How does the oxidation state of a metal affect complex formation?

The oxidation state of a metal affects how strongly it can bind ligands and what type of complex it can form.

Effects of Oxidation State

1. Charge on the metal ion
   • A higher oxidation state means the metal ion has more positive charge.
   • This pulls ligands closer, making stronger bonds and often more stable complexes.
2. Size of the ion
   • Higher oxidation states make the metal ion smaller (because electrons are pulled in tighter).
   • Smaller ions attract ligands more strongly.
3. Type of ligands that bind
   • Metals in low oxidation states often bind to ligands that donate electrons easily (like CO, phosphines).
   • Metals in high oxidation states usually prefer oxygen or nitrogen donors (like H₂O, OH⁻, F⁻).
4. Geometry and number of ligands
   • Oxidation state can change how many ligands fit around the metal (coordination number) and the shape of the complex.

Examples

• Fe²⁺ (low oxidation) forms pale green [Fe(H₂O)₆]²⁺ complexes.
• Fe³⁺ (higher oxidation) forms yellow/brown [Fe(H₂O)₆]³⁺ complexes, more acidic and strongly binding.
• Cu⁺ forms fewer and softer complexes, while Cu²⁺ forms stronger, more stable ones.

In short:
The oxidation state decides how tightly the metal holds ligands, which ligands it prefers, and what shape complex forms.