Dihydroxo isomers

The dihydroxo-bridged binuclear complexes are among the most common, structurally well-characterized polynuclear complexes. For symmetrical bidentate nonbridging ligands, LL, the cation (LL)2M(OH)2M(LL)2"+ may exist in three isomeric forms a meso isomer (A,A) with configuration A and A, respectively, at the two metal centers, and a racemic pair of isomers (A,A and A, A) with configuration A or A at each of the two metal centers (cf. Fig. 2). If the bidentate... 

The yields of the dihydroxo-bridged complexes vary and are often relatively low, probably owing to the formation of different isomers and higher polynuclear complexes and to hydrolysis of the ligand L. For many of the complexes listed above, the preparative procedures involved the preparation of the mononuclear species in situ. 

Hydrolysis of ML3(H20)33 + by reaction Eq. (1) gives dihydroxo-bridged species which may exist in several isomeric forms. For (L)3 = (NH3)3 there are five possible isomers if only facial coordination is considered then the number of isomers is reduced to two, the so-called cis and trans isomers (see Fig. 9). The ammonia, tame, and tacn chromium(III) complexes mentioned previously have all been isolated as their trans isomers. Similarly it has been found that... 

The cis/trans isomerization reaction, Eq. (24), has been applied in the preparation of salts of the cis isomers of the chromium(III) complexes with L3 = (NH3)3 or tacn (319). For these species Eq. (24) equilibrium is shifted to the right, while the corresponding equilibria with the diaqua or dihydroxo species, respectively, are shifted to the left (Table X). The increased stability of the cis aqua hydroxo species can be explained in terms of intramolecular hydrogen bond formations (Section VI,C). As mentioned above, the corresponding cobalt(III) and rhodium(III) complexes have been isolated as salts only in the case of the trans-(H20)L3M(0H)2ML3(H20)4+ cations, but it seems very probable that their cis isomers could be prepared by reaction Eq. (24). 

Fig. 1 shows the distribution curves for cis-(NH3)2Ptn at 4 mM chloride-ion concentration representative of that in the cell nucleus. Fig. 2 shows the analogous plot for the trans-isomer. For both isomers the three dominant species at pH 7.4 are the chloro-hydroxo, dihydroxo, and reactive hy-droxo-aqua complexes. 

The base hydrolysis of D -[Co(en)2Cl(OH)] produces mainly cis product with essentially complete (> 90 %) retention of configuration. The results of the D -[Co(en)2Cl2] base hydrolysis reaction indicate that the D -[Co(en)2Cl(OH)] ion predominates over the corresponding l optical isomer at all concentrations, while the reverse is true for the dihydroxo enantiomers—under conditions which give dihydroxo species to a greater extent than predicted by rate data in the literature. Some L -[Co(en2-(0H)2] and trflns-[Co(en)2(OH)2] appear to be formed above that predicted by the individual reaction sequences deduced from kinetics measurements in dilute solution. This effect had been noted earlier by Pearson, Meeker, and Basolo, but was later discounted by Chan and Tobe, who did not observe this phenomenon when hydroxide ion is slowly added at 0°. The results (Table 1) agree with both observations. 

---