Transition metal silyl complexes

In chemistry, transition metal silyl complexes describe coordination complexes in which a transition metal is bonded to an anionic silyl ligand, forming a metal-silicon sigma bond. This class of complexes are numerous and some are technologically significant as intermediates in hydrosilylation.^{[1][2]: 246} These complexes are a subset of organosilicon compounds.

Silyl halides and hydrides easily add oxidatively to "low-valent, electron-rich complexes". Other reagents for oxidative additions are rare, and typically require a strained bond for the metal to insert into.^[2]: 249

Electron-poor complexes form when a silanide displaces an X-type ligand, and often form oligomeric ring clusters. The complexes are extremely reactive with oxygen, and must be investigated air-free. If they also bear organyl ligands, the complex may rearrange to an organosilane and a hydride ligand.^{[2]: 254–255}

Beyond simple ligands like SiR₃–, silyl ligands with Si-Si bonds are known. (C₅H₅)Fe(CO)₂-SiMe₂SiPh₃ is one example (Me = CH₃, Ph = C₆H₅).^[4] Another example is the metalacycle derived from titanocene dichloride, (C₅H₅)₂Ti(SiPh₂)₅.^[5]

Compounds containing a 3-membered metal-silicon-carbon ring are formally η² complexes of silenes, although they are not prepared from such unstable precursors. Instead, silenes are produced from Grignard or Barbier reagents, e.g.:^{[2]: 332–336}

Cp*(PMe₃)IrClMe + ClMgCH₂SiPh₂H → Cp*(PMe₃)Ir(η²-SiR′₂CH₂) + MgCl₂ + CH₄

Cp₂W(Cl)(CH₂SiMe₂Cl) + Mg → Cp₂W(η²-SiR′₂CH₂) + MgCl₂

Disilene complexes are typically produced by dehydrogenation of the corresponding hydrodisilanes.^{[2]: 337–340}

Electron-rich metal complexes undergo nucleophilic attack at silicon, and Brønsted acids usually convert silyl complexes to hydride complexes. The product may eliminate with retention or inversion of stereochemistry at silicon, or may form a 3-membered ring in which silicon is hypercoordinate.^{[2]: 273–274}

Geminal dihalides react with silylmetal anions to give a halide anion, a silyl halide, and a metal carbene complex.^{[2]: 346–347}

Insertions between the metal and silicon are hindered by the partial π bond formed through negative hyperconjugation. The process proceeds with alkenes and alkynes,^{[2]: 276–280} possibly through [2+2] addition to form a metallasilacyclobutane intermediate.

Ketones and aldehydes react extensively with metal silanes, producing either M–C–O–Si or Si–C–O–M, depending on the reagents.^{[2]: 280–283}

Transition metal silane complexes are coordination compounds containing hydrosilane ligands. An early example is (MeC₅H₄)Mn(CO)₂(η²-HSiPh₃) (Ph = C₆H₅).^[7]

The bonding in silane sigma complexes is similar to that invoked in agostic interactions. The metal center engages the Si-H entity via a 3-center, 2-electron bond. It is widely assumed that these sigma complexes are intermediates in the oxidative addition of hydrosilanes to give metal silyl hydrides. This transformation is invoked in hydrosilylation catalysis.

Evidence for sigma-silane complexes is provided by proton NMR spectroscopy. For (MeC₅H₄)Mn(CO)₂(η²-HSiPh₃), J(²⁹Si,¹H) = 65 Hz compared to 180 Hz in free diphenylsilane. In silyl hydride complexes, the coupling in about 6 Hz. Neutron diffraction studies reveal a Si-H distance of 1.802(5) Å in the corresponding η²-HSiFPh₂ complex vs 1.48 Å in free HSiFPh₂. Elongated Si-H bonds are characteristic of these sigma complexes.^[8]