Organometallics, Catalysis, and Reactive Intermediates
Catalysis as controlled reaction energetics
Electronic structure of metal complexes
Transition-metal chemistry is governed by ligand field splitting, covalency, oxidation-state ambiguity, and spin-state energetics. The balance of -donation, -backbonding, and steric environment determines reactivity and selectivity.
Mechanistic caution
Formal oxidation states are bookkeeping devices; the true electronic structure is often delocalized and method-dependent.
Catalytic cycles
A catalytic cycle must be analyzed in terms of elementary steps, turnover-limiting barriers, off-cycle resting states, and selectivity-determining transitions. The best catalyst is not merely the one with the lowest barrier, but the one with the most favorable full energetic landscape under realistic conditions.
What is a common reason a catalyst with a low intrinsic barrier still performs poorly?
A deep off-cycle minimum can trap the catalyst and reduce effective turnover frequency.
Correct answer: It forms a strongly bound resting state that suppresses turnover
Define chemoselectivity.
Selectivity is central in catalyst design and synthetic planning.
Correct answer: The preference for one functional group or bond to react in the presence of others.
Catalyst classes
| Class | Advantages | Challenges |
|---|---|---|
| Homogeneous catalysts | High selectivity and mechanistic tunability | Separation and stability issues |
| Heterogeneous catalysts | Recyclability and process compatibility | Active-site heterogeneity |
| Enzymes | Exceptional selectivity under mild conditions | Substrate scope and robustness limits |
| Single-atom catalysts | Isolated active sites with tunable coordination | Characterization and stability |
Key mechanistic questions
Homogeneous catalysis
- Which ligand controls reactivity?
- Is oxidation-state change essential?
- What is the turnover-limiting step?
Heterogeneous catalysis
- Which surface site is active?
- How do adsorbates alter the barrier?
- What determines deactivation?
Which description best fits ligand-field splitting?
Ligands break degeneracy of metal d orbitals, creating distinct orbital energies.
Correct answer: Energy separation of metal d orbitals in a ligand environment
Frontier issues
Open challenges include C-H functionalization under mild conditions, redox-active ligands, catalyst deactivation pathways, and predictive models linking microscopic descriptors to macroscopic selectivity and lifetime.