Transition metal alkyl complexes

Source: Wikipedia, the free encyclopedia.
Vitamin B12 is a naturally occurring metal-alkyl complex.

Transition metal alkyl complexes are

alkyl ligand. Such complexes are not only pervasive but are of practical and theoretical interest.[1][2]

Scope

Most metal alkyl complexes contain other, non-alkyl ligands. Great interest, mainly theoretical, has focused on the homoleptic complexes. Indeed, the first reported example of a complex containing a metal-sp3 carbon bond was the homoleptic complex diethylzinc. Other examples include hexamethyltungsten, tetramethyltitanium, and tetranorbornylcobalt.[3]

Structure of diethylzinc. The Zn-C bonds measure 194.8(5) pm, while the C-Zn-C angle is slightly bent with 176.2(4)°.[4]

Mixed ligand, or heteroleptic, complexes containing alkyls are numerous. In nature, vitamin B12 and its many derivatives contain reactive Co-alkyl bonds.

Hexamethyltungsten is an example of a "homoleptic" (all ligands being the same) metal alkyl complex.[3]
Homoleptic metal alkyl complexes
example comment
Ti(CH3)4 only observed as monoetherate, d0[5]
[Ti(CH3)5]− trigonal bipyramidal, d0[5]
[Ti2(CH3)9]− one bridging methyl ligand, d0,d0[5]
[Zr(CH3)6]− trigonal prismatic, d0[6][5]
[Hf(CH3)6]− trigonal prismatic, d0[6]
[Nb(CH3)6]− d0[3][6]
[Ta(CH3)6]− d0[3]
Mo(CH3)5 d1[7]
W(CH3)6 trigonal prismatic, d0[3]
[Mn(CH3)4]2- d5[8]
[Mn(CH3)6]2- d3[9]
[Re(CH3)6] d1[3]
[Fe(CH3)4]− low-spin, d5, square planar[10]
[Co(CH3)4]− square planar, d6[11]
[Rh(CH3)6]3- d6[12]
[Ir(CH3)6]3- d6[12]
[Ni(CH3)4]2- d8[13]
[Pt(CH3)4]2-[12] d8[14]
[Au(CH3)2]− d10[15]
[Au(CH3)4]− d8[15]
Zn(CH3)2 d10
Cd(CH3)2 d10
Hg(CH3)2 d10

Preparation

Metal alkyl complexes are prepared generally by two pathways, use of alkyl nucleophiles and use of alkyl electrophiles. Nucleophilic sources of alkyl ligands include

organoaluminium compounds
are such milder reagents.

Electrophilic alkylation commonly starts with low valence metal complexes. Typical electrophilic reagents are

alkyl halides. Illustrative is the preparation of the methyl derivative of cyclopentadienyliron dicarbonyl anion:[16]

CpFe(CO)2Na + CH3I → CpFe(CO)2CH3 + NaI

Many metal alkyls are prepared by oxidative addition:[2]

An example is the reaction of a

methyl iodide
.

dmpe), highlighting an agostic interaction between the methyl group and the Ti(IV) center.[17]

Agostic interactions and beta-hydride elimination

Some metal alkyls feature agostic interactions between a C-H bond on the alkyl group and the metal. Such interactions are especially common for complexes of early transition metals in their highest oxidation states.[18]

One determinant of the kinetic stability of metal-alkyl complexes is the presence of hydrogen at the position beta to the metal. If such hydrogens are present and if the metal center is

beta-hydride elimination
to form a metal-alkene complex:

These conversions are assumed to proceed via the intermediacy of agostic interactions.

Catalysis

Many

olefin polymerization. It is assumed that the corresponding heterogeneous reactions also involve metal-alkyl bonds.[19]

References

  1. ISBN 978-3-527-29390-2
    .
  2. ^
    ISBN 978-1-891389-53-5
    .
  3. ^
    doi:10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R
    .
  4. PMID 21919175
    .
  5. ^
    doi:10.1002/(SICI)1521-3765(19991203)5:12<3573::AID-CHEM3573>3.3.CO;2-F
    .
  6. ^
    doi:10.1021/ja00193a061
    .
  7. doi:10.1039/b000987n
    .
  8. PMID 22148809
    .
  9. doi:10.1021/om00049a047
    .
  10. PMID 25333789
    .
  11. S2CID 203144530
    .
  12. ^
    doi:10.1039/C39890001436
    .
  13. doi:10.1021/om000960u
    .
  14. doi:10.1002/(SICI)1521-3749(199911)625:11<1904::AID-ZAAC1904>3.0.CO;2-2
    .
  15. ^
    doi:10.1021/om990043s
    .
  16. doi:10.1021/ar50036a004
    .
  17. doi:10.1039/dt9860001629
    .
  18. PMID 15054779
    .
  19. ISSN 0002-7863
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Transition_metal_alkyl_complexes&oldid=1217551180"