Organometallic chemistry - Substitution reactions

The first step in building a bond between a transition metal atom and a carbon monoxide molecule is to donate the doublet carried by the carbon atom. The return of electron density from the transition metal to the carbon monoxide actually enables this doublet to be donated, as the carbon monoxide molecule is not a Lewis base.

It's only natural that if a molecule has free electron density in the form of a doublet, and if this molecule also has empty electron orbitals that can be donated in return, a substitution should take place. This is the subject of this article. Of course, this assumes that the orbital symmetries of the metal and the ligand molecule are compatible, so that overlap can take place. In other words, this means that the orbitals of the carbon monoxide and the molecule substituting it will have equivalent symmetries; they are known as isolobals. This concept will be further developed and used.