Cobalt aqua complexes

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

The reduction potentials indicate that the cobalt(iii) aqua complex is unstable with respect to the cobalt(ii) state, whereas the cobalt(iii) ammine complex is... 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

The cationic aqua complexes of the C2-symmetric trans-chelating tridentate ligand 447 proved also highly effective chiral catalysts. The complexes involving the metal(II) perchlorates of iron, cobalt, nickel, copper and zinc produced the main endo adduct of cyclopentadiene and N - aery loy 1-1,3 -oxazo I i din -2 -one with very high ee values281. 

In the reaction of a cobalt(III) aqua complex containing 0 and 0 with another reagent,... 

It seems to be a general feature that the acid-catalyzed path plays a more dominant role in the reactions of cobalt(III) and rhodium(III) than in those of chromium(III). A rationalization for this could be that the latter species do not readily protonate, in keeping with the observation that chromium(III) aqua complexes are significantly more acidic than cobalt(III) or rhodium(III) aqua complexes (Table XVIII). 

In the case of amino acid ester and amide complexes, the intramolecular hydrolysis reaction was not observed directly, but was deduced from the results of lsO tracer studies. However, recently the cis-hydroxo and cis-aqua complexes derived from the bis(ethylenediamine)cobalt(III) system, containing glycinamide, glycylglycine and isopropylglycylglycinate, have been isolated and their subsequent cyclization studied over the pH range 0-14.160,161... 

All of the carbonato cobalt(III) complexes reported here are reddish in color and extremely soluble in water. The rhodium complex is pale-yellow, whereas the iridium salt is virtually white they are both soluble in water. Treatment with dilute acid immediately gives the corresponding aqua complex with evolution of carbon dioxide. The characterization and the mechanistic details of acid hydrolysis of these complexes have been reported.3,4,11... 

Despite the vast number of cobalt(IT) complexes known, the dipositive oxidation state is rare for rhodium. The bulk of the rhodium(ri) complexes known are dimeric, the classic examples being the diamagnetic carboxylato complexes that adopt the classic lantern structure (26). However, even the aqua complex is dimeric so the bridging carboxylato ligands are not essential for the formation of the rhodium-rhodium bond. 

The cobalt acidato complexes are readily prepared from the pentammineaquacobalt(III) perchlorate, Cp(NIl3)5H20 (0104)3. As reported elsewhere ), the aqua complex is added to a 10-fold... 

Another way of bringing reactants into close proximity, which is encountered commonly in transition metal chemistry, is through metal ion complexation. The coordination of a reactant to a metal ion complex often activates its reactivity and can bring the reactant into close proximity with a second reactant or with a catalytic group. One example, shown in Fig. 6, is a zinc (11) complex of 1,5,9-triazacyclononane, as a model for the enzyme carbonic anhydrase, which contains a zinc (11) cofactor in its active site (4). In the aqua complex, the bound water molecule has a dramatically reduced pKa value of 7.3, which is similar to the pKa of the active site nucleophihc water. The corresponding cobalt (111) complex catalyzed ester hydrolysis at twice the rate because Co(lll) can coordinate both the hydroxide nucleophile and the ester carbonyl via a... 

They represent bridged structures of known constitution whose properties may throw light upon the most common type of hydrolysis products of aqua ions within the whole periodic system. In this connection it is noteworthy that the aquation behavior of the di-ju-hydroxochromium(lll) complex, meso-[(en)2Cr(OH)2Cr(en)2] appears to be significantly different from that of the corresponding cobalt(III) complex. ... 

In both cases, the cobalt containing product is the aqua complex because H2O is present in abundance, and high-spin d complexes of Co(II) are substitution labile. However, something that distinguishes the two pathways is the composition of the vanadium-containing product. If [V(N3)(OH2)s] is the product, then the reaction has proceeded via an inner-sphere pathway. If [V(OH2)6] " is the product, then the electron-transfer reaction is outer-sphere. The complex [V(N3)(OH2)5] is inert enough to be experimentally observed before the water molecule displaces the azide anion to give [V(OH2)6]. ... 

In addition to the cis and trans dibromo complexes, the syntheses of c/s-aqua-bromobis(l,2-ethanediamine)cobalt(III) dibromide monohydrate and trans-aquabromobis(l,2-ethanediamine)cobalt(III) dithionate monohydrate are described. It is of historical interest that of the hundreds of cis/trans isomeric pairs of diacidobis(l,2-ethanediamine)cobalt(III) complexes known to Werner,3 the trans isomers in the classical aquahalo series eluded him. The procedures described here for their isolation are a modification of much later works, 8 in which the more soluble trans complexes were obtained pure as their sulfate salts by fractional crystallization of cis/trans mixtures. The syntheses of cis-[Co(en)2(OH2)Cl]S04-2H201-9 and ds-[Co(en)2(0H2)Cl]Br2"H209 are described in previous volumes of this series. The preparation of frans-aquachloro-bis(l, 2-ethanediamine)cobalt(III) dithionate monohydrate is given here to complete the series. 

Because we live and carry out our chemistry in a world in which water is ubiquitous, we tend to forget that transition metal ions in aqueous solutions are present as aqua complexes and should not be regarded as free ions . In aqueous solution many transition metal ions are reduced by L-ascorbic acid. A typical example in which the mechanism of the reaction has been studied is that with cobalt (iii) ions in water. Cobalt (iii) ions have only a comparatively brief existence in water, but exist long enough to allow kinetic experiments to be performed on their reaction with L-ascorbic acid. The high positive charge on the cobalt causes the water molecules in the aqua complex to be acidic, so that hydroxo species are formed at higher pH values. The effect of this on the reduction reaction is reflected in the mechanism shown in equations (30)-(34). 

---