Inorganic chemistry
Homogeneous catalysis: Ziegler–Natta, metathesis and Wilkinson chemistry
A homogeneous catalyst moves through a closed sequence of elementary steps in the same phase as the substrate. Small changes in ligands, oxidation state or vacant sites can redirect an entire cycle and change selectivity.
IntuitionIntuition: the catalyst is an active participant that returns
A homogeneous catalyst binds a substrate, rearranges it through several well-defined organometallic states, releases product and regenerates itself. Unlike a stoichiometric reagent, it is not consumed over the full cycle.
SchoolSchool level: catalysts change the path, not the endpoint
A catalyst lowers an effective activation barrier by supplying an alternative pathway; it does not alter the thermodynamics of the products. Turnover number counts product molecules per catalyst molecule before deactivation; turnover frequency normalises this by time.
Definition: Catalytic cycle
A catalytic cycle is a closed sequence of elementary reactions in which the catalytic species is transformed and regenerated. Intermediates may be detected, but drawing them as a cycle is a model of the dominant pathway, not a proof of every microscopic step.
Example: Estimate turnover frequency
1.0 mmol of catalyst converts 0.50 mol of substrate in 10 h. Find TON and TOF.
Solution
TON = 0.50/0.0010 = 500. TOF = 500/10 = 50 h⁻¹, an average value that can hide induction, deactivation or changing rate during the run.
UndergraduateUndergraduate: elementary organometallic steps
Oxidative addition increases metal oxidation state by two and adds two ligands; reductive elimination is its reverse. Migratory insertion places an X-type ligand adjacent to a π-bound substrate, often advancing a chain or forming a new bond. β-Hydrogen elimination competes when an alkyl group has an accessible β-H.
In Wilkinson hydrogenation, Rh(I) oxidatively adds H₂, alkene coordinates, insertion forms a rhodium alkyl, and reductive elimination releases alkane while regenerating Rh(I). Electron counts and vacant sites help explain why ligands tune rate and selectivity.
Example: Classify an elementary step
A square-planar Rh(I) complex adds H₂ to form an octahedral Rh(III) dihydride. What elementary step is this?
Solution
Oxidative addition: two M–H bonds form, H₂ cleaves, and the metal is formally oxidised from +1 to +3.
AdvancedCycle design, selectivity and deactivation
The Cossee–Arlman picture describes alkene polymerisation by coordination to a metal alkyl with a vacant site followed by chain migratory insertion. Stereospecific Ziegler–Natta catalysis depends on the local geometry of insertion, not merely on whether a monomer binds.
Grubbs olefin metathesis uses a metal carbene and a metallacyclobutane intermediate; each formal [2+2] cycloaddition/cycloreversion swaps alkylidene partners. The practical challenge is designing catalysts with high activity, functional-group tolerance and controlled stereochemistry.
Catalysts deactivate by ligand loss, metal aggregation, impurity poisoning or side reactions. Kinetics must therefore distinguish intrinsic cycle rate from catalyst lifetime; a high TOF under ideal conditions may mask rapid irreversible deactivation in a real stream.
ResearchResearch: sustainable catalysis and earth-abundant metals
References
- The Organometallic Chemistry of the Transition Metals · R. H. Crabtree, 2014
- The mechanism of olefin metathesis · J.-L. Hérisson, Y. Chauvin, 1971