Hydroxo-bridged species

The oxide (p. 1209), chalcogenides (p. 1210) and halides (p. 1211) have already been described. Of them, the only ionic compound is HgF2 but other compounds in which there is appreciable charge separation are the hydrated salts of strong oxoacids, e.g. the nitrate, perchlorate, and sulfate. In aqueous solution such salts are extensively hydrolysed (HgO is only very weakly basic) and they require acidification to prevent the formation of polynuclear hydroxo-bridged species or the precipitation of basic salts such as Hg(OH)(N03) which contains infinite zigzag chains ... 

Bimetallic zinc complexes formed with hexaazamacrocycles were studied in the hydrolysis of activated carboxyesters. Potentiometric titration demonstrated the dominant presence of a dinuclear hydroxo bridged species at pH >7. /)-Nitrophenyl acetate is hydrolyzed with no loss of catalytic activity for at least 2.7 catalytic cycles 4... 

At higher temperatures the monomer is the predominant species although the rate of hydrolysis to U03 is increased. U03 dissolves in uranyl solutions to give U02OH+ and polymerised hydroxo-bridged species. Polynuclear species could arise from U4+ as it hydrolyses in dilute acid solutions. Complex ions are formed with thiocyanate, phosphate, citrate and anions of other organic acids. 

Details of Visible Absorption Spectra for Oligomeric Cr(III) Hydroxo Bridged Species (70)... 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt.17 The complexes obtained as hydroxides, Mm(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, MIU(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1 % acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The Mnl(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10 4 M concentration. The dimer is considered to be a di-p-hydroxo-bridged species, different from the p-oxo dimer, Scin(OEP) 20 (Scheme 6). 

The inert hydroxo-bridged species were also a product of (very fast) hydrolysis of p-cymene osmium complexes with glycinate, L-alaninate, a-aminobutyrate and p-alaninate. However, complexes with picolinate as the chelating ligand, [Os(r 6-/> cym)Cl(pic)] 8 and [Os(r 6-biph)Cl(pic)] 9, with pyridine as /V-donor and carboxy-late as O-donor, hydrolyzed with half-lives of 0.20 and 0.52 h (298 K), and aqua adduct pKa values (pk L value for solutions in D20) of 6.67 and 6.33, respectively. Complexes 8 and 9 were cytotoxic towards A2780 human ovarian cancer cells, with IC50 values of 8 and 4.2 pM, respectively [64],... 

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. 

We are faced, therefore, with the puzzling problem of why it is not possible to isolate any aquo-species and why all hydroxo-species are polymerized through hydroxide bridges. It is accepted that aquo-species exist in solution Raman spectroscopy (Figure 8) shows that the coordinated nitrate groups are replaced immediately on solution in water (32), and n.m.r. studies have shown that these aquo-species are more slowly converted into the polymerised dimer and trimer hydroxo-bridged species(30). 

Thus we can understand that, while aquo-complexes of platinum(ll) exist in solution, it will be difficult to isolate them as solids unless a strong hydrogen-bonding species is used as a counter-ion. Further, monomeric hydroxo-species formed by the loss of a proton from the aquo-species will be thermodynamically unstable and will tend to oligomerize to hydroxo-bridged species. 

Nevertheless at the Cu(II) state, in the presence of base, deprotonation of the exo-bound water molecule occurs and leads to the formation of polynuclear hydroxo bridged species (Figure 7). This results from the combined effect of host flexibility, basicity of the exo site, and reluctance of the complex to bind anions in endo position (see below). 

Catalysis by chromium(n) involves the formation of a Cr —OH—Cr hydroxo-bridged species. Some preliminary data are also reported for the analogous nicotinamide system. A similar study involves the decomposition of a uranium(v)-chromium(ra) dinuclem complex in the presence of excess chromium(n). Approximate rate data for the uncatalysed reactions are shown in Table 12. 

Oxidation of mandelic acid by H2O2, catalyzed by Fe ", is only 50% inhibited by the presence of OH radical traps and formation of an active Fe(IV) species may be a possibility. However it is thought more likely that the uninhibited pathway involves cage reaction of newly formed OH radicals. Peroxytungstic acids are formed in the H2O2 oxidation of dmso catalyzed by WO4 . An extensive study of the oxidation of cobalt(II) by m-Cl-perbenzoic acid in 90% acetic acid media has appeared. The mechanism is complex involving a second-order dependence on [Co(II)] and various dimeric and trimeric 0x0- and hydroxo-bridged species, are formed in the course of the reaction. 

Bridged Dicobalt Complexes.—Rate constants for the forward and reverse reactions of aquation and chloride anation for the equilibrium between (7) and (8) have been determined. The amido-bridged chloro-aquo-complex (8) can Equate further to give the amido-hydroxo-bridged species (9), which... 

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