In organometallic chemistry, a tuck-in complex usually refers to derivatives of Cp* ligands wherein a methyl group is deprotonated and the resulting methylene attaches to the metal. The C<sub>5</sub>âÂÂCH<sub>2</sub>âÂÂM angle is acute. The term "tucked in" was coined to describe derivatives of organotungsten complexes. Although most "tucked-in" complexes are derived from Cp* ligands, other pi-bonded rings undergo similar reactions.
The "tuck-in" process is related to ortho-metalation in the sense that it is an intramolecular cyclometalation. Tuck-in complexes derived from Cp* ligands are derivatives of tetramethylfulvene, sometimes abbreviated Me<sub>4</sub>Fv. A variety of complexes are known for Me<sub>4</sub>Fv and related ligands. In these complexes, the Fv can serve as a 4-electron or as a 6-electron ligand.
The original example proceeded via sequential loss of two equivalents of H<sub>2</sub> from decamethyltungstocene dihydride, Cp*<sub>2</sub>WH<sub>2</sub>. The first dehydrogenation step affords a simple tuck-in complex:
The second dehydrogenation step affords a double tuck-in complex:
In organouranium chemistry, both tuck-in and tuck-over complexes are recognized, for example in the dihydrido diuranium complex [Cp*<sub>3</sub>(÷<sup>7</sup>-C<sub>5</sub>Me<sub>3</sub>(CH<sub>2</sub>))U<sub>2</sub>H<sub>2</sub>]. In this complex the two methylene groups bind to different uranium centers. The tuck-over mode is binding of the Cp* methylene to a metal center elsewhere in the molecule rather than the one coordinated to that Cp* ligand.
Tuck-in complexes retain nucleophilicity at the methylene carbon. They can be activated by Lewis acids to generate active catalysts for use in ZieglerâÂÂNatta catalysis. The Lewis acid attaches to the CH<sub>2</sub> group, exposing a vacant site on the electrophilic Zr(IV) centre.