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A sequence of electron transfer, chemical reaction, electron transfer and chemical reaction. Binding reactions can shift reduction potentials and separate reduction from product-release waves.

Electron transfer through a ligand shared by two metal centres. Bridge formation, electron transfer and bridge cleavage can leave transferred-ligand connectivity in a substitution-inert product.

The difference between an applied electrode potential and the relevant equilibrium potential. Increasing its magnitude can drive electron transfer toward a mass-transport limit.

The free end of N-bound $\mathrm{Co}-\mathrm{NCS}$ is sulfur. Cr(II) binds there, giving the remote bridge $\mathrm{C}{\mathrm{o}}^{\mathrm{III}}-\mathrm{N}-\mathrm{C}-\mathrm{S}-\mathrm{C}{\mathrm{r}}^{\mathrm{II}}$. Electron transfer and cleavage of the Co-N bond leave S-bound Cr(III), $[\mathrm{Cr}(\mathrm{SCN})(\mathrm{O}{\mathrm{H}}_2{)}_5{]}^{2+}$; labile Co(II) becomes hexaaquacobalt(II).

The much smaller rate constant shows that the efficiency of bridge formation/electron transmission is strongly sensitive to linkage orientation. Efficient donation and coupling in a Co-S-bound precursor need not be reproduced when cobalt is N-bound. Its reduction is still consistent with inner-sphere electron transfer; a slow rate alone is not a criterion for outer-sphere transfer.

These are inner-sphere electron transfer pathways distinguished by where Cr(II) attacks the coordinated thiocyanate. Cr(II) is substitution-labile and can replace an aqua ligand to form a bridged Co(III)/Cr(II) precursor.

In both cases the product Co(II) rapidly loses its ammines to give the hexaaqua ion, whereas substitution-inert Cr(III) retains the bridge-end information. The two chromium linkage isomers are therefore a record of two precursor geometries, rather than evidence that one outer-sphere electron-transfer event somehow selects two arbitrary new ligands afterwards.

Make heterogeneous electron transfer fast relative to delivery of reactant, usually by applying sufficient overpotential so that its surface concentration approaches zero. Use excess supporting electrolyte to suppress migration. For purely diffusional transient transport, suppress uncontrolled convection; for a rotating disk electrode, controlled convection supplies the bulk while diffusion across the thin interfacial boundary layer limits the current. A limiting current that no longer increases with driving potential is the characteristic test; a large current alone is not sufficient.

Event a probes electron transfer between the semiquinone and dianion. Benzoquinone's smaller framework localizes charge and undergoes a greater change in bonding and solvation on this reduction; a larger effective reorganization barrier can make its heterogeneous transfer insufficiently fast for Nernst equilibrium on the $50\mathrm{mV}{\mathrm{s}}^{-1}$ time scale. Anthraquinone's extended framework distributes the added charge and can support the more reversible response seen here.

This is a kinetic interpretation of quasi-reversibility, not a claim that benzoquinone cannot be reduced. Follow-up chemistry or adsorption can also distort peak separation and return-current ratios; scan-rate-dependent measurements would distinguish them from slow electron transfer. A single trace does not uniquely identify the cause.