Which of the following complex ions is paramagnetic?

• A. [Cu(CN)_4]^{3-}
• B. [Zn(NH_3)_4]^{2+}
• C. [Fe(CN)_6]^{4-}
• D. [Fe(H_2O)_6]^{2+}

Answer: (D)

Verified step by step guidance

1. Identify the metal center and its oxidation state in each complex ion. For example, determine the oxidation state of Cu in [Cu(CN)_4]^{3-}, Zn in [Zn(NH_3)_4]^{2+}, Fe in [Fe(CN)_6]^{4-}, and Fe in [Fe(H_2O)_6]^{2+}.
2. Determine the d-electron count for the metal ion in each complex by subtracting the oxidation state from the atomic number of the metal.
3. Consider the ligand field strength of the ligands attached to the metal. CN^- is a strong field ligand, NH_3 is a moderate field ligand, and H_2O is a weak field ligand. This affects whether the complex is high-spin or low-spin.
4. Use the ligand field strength and d-electron count to predict the electron configuration of the metal center in each complex, focusing on whether electrons are paired or unpaired.
5. Identify which complex has unpaired electrons (paramagnetic) and which has all electrons paired (diamagnetic). The complex with unpaired electrons is paramagnetic.