Cobalt complexes, ligand substitution

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

The bis(boratabenzene)cobalt complexes 7 and 13 may also undergo substitution of a C5H5BR ligand. With Ni(CO)4 in refluxing toluene, the substitution products Co(CO)2(C5H5BR) 15 and 11 are formed (47,49). The cyanide degradation is another important example (Section V,B,2). 

In the sixties it was recognised that ligand substitution on the cobalt carbonyl complex might influence the performance of the catalyst. Tertiary alkyl phosphines have a profound influence ... 

Dinuclear clusters ferrous site distortion, 38 175 spin ladder, 38 182-183 Dinuclear cobalt complex, 45 291-293 Dinuclear complexes osmium, electrochemistry, 37 321-323 quadruply bridged, 40 187-235 axial ligand substitution properties, 40 232-234... 

The final product is ferrocyanide and cobaltic EDTA, but this goes through an intermediate which can be isolated, and which is an adduct of these twro. Dr. Wilkins tried this system out in his rapid flow rate system and found a rate of association which was about right for substitution rates on a cobaltous ion. So this seemed to be a case where perhaps the nitrogen end of a cyanide was able to coordinate into a cobaltous complex, with either concomitant cr subsequent charge transfer. Yet no transfer of ligand occurs in the overall reaction. 

Certain octahedral complexes, particularly the acido—amine complexes of cobalt(III), undergo substitution in protonic solvents at rates that are proportional to the concentration of the conjugate base of the solvent (e.g. OH- in water) or inversely proportional to the concentration of the conjugate acid of the solvent (e.g. retardation by H30+ in water or NH4+ in liquid ammonia). Such reactions have received considerable attention since systematic studies of ligand substitution commenced, and figured amongst the earliest kinetic studies in the field.298 The subject has been... 

There are a few examples of spin equilibria with other metal ions which have not been mentioned above. In cobalt(III) chemistry there exist some paramagnetic planar complexes in equilibrium with the usual diamagnetic octahedral species (22). The equilibria are the converse of the diamagnetic-planar to paramagnetic-octahedral equilibria which occur with nickel(II). Their interconversions are also presumably adiabatic. Preliminary observations indicate relaxation times of tens of microseconds, consistent with slower ligand substitution on a metal ion in the higher (III) oxidation state (120). 

No terminal fluoromethylidyne complexes have been reported, but triply bridging CF ligands are observed in the cobalt complex 14 (45), in various substituted analogues (46-49), and in the osmium complex 15 (50). An iron cluster 16 containing two triply bridging CF ligands has also been reported... 

The rates of complex formation and ligand substitution reactions of the polymer-bound Co(III) complexes depend on the dynamic property of the polymer domains. Reports on the kinetics of complex formation and ligand substitution of macromolecule-metal complexes are, however, relatively scarce. They include investigations on the complexation of poly-4-vinylpyridine with Ni2+ by the stopped conductance technique 30) and on a ligand substitution reaction of the polymer-bound cobalt(III) complexes 31>. 

The most obvious examples are in the preparation of octahedral Co(III) complexes. The most readily-available cobalt compounds are Co(II) salts in the presence of suitable ligands - usually N-donors - these are oxidised to give Co(III) complexes by air or hydrogen peroxide. A few such easily-prepared complexes open up pathways to the vast number of known octahedral Co(III) complexes via substitution reactions. For example, [Co(NH3)5(H20)]CI3 is readily converted into [Co(NH3)5X]Cl2 via anation reactions of the type discussed in Section 9.5, and salts containing the [Co(NH3)4(C03)]+ ion (where the carbonate is bidentate, taking up two cis positions) are useful for the formation of cw-[Co(NH3)4X2]+. 

Fundamentally, optical isomers of the spiroheterocyclic cobalt complex must exist. Since these compounds could not be separated, we tried to synthesize the dicyano derivative under CO substitution by reaction with KCN in liquid NH3 in the hope that a separation into the optical antipodes might be possible. At 120°C, however, the reaction gave, besides K2[Co(NO)(CO)(CN)2], a complex in which the C(CH2PPh2)4 ligand is tridentate (139) ... 

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