Questions: Organometallic Chemistry Fundamentals

Question 1

Cr(CO)₆ has 6 CO ligands coordinated to chromium(0). Using the ionic or covalent electron counting method, how many valence electrons surround the chromium?

• A. 12 electrons (Cr has 6, each CO donates 1)
• B. 18 electrons (Cr has 6, each CO donates 2)
• C. 16 electrons (Cr has 4 in the d-shell, each CO donates 2)
• D. 24 electrons (Cr has 6, each CO donates 3)

Answer: (B)

In the covalent electron counting method: Cr⁰ contributes 6 valence electrons (Group 6 metal). Each CO is a two-electron donor through its carbon lone pair. Six CO ligands contribute 12 electrons. Total: 6 + 12 = 18 electrons. This satisfies the 18-electron rule — the metal achieves a filled valence shell analogous to a noble gas configuration. The 18-electron rule explains why Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ all exist as stable, isolable compounds with different numbers of CO ligands.

Question 2

The 18-electron rule states that stable organometallic complexes tend to have 18 valence electrons around the metal center, analogous to the octet rule for main group elements.

• T. True
• F. False

Answer: True

The 18-electron rule arises because the metal valence shell consists of nine orbitals (five d, one s, three p) that can accommodate a maximum of 18 electrons. When all nine orbitals are filled through a combination of metal valence electrons and ligand donations, the complex achieves maximum stability — a closed-shell configuration. Like the octet rule, this is a guideline with exceptions: many stable organometallic compounds have 16 electrons (especially d⁸ square planar complexes like Vaska's compound) and some have fewer. But the 18-electron rule is the single most useful predictor of organometallic stoichiometry and reactivity.

Question 3

Oxidative addition increases both the oxidation state and the coordination number of the metal center by 2.

• T. True
• F. False

Answer: True

In oxidative addition, a substrate A-B adds across the metal center, breaking the A-B bond and forming two new M-A and M-B bonds. The metal formally goes from M^n to M^(n+2) (oxidized by 2) and from x-coordinate to (x+2)-coordinate. For example, when Vaska's compound IrCl(CO)(PPh₃)₂ (Ir⁺, 16e, 4-coordinate square planar) undergoes oxidative addition of H₂, it forms IrH₂Cl(CO)(PPh₃)₂ (Ir³⁺, 18e, 6-coordinate octahedral). The metal has been oxidized (given up electron density to form two new bonds) while gaining two new ligands.

Question 4

Explain the 18-electron rule in terms of molecular orbital theory and identify two important classes of exceptions.

Model answer: In an octahedral complex, the nine metal valence orbitals (s + 3p + 5d) combine with ligand orbitals to form nine bonding MOs and nine antibonding MOs. The 18 electrons fill all nine bonding MOs, achieving maximum stability with no antibonding occupation. This is analogous to how the octet rule fills four bonding orbitals in main group chemistry. Two important classes of exceptions: (1) 16-electron complexes of d⁸ metals in square planar geometry (Rh⁺, Ir⁺, Ni²⁺, Pd²⁺, Pt²⁺), where one of the nine orbitals (d_x²−y²) is pushed so high in energy by the strong equatorial field that it remains empty — these are stable because the 'missing' orbital is antibonding. (2) Bulky ligands that sterically prevent enough ligands from coordinating to reach 18 electrons, as in many early transition metal complexes.

The 18-electron rule works best for middle and late transition metals with strong-field ligands (CO, PR₃, Cp). It is less reliable for early transition metals (which often have fewer than 18 electrons due to limited d-electron count and high oxidation states) and for f-block metals (where the f-orbitals add complexity).