Inorganic Chemistry, Coordination Compounds, and Organometallic Bonding
Bonding in Metal Complexes
Crystal Field and Ligand Field Theory
Crystal field theory models electrostatic splitting of metal orbitals, while ligand field theory incorporates covalency and molecular orbital interactions. The splitting pattern depends on geometry, ligand donor-acceptor properties, and metal oxidation state.
Important Coordination Geometries
Octahedral
- Common for six-coordinate complexes
- Splits orbitals into and sets
- Supports low-spin and high-spin regimes
Tetrahedral
- Typically smaller splitting than octahedral
- Often high-spin
- Frequently found for and first-row metals
18-electron rule
The 18-electron rule is a useful guide for many organometallic complexes, but it is not universal. Electron count, geometry, oxidation state, and ligand type all matter.
Which geometry is especially common for many square-planar complexes?
Many metals such as Pt(II) and Pd(II) prefer square-planar coordination.
Correct answer: Square planar
Organometallic Elementary Steps
Catalytic cycles are often analyzed through oxidative addition, reductive elimination, migratory insertion, -hydride elimination, and transmetalation. These steps reshape oxidation state and coordination number in controlled ways.
What is oxidative addition in organometallic chemistry?
It is a key step in many cross-coupling and hydrogenation cycles.
Correct answer: An increase in metal oxidation state and coordination number by insertion of a ligand bond into the metal center.
Analyzing a Catalytic Cycle
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Step 1: Identify the catalyst resting state.
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Step 2: Assign oxidation states and electron counts.
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Step 3: Determine possible elementary steps.
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Step 4: Locate the rate-determining transition state.
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Step 5: Relate ligands and geometry to selectivity.
Which step typically decreases the electron count at the metal after bond cleavage and bond formation?
Reductive elimination lowers the oxidation state and often decreases the electron count at the metal center.
Correct answer: Reductive elimination
Why do strong-field ligands often promote low-spin complexes?
High splitting can outweigh pairing energy, favoring low-spin arrangements.
Correct answer: Because large ligand-field splitting makes pairing electrons in lower orbitals energetically favorable compared with occupying higher orbitals.