Pre-lab 5.3.a Thermodynamic Stability Considerations

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. 

The equilibrium expression and accompanying Nemst equation for the Co(en)3+,/2+ couple in this experiment are given in equations (5.4) and (5.5)  

Remember that, although cobalt(III) complexes are very kinetically stable and inert to substitution, the reduced cobalt(II), d1, complexes are less stable and extremely labile in aqueous solution. Because of this, we must use stability constant data to determine the excess concentration of ligand that will be necessary to maintain the binding of all three bidentate ligands (as opposed to two bidentate ligands and two H20 ligands for example) to the metal center. We consider the Co(en)3 2+ system below. 

If we assume that a ratio of tris. bis complex of 100 1 is good enough for our experimental purposes, we can calculate the excess en needed to maintain this ratio. 

This is the excess [en] that must be in solution over and above the en bound to the metal center. To be on the safe side, and also to leave room for buffering capacity, 0.1 Men will be used in the [Co(en)3]3+ 2+ system. Taking advantage of the first protonation of en, the solution will be buffered by the addition of a small amount of HNO3 (0.01 M final concentration). In Procedure 5.3.b you will solve for the excess oxalate2- needed for thermodynamic stability in the Co(ox)33-,/4- system. 

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