title: Tuck-in complex
text: # Tuck-in complex
  
  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 $C_5–CH_2–M$ 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.
  
  ## Scope and bonding
  
  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 $Me_4Fv.$ A variety of complexes are known for $Me_4Fv$ and related ligands. In these complexes, the Fv can serve as a 4-electron or as a 6-electron ligand.
  
  ## Examples
  
  The original example proceeded via sequential loss of two equivalents of $H_2$ from decamethyltungstocene dihydride, $Cp*_2WH_2.$ The first dehydrogenation step affords a simple tuck-in complex:
  
      $(C_5Me_5)_2WH_2$ → $(C_5Me_5)(η^6-C_5Me_4CH_2)WH$ + $H_2$
  
  The second dehydrogenation step affords a double tuck-in complex:
  
      $(C_5Me_5)(η^6-C_5Me_4CH_2)WH$ → $(C_5Me_5)(η^7-C_5Me_3(CH_2)_2)W$ + $H_2$
  
  In organouranium chemistry, both tuck-in and tuck-over complexes are recognized, for example in the dihydrido diuranium complex $[Cp*_3(η^7-C_5Me_3(CH_2))U_2H_2].$ 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.
  
  ## Reactions
  
  Tuck-in complexes retain nucleophilicity at the methylene carbon. They can be activated by Lewis acids to generate active catalysts for use in Ziegler–Natta catalysis. The Lewis acid attaches to the $CH_2$ group, exposing a vacant site on the electrophilic Zr(IV) centre.
  
  