chemistry

Transition metal silyl complexes

Transition metal silyl complexes is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Transition metal silyl complexes rather than just read about it. In short: 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.

Key takeaways

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Reference excerpt

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. These complexes are a subset of organosilicon compounds.

Synthesis 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. 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.

From silyl halides The first silyl complexes were prepared by treatment of sodium cyclopentadienyliron dicarbonyl with trimethylsilyl chloride:

(C5H5)Fe(CO)2Na + Me3SiCl → (C5H5)Fe(CO)2SiMe3 + NaCl Although metal carbonyl anions are convenient reaction substrates, polar solvents promote nucleophilic attack on the carbonyl oxygen instead. In such cases, the result is instead a Fischer carbene that usually decomposes to a siloxane.

From hydrosilanes

Hydrosilanes oxidatively add to low-valent metal complexes to give silyl metal hydrides. Such species are assumed intermediates in hydrosilylation catalysis. Compact, electronegative substituents on the silicon favor the addition; hence a H–Si–I moiety will add across H–Si before Si–I. The process begins when the intact hydrosilane associates to the unsaturated metal center, affording an agostic σ-silane complex (see § Silane complexes). Alternatively, the hydrosilane may reduce another ligand. Diphenylsilylating the Petasis reagent eliminates methane (Me = CH3, Ph = C6H5):

2 (C5H5)2TiMe2 + 2 Ph2SiH2 → [(C5H5)2TiSiPh2]2 + 4 MeH Likewise, certain early transition metal hydrides react with hydrosilanes at high temperature, eliminating H2.

From disilanes Low valent metals insert into the Si-Si bond of disilanes. The main limitation of this reaction is the paucity of disilanes as reagents. Bis(silyl)mercury reagents behave similarly.

Acid-base metathesis Acidic metal hydrides can condense with silazanes, but the reverse-polarity reaction between a silane and an amino complex is not possible.

Silyl complexes with Si–Si bonds 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). Another example is the metalacycle derived from titanocene dichloride, (C5H5)2Ti(SiPh2)5.

Silene and disilene complexes 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.:

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.

Reactions

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. Geminal dihalides react with silylmetal anions to give a halide anion, a silyl halide, and a metal carbene complex. Insertions between the metal and silicon are hindered by the partial π bond formed through negative hyperconjugation. The process proceeds with alkenes and alkynes, 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.

Silane complexes

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

References

Worked examples

Example 1 — a first encounter with Transition metal silyl complexes

Start with the simplest possible case. Write down what Transition metal silyl complexes claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Transition metal silyl complexes before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Transition metal silyl complexes ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Transition metal silyl complexes

In research: Transition metal silyl complexes appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Transition metal silyl complexes in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Transition metal silyl complexes is common in secondary-school and first-year university syllabi. It links to neighbouring topics Organosilicon compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Transition metal silyl complexes outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

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