Lecture 32: Alkene Complexes

Read section 23.10 from your textbook.

Bonding in Alkene Complexes

Alkenes are pi-acid ligands like CO.

Ligand sigma donor orbital: filled C-C pi orbital
Ligand pi acceptor orbital: empty C-C pi antibonding orbital

Synthesis

There are 2 principle methods:

substitution reactions
reduction of the metal in the presence of the alkene

Examples

COD is an abbreviation for cyclooctadiene and NBD stands for norbornadiene. For those dienes, the coordination to the metal is tetrahapto. The metal is bonded to 4 carbon atoms. In the ethylene complex, the metal is eta-2 or dihapto and bonded to 2 carbon atoms.

Nucleophilic Attack on a Coordinated Alkene

External nucleophiles attack coordinated alkenes. Note that this is the opposite reactivity of free alkenes that are attacked by electrophiles.

For substituted alkenes, there are two sites of nucleophilic attack:

Above: at the more substituted carbon (favored sterically, when the metal has very bulky ligands)
Below: at the less substituted carbon (favored electronically)

Beta-Hydride Elimination

The most common internal nucleophilic attack on a coordinated alkene is by hydride. This is a reversible reaction.

Forward direction: hydride migration
Reverse direction: beta-hydrogen elimination