An insertion reaction is a chemical reaction where one chemical entity (a molecule or molecular fragment) interposes itself into an existing bond of typically a second chemical entity e.g.:
The term only refers to the result of the reaction and does not suggest a mechanism.  Insertion reactions are observed in organic, inorganic, and organometallic chemistry.  In cases where a metal-ligand bond in a coordination complex is involved, these reactions are typically organometallic in nature and involve a bond between a transition metal and a carbon or hydrogen.  It is usually reserved for the case where the coordination number and oxidation state of the metal remain unchanged.  When these reactions are reversible, the removal of the small molecule from the metal-ligand bond is called extrusion or elimination.
There are two common insertion geometries— 1,1 and 1,2 (pictured above). Additionally, the inserting molecule can act either as a nucleophile or as an electrophile to the metal complex.  These behaviors will be discussed in more detail for CO, nucleophilic behavior, and SO, electrophilic behavior.