Aqua-bridged hydroxo complexes

ApoSOD, 45 186, 190 Apo VFe protein, 47 204 APS reductase, see Adenylylsulfate reductase Aqua-bridged hydroxo complexes acid-base equilibria, 32 110 of dihydroxo-bridged species, 32 114-115... 

There has been a continuing discussion without resolutions of whether the aqua group acts as a (weak) bridge in the situations where the corresponding hydroxo complex reacts inner-sphere. 

There are several examples of well-characterized tri- and tetranu-clear hydroxo-bridged complexes of chromium(III) and cobalt(III). Penta- and hexanuclear aqua chromium(III) complexes have been prepared in solution, but their structure and properties are unknown. Oligomers of nuclearity higher than four have not been reported for cobalt(IIl), with the exception of some hetero-bridged heteronuclear species (193, 194). There appear to be no reports of rhodium(III) or iridium(III) complexes of nuclearity higher than two. 

Fig. 11. Intermolecular hydrogen-bonded pairs of aqua hydroxo complexes have been found in several crystal structures and are probably always present in those salts of the type cis-[ML4(H20)(0H)lX2, which yield dinuclear dihydroxo-bridged complexes upon heating in the solid state.

The [H + ] dependence of kobs has been interpreted in terms of an acid-catalyzed and an uncatalyzed reaction path, as shown in Scheme 6. The acid-catalyzed path has been proposed to involve protonation of one hydroxo bridge to give a labile aqua-bridged intermediate. The aqua-bridged complexes have in no case been identified, but spectroscopic results indicate that such species are very strong acids with Ka3 1. The kinetic parameters calculated from the rate expression, Eq. (55), are... 

---