Transition metal alkyl complexes explained

Transition metal alkyl complexes are coordination complexes that contain a bond between a transition metal and an alkyl ligand. Such complexes are not only pervasive but are of practical and theoretical interest.^[1]

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.^[2]

200 px|thumb|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)°.^[3]

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

thumb|right|180 px|Hexamethyltungsten is an example of a "homoleptic" (all ligands being the same) metal alkyl complex.

Homoleptic metal alkyl complexes

example	comment
Ti(CH3)4	only observed as monoetherate, d^0^[4]
[Ti(CH<sub>3</sub>)<sub>5</sub>]^−	trigonal bipyramidal, d^0
[Ti<sub>2</sub>(CH<sub>3</sub>)<sub>9</sub>]^−	one bridging methyl ligand, d^0,d^0
[Zr(CH<sub>3</sub>)<sub>6</sub>]^−	trigonal prismatic, d^0^[5]
[Hf(CH<sub>3</sub>)<sub>6</sub>]^−	trigonal prismatic, d^0
[Nb(CH<sub>3</sub>)<sub>6</sub>]^−	d^0
[Ta(CH<sub>3</sub>)<sub>6</sub>]^−	d^0
Mo(CH3)5	d^1^[6]
	trigonal prismatic, d^0
[Mn(CH<sub>3</sub>)<sub>4</sub>]2-	d^5^[7]
[Mn(CH<sub>3</sub>)<sub>6</sub>]2-	d^3^[8]
[Re(CH<sub>3</sub>)<sub>6</sub>]	d^1
[Fe(CH<sub>3</sub>)<sub>4</sub>]^−	low-spin, d^5, square planar^[9]
[Co(CH<sub>3</sub>)<sub>4</sub>]^−	square planar, d^6^[10]
[Rh(CH<sub>3</sub>)<sub>6</sub>]^3-	d^6^[11]
[Ir(CH<sub>3</sub>)<sub>6</sub>]^3-	d^6
[Ni(CH<sub>3</sub>)<sub>4</sub>]^2-	d^8^[12]
[Pt(CH<sub>3</sub>)<sub>4</sub>]^2-	d^8^[13]
[Au(CH<sub>3</sub>)<sub>2</sub>]^−	d^10^[14]
[Au(CH<sub>3</sub>)<sub>4</sub>]^−	d^8
	d^10
	d^10
	d^10

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 Grignard reagents and organolithium compounds. Since many strong nucleophiles are also potent reductants, mildly nucleophilic alkylating agents are sometimes employed to avoid redox reactions. Organozinc compounds and 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:^[15]

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

Many metal alkyls are prepared by oxidative addition:^[16]

An example is the reaction of a Vaska's complex with methyl iodide.

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.^[17]

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 coordinatively unsaturated, then the complex can undergo beta-hydride elimination to form a metal-alkene complex:These conversions are assumed to proceed via the intermediacy of agostic interactions.

Catalysis

Many homogeneous catalysts operate via the intermediacy of metal alkyls. These reactions include hydrogenation, hydroformylation, alkene isomerization, and olefin polymerization. It is assumed that the corresponding heterogeneous reactions also involve metal-alkyl bonds.^[18]

Notes and References

1. Book: Oliveira. José. Elschenbroich. Christoph. Organometallics. 2006. Wiley-VCH-Verl.. Weinheim. 978-3-527-29390-2. 3., completely rev. and extended.
2. Kleinhenz, S. . Pfennig, V. . Seppelt, K. . Preparation and Structures of [W(CH₃)₆], [Re(CH₃)₆], [Nb(CH₃)₆]⁻, and [Ta(CH₃)₆]⁻. Chemistry – A European Journal. 1998. 4. 9. 1687–1691. 10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R.
3. John Bacsa . Felix Hanke . Sarah Hindley . Rajesh Odedra . George R. Darling . Anthony C. Jones . Alexander Steiner . The Solid State Structures of Dimethylzinc and Diethylzinc . . 2011 . 50. 49 . 11685–11687. 10.1002/anie.201105099. 21919175 . 3326375.
4. 10.1002/(SICI)1521-3765(19991203)5:12<3573::AID-CHEM3573>3.3.CO;2-F. Preparation and Structures of Methyltitanium Compounds. 1999. Kleinhenz. Sven. Seppelt. Konrad. Chemistry - A European Journal. 5. 12. 3573–3580.
5. 10.1021/ja00193a061. Are d0 ML6 complexes always octahedral? The x-ray structure of trigonal-prismatic [Li(tmed)]2[ZrMe6]. 1989. Morse. Paige M.. Girolami. Gregory S.. Journal of the American Chemical Society. 111. 11. 4114–4116.
6. Chemical Communications. 12. Pentamethylmolybdenum . Beatrice Roessler . Sven Kleinhenza . Konrad Seppelt . 10.1039/b000987n. 2000. 1039–1040.
7. 10.1021/ja00226a049. Permethylmanganates. Synthesis and characterization of divalent [MnMe42-], trivalent [MnMe52-], and tetravalent [MnMe62-]. 1988. Morris. Robert J.. Girolami. Gregory S.. Journal of the American Chemical Society. 110. 18. 6245–6246. 22148809.
8. 10.1021/om00049a047. High-valent organomanganese chemistry. 1. Synthesis and characterization of manganese(III) and -(IV) alkyls. 1991. Morris. Robert J.. Girolami. Gregory S.. Organometallics. 10. 3. 792–799.
9. 10.1021/ja5080757. Isolation and Characterization of a Tetramethyliron(III) Ferrate: An Intermediate in the Reduction Pathway of Ferric Salts with MeMg Br. 2014. Al-Afyouni. Malik H.. Fillman. Kathlyn L.. Brennessel. William W.. Neidig. Michael L.. Journal of the American Chemical Society. 136. 44. 15457–15460. 25333789. 4227835.
10. 10.1021/acs.organomet.9b00387. Isolation and Characterization of a Homoleptic Tetramethylcobalt(III) Distorted Square-Planar Complex. 2019. Carpenter. Stephanie H.. Brennessel. William W.. Neidig. Michael L.. Organometallics. 38. 19. 3486–3489. 203144530. free.
11. 10.1039/C39890001436. Homoleptic methyl compounds of rhodium and iridium(III). X-Ray crystal structures of tetramethylethylenediamine lithium hexamethyl-rhodate(III) and -iridate(III). 1989. Hay-Motherwell. Robyn S.. Wilkinson. Geoffrey. Hussain. Bilquis. Hursthouse. Michael B.. Journal of the Chemical Society, Chemical Communications. 19. 1436.
12. 10.1021/om000960u. Dinuclear [RNi(oxam)NiR] Complexes (Oxam = N1,N2-bis(2-pyridylmethyl)-N3,N4-bis(2,4,6-trimethylphenyl)oxalamidinate; R = Me, Ph, C⋮CH, C⋮CPH): Reactions of the Methyl Complex and Formation of [Li(THF)]2Li2Ni2Me8 and [Li(THF)]4Ni2Me8. 2001. Walther. Dirk. Stollenz. Michael. Görls. Helmar. Organometallics. 20. 20. 4221–4229.
13. 10.1002/(SICI)1521-3749(199911)625:11<1904::AID-ZAAC1904>3.0.CO;2-2. Ungewöhnliche Molekülstrukturen für Platin(II)-at-Komplexe. 1999. Wyrwa. R.. Görls. H.. Zeitschrift für Anorganische und Allgemeine Chemie. 625. 11. 1904–1907.
14. 10.1021/om990043s. X-ray Crystal Structures and the Facile Oxidative (Au−C) Cleavage of the Dimethylaurate(I) and Tetramethylaurate(III) Homologues. 1999. Zhu. Dunming. Lindeman. Sergey V.. Kochi. Jay K.. Organometallics. 18. 11. 2241–2248.
15. King. B. . Applications of Metal Carbonyl Anions in the Synthesis of Unusual Organometallic Compounds . Acc. Chem. Res. . 1970 . 3 . 12 . 417–427 . 10.1021/ar50036a004.
16. Book: Hartwig, J. F. . Organotransition Metal Chemistry, from Bonding to Catalysis . University Science Books . New York . 2010 . 978-1-891389-53-5.
17. Scherer, W. . McGrady, G. S. . Agostic Interactions in d⁰ Metal Alkyl Complexes. Angew. Chem. Int. Ed.. 2004. 43. 14. 1782–1806. 10.1002/anie.200200548. 15054779.
18. Forbes. Jeffrey G.. Gellman. Andrew J.. 1993-07-01. The .beta.-hydride elimination mechanism in adsorbed alkyl groups. Journal of the American Chemical Society. 115. 14. 6277–6283. 10.1021/ja00067a048. 0002-7863.