Aqua ligands, deprotonation

Metal-oxo complexes are effective oxidizing agents via both hydrogen abstraction and 0x0 transfer (46,119). Meyer etal. have developed a series of 0x0 complexes that can be isolated in the oxidized form (120). These complexes are prepared by changing one pyridyl nitrogen from a tris-chelate type complex to a water molecule, as in Ru(tpy)(bpy) 0H2 or Ru(bpy)2(py)OH2. Upon oxidation at moderate potentials, the coordinated aqua ligands deprotonate to provide successive hydroxo and 0x0 complexes ... 

Conversely, the stabilization of Os(IV) by 1 V per deprotonation in [Os(en)3]n+ (204,205) and the stabilization of higher oxidation states by deprotonation of aqua ligands (325) illustrate well the large effects brought about by it donation. 

To build the Mn4Ca cluster from its free metal ions, it is necessary to deprotonate four or more aqua ligands attached to the initially bound Mn2+ [29]. 

The deprotonation of the aqua ligands is primarily responsible for stabilizing the high oxidation state at pH 7 the relevant potentials... 

Two proposals for the mechanism of YIP are quite similar (34,95), both include metal ion binding to pyrophosphate to produce a substrate, both have proposed a metal bound hydroxo ligand as a nucleophile (the hydroxo ligand is produced by deprotonation of an aqua ligand by an adjacent basic amino acid residue), both involve stabilization of the substrate by interaction with the protonated arginine residue. The mechanisms differ in the spatial arrangement of the substrate, the various metal ions, and essential amino acid residues. 

Na2[V0(pida)(H20)]-3.5H20 (pida = ((phosphonomethyl)imino)diacetate), which undergoes protonation at the phosphonate oxygen with pXa, = 3,8 and deprotonation from the coordinated water with = 8.4. All kinetic parameters were considered consistent with a hydrogen-bonded aqua ligand in [VO(pida)(H20)] having an intermediate character between aqua and hydroxo ligands. 

Vibrational spectra are sensitive to qualities of metal complexes, such as changes in the mass of the ligands, that are key to understanding the mechanism of the OEC. For example, the change in mass of an aqua ligand to Mn that deprotonates to form hydroxide can be detected by vibrational... 

The compound [Cp Ir(H20)3] (Cp = ri -CjMej) was found to be an efEcient catalyst precursor for the hydrogenation of water-soluble compounds [25]. Such organometallic aqua complexes are very interesting, since their properties change drastically with variations of the solution pH, due to deprotonation of the aqua ligands [26]. The active catalyst was foimd to be a binuclear i-hydride complex [(Cp Ir)2(p-H)(ii-OH)( i-HCOO)] (5) that could be isolated and characterized. 

As O/OH groups are formally derived from deprotonation of an aqua ligand, which is greatly enhanced by coordination with the Lewis acidic lanthanide ion, conducting lanthanide coordination chemistry in aqueous solutions at low pH (often below 4) is a common practice in order not to produce precipitates of lanthanide oxide/hydroxides due to extensive hydrolysis of the metal ion. This concern is easily understandable, and analogous chemistry is well established in the precipitation of gel-like oxide/hydroxides of Fe and Al when salts... 

Tanaka and coworkers (777) proposed that the ruthenium oxyl radical complex [Ru (0 )(SQ)]° was prepared by the double deprotonation of the aqua ligand of [Ru (OH2)(SQ)] through [Ru (OH)(SQ)] as shown in Scheme 2 with revised assignments (116) by the first two sets of arrows. 

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