Co-III complexes

Several complexes with cobalt in the unusually high oxidation state of (-(- 4) were reported (198,202,280) in 1974. All of the complexes reported were prepared by reaction of [Codlidtcls] with BF3 or Et20BF3 in the presence of air. The complexes were formulated (202, 280) as [Co(R2dtc)3]BF4 (R = Me, Et, Pr, or cyclohexyl), but Hendrickson and Martin (198) suggested that dimeric [Co2(R2dtc)5]BF4 (Rj = Me2, Et2 pyrrolidyl, MeBu", or Bzj), Co(III) complexes, form that are analogous with the ruthenium(III) complex discussed earlier. 

The simple Co(I) complexes may reversibly pick up a proton to give what is essentially a Co(III) complex with a coordinated hydride, i.e.,... 

For the Co(III) complex Co(NH3)jN02 , Halpern and Nakamura have obtained spectrophotometric evidence for the inner-sphere reaction occurring via Co(CN)sONO which isomerises to give the product Co(CN)sN02 . The species Co(NH3)5CN also reacts in this manner to give Co(CN)sNC " and finally Co(CN). ... 

This mechanism gives a quantitative fit with the observed kinetics at 25 °C, is 1280 and is 4.4I.mole sec . Furthermore, experiments in which the Co(III) complex, Co(YOH)H20, is included in the reacting system provide evidence for the presence of Sn(III) intermediates (H4Y = EDTA). 

SECOND-ORDER RATE COEFFICIENTS (l.molc" LsCC" at 25 °C) FOR THE REDUCTION OF VARIOUS Co(III) COMPLEXES... 

Fig. 5. Relative rate coefficients (/crx/ rohj ) at 25 °C, referred to the rate for ROH2 [R = Co(NH3)j], for the reduction of various Co(III) complexes arranged in approximate order of increasing reactivity. Ionic strengths Cr(bipy)3, 0.1 M 1.0 Af dropping mercury...

Espenson has shown that the reaction of c/j-Co(en)2(N3)2 with takes place by an inner-sphere mechanism. This Co(III) complex was selected for investigation because it is particularly reactive towards and also the dissociation of monoazido vanadium(lll) is relatively slow. At low concentrations (2-20 X 10 M) the second-order rate coefficient is 32.9 l.mole . sec at 25 °C, [H ] = 0.10 M and [i = 1.0 M. At higher concentrations ( 0.1 M), using a stopped-flow apparatus, the kinetics are apparently first order at 520 mfi, a wavelength where shows negligible absorbance. The rate coefficient under... 

RATE PARAMETERS FORREDUCTION OF SELECTED C ARBOX YLATOPENTA A MMINE Co(III) COMPLEXES BY Ct, SHOWING EFFECT OF CHELATION ... 

Despite the lability of Fe(ni) complexes, Haim and Sutin have been able to study the rapid reductions of some Co(III) complexes by Fe(fr) and identify the primary Fe(III) products. A flow apparatus, described by Dulz and Sutin", enabled the rate of formation and decay of the spectra of the Fe(III) products... 

The Co(III) complexes Co(NH3)6 " and Co(NH3)sOH bring about oxidation of stannate(II) ion in strongly basic solution . The rates were found to be independent of the concentration of the Co(III) complex. It is proposed that stannate(Il) exists as a dimer, and that the monomer is the reactive species, the rate being close to half-order in stannate(II). Cyanide and thiosulphate catalyse the reaction but Co(CN)g is immune to attack by stannate(II) ion. The experimental difficulties encountered in this study preclude a full analysis as regards mechanism. 

Cerium(IV) oxidises tin(II) in aqueous sulphuric acid probably by a two-step path involving Sn(III). At low Sn(IV) concentrations and low sulphate concentration the reaction is second order, and the suggestion is made that the reactant species are Ce(S04)3 and SnS04. In mixed chloride-sulphate media the Ce(IV)- -Sn(II) reaction, in the presence of trioxalatocobaltate(III), produces an intermediate which consumes the Co(III) complex . This result is interpreted as being evidence for the presence of Sn(III) in the reacting system. 

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

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