Equilibria between Tri- and Dihydroxo-Bridged Complexes

Equilibria between Tri- and Dihydroxo-Bridged Complexes A. Cobalt(III) 

The kinetics of the acid cleavage of the trihydroxobridged complexes with the tridentate amines dien and tach have also been studied (186). These systems are very similar to the ammonia system, and the kinetic and thermodynamic data are therefore likely to be interpretable in the same way (Table XXXV). The cleavage of the NH3, dien, and tach complexes in chloride media has also been studied (186). These reactions were found to give dihydroxo-bridged species in two 

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation 

Bridge Cleavage of L3Co(OH)3CoL33+ Species at20°C in 1.0 M(Li,H)C104 

Only a few trihydroxo-bridged chromium(III) species are known. The (tacd)Cr(OH)3Cr(tacd)3+ and the (metacn)Cr(OH)3Cr(metacn)3+ ions are unusually kinetically and thermodynamically stable in acidic solution, i.e., they do not undergo hydroxo-bridge cleavage in 1 M 

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