18 Electron Practice Problems

Practice Problems: 18-Electron Rule

Question 1: Hexacarbonyl Manganese ([Mn(CO)₆]⁺)

Determine the electron count for the complex [Mn(CO)₆]⁺. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Manganese (Mn) is in Group 7. The +1 charge means Mn is in the +1 oxidation state.
    • Mn contributes 6 electrons (7 − 1 = 6).
  2. Ligand contributions:
    • Each CO ligand contributes 2 electrons. With 6 CO ligands: 6 × 2 = 12 electrons.
  3. Total electron count:
    • 6 (Mn) + 12 (CO ligands) = 18 electrons.
  4. Conclusion:
    • [Mn(CO)₆]⁺ satisfies the 18-electron rule and is stable.

Question 2: Tetrachloronickelate ([NiCl₄]²⁻)

Determine the electron count for [NiCl₄]²⁻. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Nickel (Ni) is in Group 10. The 2⁻ charge means Ni is in the +2 oxidation state.
    • Ni contributes 8 electrons (10 − 2 = 8).
  2. Ligand contributions:
    • Each Cl⁻ ligand contributes 2 electrons. With 4 Cl⁻ ligands: 4 × 2 = 8 electrons.
  3. Total electron count:
    • 8 (Ni) + 8 (Cl⁻ ligands) = 16 electrons.
  4. Conclusion:
    • [NiCl₄]²⁻ does not satisfy the 18-electron rule but is stable as a 16-electron tetrahedral complex.

Question 3: Bis(benzene)chromium ([Cr(η⁶-C₆H₆)₂])

Determine the electron count for [Cr(η⁶-C₆H₆)₂]. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Chromium (Cr) is in Group 6 and is in the 0 oxidation state.
    • Cr contributes 6 electrons.
  2. Ligand contributions:
    • Each η⁶-C₆H₆ (benzene) ligand contributes 6 electrons. With 2 benzene ligands: 2 × 6 = 12 electrons.
  3. Total electron count:
    • 6 (Cr) + 12 (benzene ligands) = 18 electrons.
  4. Conclusion:
    • [Cr(η⁶-C₆H₆)₂] satisfies the 18-electron rule and is stable.

Question 4: Tricarbonyl Iron Hydride ([HFe(CO)₃])

Determine the electron count for [HFe(CO)₃]. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Iron (Fe) is in Group 8 and is in the +1 oxidation state (H⁻ is treated as a ligand, making Fe effectively +1).
    • Fe contributes 7 electrons (8 − 1 = 7).
  2. Ligand contributions:
    • Each CO ligand contributes 2 electrons. With 3 CO ligands: 3 × 2 = 6 electrons.
    • The H⁻ ligand contributes 2 electrons.
  3. Total electron count:
    • 7 (Fe) + 6 (CO ligands) + 2 (H⁻) = 15 electrons.
  4. Conclusion:
    • [HFe(CO)₃] does not satisfy the 18-electron rule and is a 15-electron complex, which is less stable and reactive.

Question 5: Rhodium Carbonyl Chloride ([RhCl(CO)(PPh₃)₂])

Determine the electron count for [RhCl(CO)(PPh₃)₂]. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Rhodium (Rh) is in Group 9 and is in the +1 oxidation state.
    • Rh contributes 8 electrons (9 − 1 = 8).
  2. Ligand contributions:
    • The CO ligand contributes 2 electrons.
    • The Cl⁻ ligand contributes 2 electrons.
    • Each PPh₃ ligand contributes 2 electrons. With 2 PPh₃ ligands: 2 × 2 = 4 electrons.
  3. Total electron count:
    • 8 (Rh) + 2 (CO) + 2 (Cl⁻) + 4 (PPh₃ ligands) = 16 electrons.
  4. Conclusion:
    • [RhCl(CO)(PPh₃)₂] is a stable 16-electron complex due to steric and electronic factors.

Question 6: Hexacarbonyl Chromium ([Cr(CO)₆])

Determine the electron count for [Cr(CO)₆]. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Chromium (Cr) is in Group 6 and is in the 0 oxidation state.
    • Cr contributes 6 electrons.
  2. Ligand contributions:
    • Each CO ligand contributes 2 electrons. With 6 CO ligands: 6 × 2 = 12 electrons.
  3. Total electron count:
    • 6 (Cr) + 12 (CO ligands) = 18 electrons.
  4. Conclusion:
    • [Cr(CO)₆] satisfies the 18-electron rule and is stable.

Question 7: Tetracarbonyl Nickel ([Ni(CO)₄])

Determine the electron count for [Ni(CO)₄]. Does it satisfy the 18-electron rule?

Answer:

  1. Metal and oxidation state:
    • Nickel (Ni) is in Group 10 and is in the 0 oxidation state.
    • Ni contributes 10 electrons.
  2. Ligand contributions:
    • Each CO ligand contributes 2 electrons. With 4 CO ligands: 4 × 2 = 8 electrons.
  3. Total electron count:
    • 10 (Ni) + 8 (CO ligands) = 18 electrons.
  4. Conclusion:
    • [Ni(CO)₄] satisfies the 18-electron rule and is stable.

Question 8: Determine Oxidation State of the Metal

Question: The complex [Mo(CO)₄(PPh₃)₂] is known to have a total of 18 electrons. Using this information, determine the oxidation state of molybdenum in this complex.

Answer:

  1. Ligand contributions:

    • Each CO contributes 2 electrons: 4 × 2 = 8 electrons.

    • Each PPh₃ contributes 2 electrons: 2 × 2 = 4 electrons.

    • Total from ligands: 8 + 4 = 12 electrons.

  2. Metal contribution:

    • For the complex to have 18 electrons, the metal must contribute: 18 − 12 = 6 electrons.

  3. Determine the oxidation state:

    • Molybdenum (Mo) is in Group 6, so a neutral Mo atom contributes 6 electrons.

    • Since Mo contributes all 6 electrons, it must be in the 0 oxidation state.

  4. Conclusion:

    • Molybdenum is in the 0 oxidation state in [Mo(CO)₄(PPh₃)₂].

Question 9: Determine Oxidation State of the Metal

Question: The complex [FeCp(CO)₂]⁺ has a total of 18 electrons. What is the oxidation state of iron in this complex?

Answer:

  1. Ligand contributions:

    • The Cp ligand contributes 6 electrons.

    • Each CO ligand contributes 2 electrons: 2 × 2 = 4 electrons.

    • Total from ligands: 6 + 4 = 10 electrons.

  2. Metal contribution:

    • For the complex to have 18 electrons, the metal must contribute: 18 − 10 = 8 electrons.

  3. Determine the oxidation state:

    • Iron (Fe) is in Group 8, so a neutral Fe atom contributes 8 electrons.

    • Since Fe contributes all 8 electrons, it is in the 0 oxidation state.

  4. Conclusion:

    • Iron is in the 0 oxidation state in [FeCp(CO)₂]⁺.

Question 10: Determine Oxidation State of the Metal

Question: The complex [Co(NH₃)₆]³⁺ is a 16-electron species. What is the oxidation state of cobalt in this complex?

Answer:

  1. Ligand contributions:

    • Each NH₃ contributes 2 electrons: 6 × 2 = 12 electrons.

  2. Metal contribution:

    • For the complex to have 16 electrons, the metal must contribute: 16 − 12 = 4 electrons.

  3. Determine the oxidation state:

    • Cobalt (Co) is in Group 9, so a neutral Co atom contributes 9 electrons.

    • To contribute only 4 electrons, cobalt must lose 5 electrons.

    • Oxidation state = +5.

  4. Conclusion:

    • Cobalt is in the +5 oxidation state in [Co(NH₃)₆]³⁺.