Understanding the Complex Ion [Fe(EDTA)]-

The complex ion [Fe(EDTA)]- consists of a central metal ion, Fe, coordinated to the ligand EDTA (ethylene diamine tetraacetic acid). In order to determine the oxidation state of the central metal ion Fe, we need to consider the charges and the overall charge of the complex ion.

1. Ligand EDTA
- EDTA is a hexadentate ligand, meaning it can form six coordination bonds with the central metal ion.
- Each of the four oxygen atoms from the carboxylate groups (-COO-) of EDTA can form a coordination bond with the metal ion.
- The two nitrogen atoms from the ethylenediamine group (-NH2CH2CH2NH2) can also form coordination bonds.
- The coordination bonds involve the donation of electron pairs from the ligand to the metal ion.

2. Charge of the Complex Ion
- The overall charge of the complex ion [Fe(EDTA)]- is -1, as indicated by the superscript -.
- This means that the total charge of the ligand and the central metal ion must add up to -1.

3. Determining the Oxidation State
- In order to determine the oxidation state of the central metal ion Fe, we need to assign oxidation states to each atom in the complex ion.
- Let's assume the oxidation state of the central metal ion Fe is x.

4. Oxidation State of the Ligand
- Oxygen typically has an oxidation state of -2 in compounds.
- Nitrogen typically has an oxidation state of -3 in compounds.
- Each oxygen atom in the carboxylate groups of EDTA contributes a total oxidation state of -2 x 4 = -8.
- Each nitrogen atom in the ethylenediamine group of EDTA contributes an oxidation state of -3 x 2 = -6.
- Therefore, the total oxidation state of the ligand EDTA is -8 - 6 = -14.

5. Oxidation State of the Central Metal Ion
- The total oxidation state of the complex ion is -1.
- The oxidation state of the central metal ion Fe can be calculated by subtracting the oxidation state of the ligand from the total oxidation state of the complex ion.
- Therefore, x - 14 = -1.
- Solving for x, we get x = 13.

6. Conclusion
- The oxidation state of the central metal ion Fe in the complex ion [Fe(EDTA)]- is +3.
- This means that the central metal ion Fe has lost three electrons and has a positive charge of +3.