Aqua and hydroxo ligands

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. 

Crystal-field splitting and sensitized emission of Eu Co-ordination of aqua and hydroxo ligands to Eu Kinetics of the sensitized luminescence of Eu Involvement of the O M Imct triplet states in energy transfer... 

TABLE 4 Approximate co-ordination geometry (CG) around average (TC) of the total charge of ligands bound to Eu total number (r/) of aqua and hydroxo ligands bound to Eu, Do Fo energy (0—0 in cm ), and x (in ms) and of the emission upon 0—>M Imct band... 

The rate constant for the redox step, kr, is unlikely to reflect a simple electron transfer from the monodentate diimine ligand to the metal center because replacement of coordinated water with coordinated hydroxide would be expected to decrease the oxidizing power of the metal-(III) center. This is well documented by the standard reduction potentials of the aqua and hydroxo complexes in Table IV, and it would seem... 

Some small peptide-heme complexes have been prepared, including an undecapeptide (residues 11-21)668"669 and an octapeptide (residues 14-21). TTiese are useful models as they include the two cysteine residues that covalently link the heme to the peptide, and one of the axial ligands. The axial Met-80 residue is absent, but the position can be filled by methionine or by other ligands as required.670 Work with several octapeptide complexes shows that the rates of outer-sphere electron transfer appear to be independent of the axial ligand, and faster than the reaction for cytochrome c. Other comparisons show that the orientation of the axial methionine in cytochrome c and the contacts between heme and protein are important controlling factors in the electronic structure of the heme. Aqua and hydroxo complexes of iron(III) octapeptide complexes are also useful models for studying spin equilibria in iron(III) hemoproteins.671... 

OXYGEN-CONTAINING LIGANDS 46.6.1 Aqua and Hydroxo Complexes... 

There is only one binary halide, Os2Cl10, which can be classed as a complex in the sense that it has a discrete molecular structure, and we consider it on p. 615 below. In this section we consider unsubstituted halo complexes [OsX6]"-, mixed species [OsX Y6 ]x (X and Y both halides) and aqua and hydroxo halides. Other halo species substituted with other ligands are considered under the section dealing with the substituting ligand (e.g. oxo halo species on p. 584, nitrido halo complexes on p. 563, halo ammines on p. 529). 

On the basis of the fact that only the aqua (or hydroxo) ligand is substituted by monodentate ligands and the pH profiles (see also Figs. 19-21), the reaction for the substitution reactions of these complexes with monodentate ligands may be represented by Scheme 3. 

With respect to the linearity of uranyl, it is interesting to note that significantly bent 0=U=0 moieties (-160-170°) were predicted theoretically for the hydroxo complexes U02(0H)(H20) (n = 0,3-5), U02(0H)2, and dimeric (U02)2(0H)2(H20)n (n=0,6) [228-230], In the case of U02(0H)2(H20)3, bending of the uranyl unit was observed when two OH groups were not separated by an aqua ligand. This was rationalized [231] by a strong 7i-donation from OH to the uranium atom, which results in a competition between oxo and hydroxo ligands. 

If the ligand is an anion, add the suffix -o to the stem name bromo (Br), cyano (CN ),and hydroxo (OH). The simplest neutral ligands have special names aqua (H2O), ammine (NH3), and carbonyl (CO). Other neutral ligands retain their usual names. Some common ligands and their names appear in Table 20-3. 

S.4.4 PlatiiHim(II) Complexes with Oxygen-donor Ligands 6.5.4.4.1 Aqua, hydroxo, and oxo ligands... 

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