Cobalt ligand displacement

The title complex (as the chloride) was first reported by Kyuno and Bailar,1 who obtained cis-a and m-(i isomers by the ligand displacement reaction of cis-a-dichloro(N,N -bis(2-aminoethyl)-l,2-ethanediamine)cobalt(lll) chloride in gaseous and liquid ammonia. However, its trans isomers were not prepared until 1980.2... 

The pyridine ligand in some alkenyl cobalt(III) DMG complexes may be replaced by methyl or ethyl phosphite by addition of the phosphite to a solution of the (DMG)2 complex in ethanol. In a similar manner aniline may be displaced by (CH30)3P from [CH2=CHCo(DMG)2C6HjNH2] (129). 

There is a large amount of data available concerning the thermodynamic effects of ligands on other coordination sites (i. e., the thermodynamic cis- and iraws-effects). However, very little is known about the effects of ligands on the kinetic lability of other coordination sites. In fact, very little work has been carried out, directly with Bi2-derivatives, or with models of B12, on the kinetics of ligand substitution at the cobalt center. Of particular biochemical interest would be studies on the rate of displacement of coordinated benzimidazole by various ligands. Such work has not been reported at present. If the benzimidazole is replaced during enzymatic catalysis so that the lower axial position is occupied by some other Lewis base, one would expect this displacement, and the reverse step, to be very facile. This appears to be qualitatively true in that when water displaces benzimidazole as the benzimidazole is pro-... 

It is quite certain that the two cobalamin species with coordinated and free benzimidazole are in rapid equilibrium. However, the ligands bound to the lower coordination site are apparently not always easily displaced. As mentioned above, Brodie and Poe (130) have found that in DMSO, a water molecule is firmly bound to the sixth coordination position of alkyl cobinamides. Thus, even though DMSO is a good Lewis base, it will not easily displace water from the primary coordination sphere of the cobalt. 

The XPS results for cobalt at pH 4, particularly the Co 2p splitting (15 eV) and the absence of shake-up satellite structure, are indicative of cobalt(III). However, the N(amine)/Co atomic ratio of 2.7 indicates that some ammonia ligands have been displaced. Since it is known (22) that hydrolysis rates for cobalt(III) complexes are very slow, the presence of cobalt with a low number of coordinated amines, suggests that hydrolysis is induced via an interaction with the birnessite surface. The cobalt to manganese ratios for bulk and surface measurements are equivalent within experimental error, a result which is consistent with a reaction process occurring primarily at the surface. It is... 

Remarkably, the catalytic cycle is not controlled by the presence of phosphine ligands, but it is controlled by the organo group Y at the cobalt the neutral ligand L is displaced by the substrates in the initial step. Oxidative addition of two acetylenes results in a cobaltacycle that reacts with the nitrile to give the pyridine derivative with regeneration of the active [YCo] species. 

The unsymmetrical nature of / -mercaptoethylamine should lead to geometric isomerism among its metal complexes, cis and trans isomers might be expected with the square planar nickel (II) and palladium (II) derivatives and facial and peripheral isomers with cobalt (III). However, during the course of the preparation of various samples in which the procedure and experimental conditions were varied, no evidence of such isomerism was apparent (6, 15). This is particularly evident in the case of the cobalt (III) complex, CoL3. Samples prepared by the addition of cobalt (II) chloride 6-hydrate to strongly basic aqueous solution of the ligand and by displacement of ammonia and (ethylenedinitrilo)-... 

---