Rh(NH3)4XY]", with X and Y variously from Cl, Br, OH2, are, as for [Rh(NH3)5X] complexes (Table 8.3), consistent with dissociative activation. The excited state is square-pyramidal [Rh(NH3)4X]" with X apical. Quantum yields have been determined for (stereoretentive) photoaquation of a series of complexes trans-[RhL4Cl2], where L = a heterocyclic amine, such as pyrazine (8) or a picoline. The relative quantum yields for chloride loss and for heterocyclic ligand (L) loss vary with the nature of L, with, a marked correlation with ligand pKa values. Relatively little of the aquation goes by chloride loss here, in contrast to ammine analogues. ... 

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