Hydroxo-bridged complexes species

Formation of hydroxo-bridged complexes by hydrolysis in aqueous solution is, not surprisingly, the most common preparative method. As a rule, such reactions give quite complex product mixtures containing species with different nuclearities, each of which may be present in many isomeric forms. The fact that most of the preparative procedures employed lead to the isolation of one single and pure isomer probably more often reflects favorable solubility properties rather than stereospecificity. In some cases ion-exchange chromatography has been used to isolate the polynuclear species, but systematic analysis of hydrolysis mixtures by this technique has been reported for only a few systems. 

There are several examples of well-characterized tri- and tetranu-clear hydroxo-bridged complexes of chromium(III) and cobalt(III). Penta- and hexanuclear aqua chromium(III) complexes have been prepared in solution, but their structure and properties are unknown. Oligomers of nuclearity higher than four have not been reported for cobalt(IIl), with the exception of some hetero-bridged heteronuclear species (193, 194). There appear to be no reports of rhodium(III) or iridium(III) complexes of nuclearity higher than two. 

Analysis of the products of these cleavage reactions has often served as proof of the structures of the polynuclear species. Cleavage of hydroxo-bridged complexes of nuclearity higher than two will in most cases yield at least two different mononuclear species. Identification of these species and determination of the relative ratio in which they are formed reduce the number of possible bridged skeletons greatly, and the studies of polynuclear ammine and amine chromium(III) made by Andersen et al. (mentioned in Section IV) provide many examples of this, one of which is shown in Eq. (48) above (see also Section II,A). 

In the presence of methanol, the oxo-bridged species is converted to methoxo and/or hydroxo bridged complexes according to the scheme (for n = 2) ... 

It is interesting that Ir-OH exhibits stable catalysis (TON of >300 observed) and is not deactivated by the irreversible formation of bridging hydroxo species (assuming thermodynamic control) [29] that is common for metal hydroxo complexes. Our attempts at synthesis of these bridging hydroxo complexes have thus far been unsuccessful. Calculations show that the formation of a //-hydroxo bridged complex from two molecules of Ir-OH is not very favorable (AGrxn= -2 kcal/mol) and may serve to explain the catalytic stabihty of Ir-OH. 

The hydroxo complexes [Au(6-Rbipy)(OH)2], postulated as intermediates in the formation of 44c-i, are not isolated nevertheless, in all the mass spectra (FAB conditions) a weak peak is found corresponding to these species. Unchanged 44c-i are quantitatively recovered from the reaction with aqueous solutions of HX (X = BF4 or PFfi) neither dihydroxo complexes similar to those observed in the vapor phase, nor hydroxo bridged dimers are obtained [25]. 

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. 

The complexes [LCo(p-02)(p-OH)CoL] [L = en, trien, dien, tetra-ethylenepentamine, or tris-(2-aminoethyl)amine] have been studied, and the new complexes [[Co(imidazole)(gly)2 202],4H20 [ Co2(imidazole)2-(gly)402 0H],3H20, and [Co(imidazole)(gly)2(02)H20] have been prepared The spectroscopic properties of various p-peroxo- and p-superoxo-cobalt(iii) complexes have been examined. The singly-bridged p-peroxo-compounds have a strong band at 300 nm, whereas this falls at 350 nm for p-peroxo-p-hydroxo-complexes and two peaks at 480 and 700 nm are observed for p-superoxo-species. The i.r. spectra of p-peroxo-bridged complexes of cobalt(iii)-cyclam have been reported. ... 

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