Chromium hydroxo-bridged complexes dinuclear

The di- i,-hydroxo bridged complexes, the preparations of which are described in Sections D and H, have several chemical and physical properties in common. The dithionates are only sparingly soluble in water, whereas the bromides are quite soluble. The dinuclear structures of the cations, as well as the A,A configuration of the 1,2-ethanediamine compounds have been established by the close similarity of the Guinier X-ray powder diffraction patterns of [(NH3)4Rh(0H)2Rh(NH3)JBr4-4H20 and [(en)2Rh(OH)2Rh(en)2] (8205)2 to those of the well-characterized cobalt(III) and chromium(III) analogs. 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

The inertness of the dinuclear complexes is greatest in slightly acidic solutions, which therefore have been employed for the reprecipitation reactions. Apparently the chromium systems are much more labile toward bridge breaking than are the cobalt systems. In aqueous solution the meso-[(en)2Cr(OH)2Ci(en)2] cation (I) enters into a rapidly established (t 1 min. at room temperature) equilibrium with the mono-ju-hydroxo complex [(OHXen)2Cr(OH)Cr(en)2-(HaO)] (n) The equilibrium constant K = [II]/[I] is 0.83 in 1 Af NaC104 at 0°. The salts (dithionate, bromide, chloride, and perchlorate) of the di-p-hydroxo cation are less soluble than the respective salts of the mono-/i-hydroxo cation. It is therefore possible to precipitate the pure salts of the di-/i-hydroxo cation from the equilibrium mixture following the procedure given above. 

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