Cobalt anations

Cobalt, aquapentaammine-, 1, 3 Cobalt, aquapentacyano-anation, 1, 310 isomerization, 1,186 Cobalt, aquatetracyanosulfito-anation, 1, 310... 

Cobalt, dichlorobis(AvY -dimethyl-1,2-ethanediamine)-chloride hydrate isomerization, 1, 468 Cobalt, dich orobis(l,2-ethanediamine)-base hydrolysis, 1, 304 chloride anation, 1, 469 halogen exchange, 1, 468 chloride hydrate isomerization, 1, 468 isomers, 1,191 nitrate... 

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

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]+. 

Pentaaniinineaquacobalt(III) halides are relatively unstable. Substitution of coordinated water by halide anation [19]) is detectable at about 360 K. Conductivity measiuements have shown [19] that the water evolved may remain temporarily as a liquid before evaporation and this endothermic effect may mask the exothermic anation. This is consistent with the observation [4] that the minimum temperatures of cobalt reduction in the reactions of [Co(NH3)5H20]X3 (X = Cl, Br and I ) were very close to those of the corresponding halopentaammines. 

Kinetically, there are inherent difficulties in distinguishing the three mechanisms. As an illustration, is the anation reactions of the inert (d low-spin) complexes of cobalt(III), as typified by... 

Cobalt, bromochlorobis(l, 2-ethanediamine)-hydrate thiocyanate aquation-anation, 469... 

Cobalt, diaquabis(l,2-ethanediamine)-trihalide anation, 469 trinitrate... 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Reviews of interest include a general review of anation reactions of cobalt(III) complexes and a discussion of the solvation of transition metal complexes/ The nature of the solvent can have a very large effect on such properties as solubilities, reactivities, redox potentials, formation constants, and various types of spectra. Such solvent effects reflect changes in the solvation of ions, complexes, initial states, transition states, and excited states. 

Cobalt(ill).—Unidentate Leaving Groups. Volumes of activation for the aquation of [Co(CN)5X] ions and for the anation of [Co(CN)5(OH)2] ion are collected in Table 1. The data are entirely consistent with the well established extreme dissociative Z)-mechanism for ligand substitution (see Introduction, ref. 19). 

Cobalt(lII) (High-pressure studies of the anation of [M(NH3)5(OH2)] + ions (M = Co, Rh, or Cr) gave the AF+ values shown in Table 1. These results, together with those for the corresponding aquation reactions enable values for A to be estimated, and the values obtained... 

---