Aqua halo

In addition to the aqua ion, a range of mixed aquo-halo complexes are known [38], including all 10 isomers of Rh(H20)6 VC1(X3 x)+. Synthetic entry into the series is possible from either end, the determining factor being the labilizing effect of chloride ... 

Os(V) complexes are relatively rare and generally unstable toward disproportionation or other reactions. Authentic complexes that have been isolated include [OsFs]4, [OS2CI10], salts of [OsX6] (X = F- or Cl-), and mixed halo/aqua or halo/oxo complexes, all of which are octahedral (2, 5, 6). Recently, the tetrahedral [Os(2-MeC6H4)4JT and five-coordinate [0s(0)(ehba)2]- complexes have been isolated and characterized (23). 

Oxygen-containing solvents such as water, alcohols or ethers are such poor donors that few complexes with palladium(II) have been isolated. The most important class of complexes of this type consists of those containing water, which are formed as intermediates in the substitution reactions of palladium(II) when carried out in aqueous solution. In these reactions their formation is in competition with the second order reaction of the complex with the incoming ligand. The aqua complexes can also be formed by reaction of halo complexes with silver salts (e.g. N03, C104, BF4) in water. These complexes are acidic, being in equilibrium with hydroxo complexes in neutral or basic media. 

In this section we consider unsubstituted complexes first, then aqua, hydroxo, halo, carbonyl and phosphine ammines. For nitrido ammines see pp. 562, 563, nitrosylammines p. 546, and dinitrogen ammines p. 554. Other substituted ammines are dealt with under the section concerned with the substituting ligand. 

There is only one binary halide, Os2Cl10, which can be classed as a complex in the sense that it has a discrete molecular structure, and we consider it on p. 615 below. In this section we consider unsubstituted halo complexes [OsX6]"-, mixed species [OsX Y6 ]x (X and Y both halides) and aqua and hydroxo halides. Other halo species substituted with other ligands are considered under the section dealing with the substituting ligand (e.g. oxo halo species on p. 584, nitrido halo complexes on p. 563, halo ammines on p. 529). 

Aqueous Chemistry. Aqua ions of low and medium valence states are not in general well defined or important for any of the heavier transition elements, and some, such as Zr, Hf, Nb, and Ta, do not seem to form simple cationic complexes. For most of them anionic oxo and halo complexes play a major role in their aqueous chemistry although some, such as Ru, Rh, Pd, and Pt, do form important cationic complexes as well. 

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. 

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. 

Substantial accelerations have been observed for aquation of halo-, thiocyanato-, azido-, and diacido-metal complexes, particularly in the presence of aqua metal ions such as Hg and Ag+ (17,20, 56, 77, 221, 286). Both the leaving groups themselves and their complexes form strong associations with such added metal ions. For associations such as that in Eq. (13), the equilibrium constant can be readily measured spectrophotometrically (171). 

Similar procedures may be used for other complexes containing halo, pseudohalo, acetato, carbonate, aqua, and many other ligands that are either relatively labile or are decomposed by strong acid. 

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