Hydroxo-bridged complexes bridging water

Adventitious water is responsible for the formation of the dimeric hydroxo complex 31 obtained by reaction of AuCb with 1,4-dilithiotetraphenylbutadiene in ether solution [97[. The hydroxo-bridged complex [Au(C6H4N02-2)2( i-0H)[2 (32) was obtained either by reaction of Na[Au(C6H4N02-2)2(0Ph)2] with traces of water in CH2Cl2/n-hexane solution or by treatment of the dichloroaurated complex with NaOH [98[. The crystal structure of 32 2Et20 shows that it is a centrosymmetric... 

Treatment of the cyclometallated complexes [Au(N,N,C)Cl][PF6] [N,N,CH = 6-methylbenzyl- (a) or 6-(l,l-dimethylbenzyl)-2,2 -bipyridine (b)] [20] with KOH or Ag20 in aqueous media affords the hydroxo complexes [Au(N,N,C)(OH)][PF6] (36) in fairly good yields [45b, 101] these are air-stable white solids, quite soluble in water and in many organic solvents. When refluxed in anhydrous THF they condense to give the oxo-bridged complexes [Au2(N,N,C)2( J--0)] (37) (Equation 2.10 in Scheme 2.5) which, in turn, can be obtained by a different route [102] (see Section 3.2) the reaction can be reversed by refluxing the 0x0 complex in water. 

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... 

The thermodynamic stability of the binuclear site has been demonstrated by the spontaneous assembly of [Fe20(02CR)2L2] (13) from ferric salts in the presence of water, an alkyl carboxylate salt, and a tridentate nitrogen donor ligand L that can cap an octahedral face on iron (8). Suitable ligands include tris(pyrazolyl)borates and 1,4,7-triazacyclononanes. Structure (13) is in essence a portion of the basic ferric acetate structure. The complexes are excellent physical and structural models of the diiron sites and model some aspects of reactivity including redox activity and interconversion of the oxo and hydroxo bridge. 

An analysis of the RDF for the acid solution shows that the In3+ ion is bonded to six water molecules at 2.17 A (Fig. 28). The same In—H20 distance is found for octahedrally coordinated In3+ in crystal structures (223, 224). According to the difference curve this coordination is not changed by the hydrolysis. A possible model for a tetranuclear complex with four octahedrally coordinated In atoms occupying the four corners of a regular tetrahedron and joined by single hydroxo bridges is shown in Fig. 28. It is consistent with the experimental data and seems to be a likely model for the hydrolysis complexes formed in solution, but has not yet been found in crystal structures. 

The complex ions containing only hydroxo bridges (III and VI) are of interest because these are the only polynuclear cobalt (III) species which can be prepared directly from mononuclear cobalt (III) complexes. These ions are prepared by removing two moles of water from two moles of solid starting material—e.g., the sulfate of III is obtained by heating hydroxo-aquotetramminecobalt(III) sulfate (30) ... 

A water molecule in these aquo-complexes can be replaced by [Al(OH) (H20)5]2+ + SO42- [Al(OH) (SO4) (HgO),] + HjjO The passage from solution to sol probably occurs by hydroxo-bridging... 

Aqua ions are prototypes illustrating the solution properties of complexes in a particular oxidation state. The properties displayed generally reflect size, charge, and effects such as crystal field splitting, as well as the tendency to form polynuclear oxo/hydroxo-bridged species, and metal-metal bonded species. Residence times of water ligands on aqua ions across the periodic table cover a remarkable 20 orders of magnitude from the most labile (<1 ns) to the most inert (>300 years), which is an important feature to understand. 

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