What is the general effect of complexing ligands on redox potentials, as illustrated by CN- complexes of iron?

• A. They do not change redox potentials, only kinetics.
• B. They shift the electron transfer rate but not stable species.
• C. They change the activity of the metal ion, shifting redox equilibria and E° values.
• D. They always make potentials more positive regardless of the system.

Answer: (C)