Section 11.2.5: Abstraction and Addition

The nucleophilic and electrophilic addition and abstraction reactions can be viewed as ways of activating a ligand toward reaction with an external reagent. The external reagent reacts directly with the ligand, not with the metal center. The external reagent may be a nucleophile (Lewis Base) or an electrophile (Lewis acid).

Nucleophilic Addition and Abstraction

The reaction of an activated ligand with a nucleophilic reagent is favored if the activated ligand is electron poor. When the metal complex is either cationic and/or has spectator ligands that are are electron withdrawing, the reactive ligand becomes activated by being depleted of electron density. The attack of the nucleophile may result in the formation of a new bond between the nucleophile and the activated unsaturated substrate, in which case it is called nucleophilic addition. Alternatively, the reaction may result in an abstraction of a part or the whole of the activated ligand, in which case it is called the nucleophilic abstraction. Some examples of nucleophilic addition and the abstraction reactions are discussed below.

Nucleophilic Addition

An example of a nucleophilic addition reaction is shown in Figure \(\PageIndex{1}\). The olefin is activated by its interaction with platinum. Pyridine is the nucleophile, and the result is a new \(\ce{C-N}\) bond.

Figure \(\PageIndex{1}\): An example of addition of a nucleophile to an activated ligand.

Nucleophilic Abstraction

An example of a nucleophilic abstraction reaction is shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): An example of abstraction by a nucleophile.

Electrophilic Addition and Abstraction

Similar to the nucleophilic addition and abstraction reactions, the electrophilic counterparts of these reactions also exist. An electrophilic attack is favored if the ligand-metal fragment is more electron rich. When the complex is anionic, the metal center is at low oxidation state, and/or when the spectator ligands are electron donating, the ligand becomes activated toward reaction with an electrophile. The reaction of activated ligands with electrophilic substrates may result in the formation of a new bond between the electrophile and the activated unsaturated substrate, in which case it is called electrophilic addition, or may result in an abstraction of a part or the whole of the activated ligand, in which case it is called the electrophilic abstraction.

Electrophilic Addition

An example of an electrophile being added to an activated ligand is shown in Figure \(\PageIndex{3}\). The allylic anion ligand uses its \(\pi\) electrons to react with hydrogen ion. This adds a hydrogen to the ligand, converting it from an anionic X-type ligand to a neutral L-type ligand .

Figure \(\PageIndex{3}\): An example of electrophilic addition to an activated ligand.

Electrophilic Abstraction

Depending on whether the abstraction occurs at the \(\alpha\) or \(\beta\) position with respect to the metal ion, the reaction is either classified as an \(\alpha\)-abstraction or a \(\beta\)-abstraction.

An example of an \(\alpha\) electrophilic abstraction reaction is shown in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): An example of an electrophilic \(\alpha\)-abstraction. (CC-BY-SA; Kathryn Haas)

Two examples of a \(\beta\) electrophilic abstraction are shown in Figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): Two examples of electrophilic \(\beta\)-abstraction. (CC-BY-SA; Kathryn Haas)