Agostic interaction

Source: Wikipedia, the free encyclopedia.

In

alkylidene
, and polyenyl ligands.

History

The term agostic, derived from the

classicist Jasper Griffin, to describe this and many other interactions between a transition metal and a C−H bond. Often such agostic interactions involve alkyl or aryl groups that are held close to the metal center through an additional σ-bond.[2][3]

Short interactions between hydrocarbon substituents and coordinatively unsaturated metal complexes have been noted since the 1960s. For example, in tris(triphenylphosphine) ruthenium dichloride, a short interaction is observed between the ruthenium(II) center and a hydrogen atom on the ortho position of one of the nine phenyl rings.[4] Complexes of borohydride are described as using the three-center two-electron bonding model.

Mo(PCy3)2(CO)3, featuring an agostic interaction

The nature of the interaction was foreshadowed in main group chemistry in the structural chemistry of trimethylaluminium.

Characteristics of agostic bonds

Agostic interactions are best demonstrated by

NMR signal that is shifted upfield from that of a normal aryl or alkane, often to the region normally assigned to hydride
ligands. The coupling constant 1JCH is typically lowered to 70–100 Hz versus the 125 Hz expected for a normal sp3 carbon–hydrogen bond.

dmpe), highlighting an agostic interaction between the methyl group and the Ti(IV) center.[5]

Strength of bond

On the basis of experimental and

Ziegler–Natta catalysis
the highly electrophilic metal center has agostic interactions with the growing polymer chain. This increased rigidity influences the stereoselectivity of the polymerization process.

Related bonding interactions

sigma complex derived from (MeC5H4)Mn(CO)3 and triphenylsilane.[7]

The term agostic is reserved to describe two-electron, three-center bonding interactions between carbon, hydrogen, and a metal. Two-electron three-center bonding is clearly implicated in the complexation of H2, e.g., in W(CO)3(PCy3)2H2, which is closely related to the agostic complex shown in the figure.[8] Silane

binds to metal centers often via agostic-like, three-centered Si┄H−M interactions. Because these interactions do not include carbon, however, they are not classified as agostic.

Anagostic bonds

Certain M┄H−C interactions are not classified as agostic but are described by the term anagostic. Anagostic interactions are more electrostatic in character. In terms of structures of anagostic interactions, the M┄H distances and M┄H−C angles fall into the ranges 2.3–2.9 Å and 110°–170°, respectively.[2][9]

Function

Agostic interactions serve a key function in alkene polymerization and stereochemistry, as well as migratory insertion.

References

  1. doi:10.1351/goldbook.AT06984
  2. ^
    doi:10.1016/0022-328X(83)85065-7
    ..
  3. PMID 17442749.Open access icon
  4. doi:10.1021/ic50028a002
    .
  5. doi:10.1039/dt9860001629
    .
  6. doi:10.1021/om050489a
    .
  7. ISBN 9780120311538
    .
  8. ISBN 978-0-306-46465-2
    .
  9. doi:10.1021/om9608364
    .

External links
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