Cobalt standard reduction potentials

Only the + 2 state of cobalt has thermodynamic stability in acid solution. The instability of Co3+ is referred to in Section 5.3. Only the + 3 states of Rh and Ir are stable in acid solution their +3/0 standard reduction potentials are quite positive, consistent with their nobility . In alkaline solutions the + 2 and + 3 states of the elements exist as insoluble hydroxides. 

Knowing and understanding standard reduction potentials, E°, also helps determine whether a particular oxidation state will be stable and appropriate oxidants and reductants to use in a synthetic scheme. In reviewing the cobalt complex syntheses in Chapters 2 and 6, for example, complex ions are formed by oxidizing cobalt(II) salts to the more stable +3 state. 

In this experiment, the electrochemistry of both [Co(en)3]3+/2+ and [Co(ox)3]3+/2+ will be investigated using cyclic voltammetry, and the standard reduction potential (E°, V) for the [Co(en)3]3+/2+ couple will be measured. For metal complex stability reasons discussed below, it is not possible to use this technique to obtain reduction potentials for the mixed ligand cobalt systems an exercise at the end of this experiment helps to estimate these. The E° values obtained will be important for experiment 5.6, in which outer-sphere electron transfer rate constants between [Co(en)3)]2+ and [Co(en)2)(ox)]+ will be mathematically modeled using Marcus theory. 

Cob(III)alamins undergo ligand exchange reactions typical of cobalt(III) complexes, most of which are not directly relevant to biological function. One-electron reduction to cob(II)alamin and two-electron reduction to cob(I)alamin are biologically relevant. The standard reduction potentials are +200 and —610 mV, respectively, for the base-on Cob(III)/Cob(II) and Cob(II)/Cob(I) couples, referenced to the normal hydrogen electrode. ... 

The reactions described above all involve the displacement of a carbanion from the cobalt atom of methylcobalamin. These reactions occur under aerobic conditions with rate constants in the order of milliseconds. It is apparent that metals which react by electrophilic attack on the Co-C bond (SE2 mechanism) occur with the more oxidized state of the metal, i.e., Pb, Tl, Hg, Pd, which have standard reduction potentials greater than 0.8 volts. Because of the "base on - base off" equilibrium these reactions are pH dependent. 

In the past, there has been controversy [1] regarding both the true identity of the cobalt(III) species present in an aqueous medium and the standard potential for its one-electron reduction to cobalt(II). However, according to Wam-qvist [7], as well as Davies and Wam-qvist [8], there is good reason to conclude that Co(H20)6 is the predominant species in a 3 M perchloric acid solution and that the cobalt( 111) -cobalt( 11) couple in this medium can be represented as follows ... 

Several investigations have been made of the reduction of cobalt(II) to cobalt(O) in molten salt media. Eor a eutectic melt of LiCl-KCl at 450°C[10], a 1 1 NaCl-KCl melt at 450°C[11], and a MgCh-NaCl-KCl (50 30 20 mol%) mixture at 475 °C [12], the apparent standard potentials for the cobalt(II)-cobalt(0) couple have been deduced to be —1.207 V, — 1.277 V, and—1.046 V, respectively, each with respect to a chlorine-chloride ion reference electrode. 

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