Aquo ligand

Similarly, analogous Rh(III) hydrolytic cations can be isolated by crystallisation with the super-anion, however these complexes show quite different hydrogen bonding networks. A second type of Russian Doll super-anion is found in the complex [Rh4(OH)6(H2O)12](20.21+8H). 33H2O [46]. The dehydrated Russian Doll Na([18]crown-6)(19)2 7 21 is skewed by comparison with the capsule structure of 20. The two trans aquo ligands are not present in the dehydrated super-anion, and the axial Na coordination positions are occupied by oxygens... 

A ferris wheel assembly involving a 1 1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule [La(H20)3([ 18]crown-6)] (19+2H) + [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18] crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. 

A robust and highly active catalyst for water-phase, acid-catalyzed THs of carbonyl compounds at pH 2.0-3.0 at 70 °C was disclosed by Ogo and coworkers [60]. The water-soluble hydride complex [Cp lr(bipy)H] (72, Cp = Tl -CsMes, bipy = 2,2 -bipyridine) was synthesized from the reaction of [Cp lr(bipy)(H20)] (73) with HCOOX (X = H or Na) in H2O under controlled pH conditions (2.0 < pH < 6.0, 25 °C). The pH control is pivotal in avoiding protonation of the hydrido ligand of 72 below pH ca. 1.0 and deprotonation of the aquo ligand of 73 above pH ca. 6.0. The rate of the reaction is heavily dependent on the pH of the solution, the reaction temperature, and the concentration of HCOOH. High TOFs of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0, ranging from 150 to 525 h, were observed for a variety of linear and cyclic ketones, as summarized in Table 4.5. 

Polynuclear anion-hridged complexes. The structure of [(NH3)3(H20)Co-(p-OH)2Co(H20XNH3)3](N03), 2H20 has been reported, although it was not possible to distinguish between the ammine and aquo ligands. When [Co(acac)2] in methanol, containing KOAc, is treated with H2O at 25°C for 1 h, [(acac)2Co(p-OH)2Co(acac)2] is formed. The structure of rac-... 

Re(CO)5Br reacts with pentane-2,4-dione in the presence of KOH to form the complex [Re2(C0)6(acac)2(H20)2], which appears to contain p-aquo-ligands and a Re—Re bond.653... 

Alternatively, it is well known (74) that oxo group removal coupled with reduction is facilitated by the presence of acid. Protonation of oxo groups produces hydroxo or aquo ligands which are superior leaving groups. The hydrolysis of ATP near pH 7 produces protons, and this hydrolysis may be enzymically controlled to produce these protons near the oxo group which would facilitate the coupled reduction and oxo removal on the molybdenum site. In either case, the role of ATP would appear to be the production of an open site on molybdenum and in some model systems (104,105), ATP may also be serving this function. Further experimental elaboration of the ATP reactions is desirable in both enzyme and model systems. 

Methyl trioxorhenium (MTO) has proven to be an efficient catalyst in the presence of hydrogen peroxide, which leads to the formation of mono- and bisperoxo compounds an additional aquo ligand has been found to stabilize the latter complex. MTO as well as the corresponding monoperoxo- and bisperoxo complexes have been studied, in both their free and monohydrated forms.96... 

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