Cobalt hydroxo-bridged complexes

HYDROXO-BRIDGED COMPLEXES OF CHROMIUM(III), COBALT(III), RHODIUM(III), AND IRIDIUM(III)... 

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. 

Hydroxo-Bridged Complexes of Chromium(III), Cobalt(III), Rhodium(III), and Iridium(III) Johan Springborg... 

The electronic spectra of bridged cobalt(III) complexes have been reported by Ohyagi (18) and by Inamura and Kondo (13), Ohyagi determined the visible and ultraviolet spectra of a large number of Werner s complexes but used relatively primitive (by modern standards) equipment. Inamura and Kondo examined the hydroxo-bridged complexes exclusively. 

Springborg J (1988) Hydroxo-bridged complexes of chromium(III), cobalt(III), rhodium(III) and iridium(III). Adv Inorg Chem 32 55-169... 

Hydroxo-Bridged Complexes of ChromiumdIIl, Cobalt(II), Rhodium(IIl), and Iridium(lII)... 

The di- i,-hydroxo bridged complexes, the preparations of which are described in Sections D and H, have several chemical and physical properties in common. The dithionates are only sparingly soluble in water, whereas the bromides are quite soluble. The dinuclear structures of the cations, as well as the A,A configuration of the 1,2-ethanediamine compounds have been established by the close similarity of the Guinier X-ray powder diffraction patterns of [(NH3)4Rh(0H)2Rh(NH3)JBr4-4H20 and [(en)2Rh(OH)2Rh(en)2] (8205)2 to those of the well-characterized cobalt(III) and chromium(III) analogs. 

It has been noted (84) that U-dioxygen (type 1) complexes formed in solution often contain a u-hydroxo bridge, in addition to the /i-dioxygen bridge, provided that each cobalt atom has a vacant coordination site. Fig. 4 illustrates the dibridged M-(02, OH)—M moitey... 

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. ... 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

In this section the methods which have been used to gain structural information are briefly summarized. The term structure is used in this context in its broadest sense, including more qualitative observations concerning the skeleton of the bridging atoms. As a general rule, the hydroxo-bridged polynuclear complexes of chromium(III) and cobalt(III) can be isolated as well-defined crystalline salts and it is therefore quite natural that single-crystal X-ray structure analysis has... 

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation... 

Fig. 13. Formation of a (x-peroxo-bridged complex from a nitrogen ligand complex of cobalt(II) and O2. The p,-peroxo-bridged complex is stabilized in the presence of a second bridging ligand, L, such as amino or hydroxo.

The complex ions containing only hydroxo bridges (III and VI) are of interest because these are the only polynuclear cobalt (III) species which can be prepared directly from mononuclear cobalt (III) complexes. These ions are prepared by removing two moles of water from two moles of solid starting material—e.g., the sulfate of III is obtained by heating hydroxo-aquotetramminecobalt(III) sulfate (30) ... 

Chem. See. 1952, HO by oxidation of solns of cobaltous salts by alkaline persulfates in the presence of acetic acid Peschanski, Wormser, Compt. Rend. 252, 1607 (1961). Generally considered to be a hydroxo -bridged binuclear complex. 

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