Osmium complexes cyclic voltammetry

Salts of [OsL(NH3)5]3+ (L = pyrazine, pyrimidine, pyridazine) can be made by displacement of nitrogen from [Os(N2)(NH3)5]2+9° or from the triflato complex [0s(0S02CF3)(NH3)J]2+ and the ligands.62 Cyclic voltammetry showed reduction to the osmium(II) species, 90 and the surface-enhanced Raman spectra (SERS) of [Os(pyz)(HNH3)5]3+, [Os(pyz)(NH3)5]2+ and of their ammine-d5 derivatives were measured, as were the IR spectra.91... 

Other redox-active polyelectrolyte films were prepared from ferrocene-derivatized polly(allylamine) and poly(vinyl pyridine) as well as an osmium complex of poly(vinyl pyridine) [44-46]. These films were synthesized to mediate electron transfer between the electrode and a charged enzyme that was a constituent of the polyelectrolyte film. In the case of ferrocene-derivatized poly(allylamine) or polyfvinyl pyridine), cyclic voltammetry of the bound ferrocene moiety showed small peak splittings (<50 mV at a scan rate of 50 and 20 mV s respectively) [45, 46). The amount of electroactive material increased with the number of deposited layers, but the first layer contained significantly more electroactive ferrocene than the later layers in the poly(allylamine) system [46]. 

Electrochemical measurements on transition metal complexes are performed for a wide variety of reasons. In this paper we discuss the synthesis of new species by electrogeneration, the measurement of rates of chemical reaction by cyclic voltammetry and the parameterisation of redox potentials within the related series of complexes [OsX. j pyjj], where X = Cl, Br, I, py = pyridine and n = 0-6. A further goal realised by this work was to increase the limited number of Ej/2 values for mononuclear osmium complexes reported in the literature. ... 

Once the M and M electrochemical parameters are established, they can be used to predict the redox potential of a species and enable the in situ identification of an (un)known complex, product, or intermediate, thus avoiding its isolation and eventual decomposition. Examples include some chloro/azole-ruthenium [73, 74] or chloro-osmium [75] complexes derived upon ligand replacement induced by electron transfer. The identification of such species, generated in situ, has allowed the establishment of the detailed mechanisms of the reactions by digital simulation of cyclic voltammetry. 

If a chemical reaction follows an electrochemical step, the cyclic voltam-mogram is typically irreversible and i jip 1 and/or A p 59/n mV. Cyclic voltammetry can be used to estimate the relative energy of the frontier orbitals of a series of complexes for each coordination site. A cyclic voltammogram for [Os(bpy)3] shows a reversible oxidation with 1/2= 0.81 V and three reversible reductions starting at —1.29 V. The oxidation is representative of Os , while the reduction represents sequential bpy for each coordinated ligand. The electrochemistry shows that frontier orbitals of [Os(bpy)3] involve an osmium-based HOMO and a bpy-based LUMO. This is consistent with the photochemical properties of [Os(bpy)3] in which the lowest energy... 

---