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Mononuclear homoleptic permethyl neutral metal complexes

Any good reviews of this chemistry?

Bonding and Chemical Properties
Strength and polarity of M–C bond. Transmetalation reactions, and strength of various compound as methylating agents.

Group 1
Methyllithium-dominated discussion. Mention heavier analogues briefly.

Group 2
Discussion of dimethylmagnesium and synthesis from Grignard reagent by Schlenk equilibrium.

Mention polymeric crystal structure of BeMe2 and MgMe2.

Ca, Sr, and Ba compounds are rarely encountered, but have been synthesized.

p-Block Compounds
Don't bother with nonmetal 'methyl complexes' (e.g., neopentane, trimethylamine, dimethyl ether, etc.). Definitely include main group metals. Should metalloids be included? Boranes are important alkylating agents, so possibly BMe3 needs to be included.

If we include metalloids: AsMe5 and SbMe5 ; SbMe5

BiMe5 is an unstable violet solid that decomposes explosively upon heating to room temperature.

TeMe6 is a volatile white solid and is thermodynamically very stable. TeMe2 and TeMe4 are also known. TeMe6 appears to adopt an octahedral geometry by gas-phase electron diffraction. TeMe4 is a yellow-orange liquid that is unstable to light. It reacts explosively with oxygen, and decomposes above 100 °C.

d-Block Compounds
Methyl groups are common ligands that are ubiquitous in organometallic chemistry, and can display a wide range of chemistry based on the metal that it is bonded to. Early transition metals, especially d0 metals, tend to have quite polarized M–C bonds, with higher electron density localized on the methyl group. On the other hand, late transition metals are more electronegative (although not more so than carbon) so their M–C bonds have increased covalent character.

Generally, only groups 4–7 are able to give neutral binary homoleptic methyl complexes of any stability, and even many of these are prone to rapid decomposition at modest temperatures. However, methyl groups' lack of β-hydrogens lends stability to these structures by preventing intramolecular decomposition via β-hydride elimination. For example, this means methyl complexes are far more stable than analogous ethyl complexes.

Coordinatively unsaturated complexes are less stable than their saturated counterparts because the open sites allow bimolecular decomposition pathways, thought to occur through α-hydride abstraction and release of methane. Homoleptic complexes of bulkier alkyl chains (often neopentyl, benzyl, or trimethylsilylmethyl) can be stabler because the steric effects block access to the open coordination site and inhibit intermolecular reactions. An intermolecular decomposition pathway has been supported by computational chemistry in the case of Nb(CH3)5 and Ta(CH3)5, which decompose autocatalytically at –30 °C and room temperature, respectively. Additional coordinatively unsaturated complexes include Ti(CH3)4, which decomposes just over –78 °C, and Zr(CH3)4, which decomposes above –20 °C. It is postulated that the tetramethylhafnium complex could also be made, and possibly be stabler than its lighter analogues. Sometimes, neutral group 6 complexes can be made with an open coordination site, such as Mo(CH3)5, which is a paramagnetic turquoise solid that decomposes over –10 °C. The structure is very close to that of TaMe5.

Hexamethyl complexes are more stable, in general, and can sometimes be stored at or near room temperature. Interestingly, many homoleptic methyl complexes are not in the standard octahedral coordination geometry; instead, the complexes adopt trigonal prismatic structures, including Nb(CH3)6–, Ta(CH3)6–, W(CH3)6, and Re(CH3)6. Three neutral hexamethyl complexes have been made: Mo(CH3)6, W(CH3)6, and Re(CH3)6. Hexamethylmolybdenum is a volatile orange-brown crystalline solid thermally stable below ca. 10 °C, and has a trigonal prismatic structure. Hexamethyltungsten is a volatile red crystalline solid with m.p. ca. 30 °C that can be easily sublimed, and it is thermally unstable at room temperature. Hexamethylrhenium is a volatile paramagnetic green solid that is reasonably stable at 25 °C.

Homoleptic methyl complexes are also known for chromium and manganese, but their structures are often unknown or polymeric and they are all quite unstable.

Although they are not commonly considered to be transition metals, group 12 elements are also in the d-block and can form homoleptic methyl species: Zn(CH3)2, Cd(CH3)2, and Hg(CH3)2.

Safety
Some of these compounds must be handled with extreme caution due to their propensity to explosively decompose with elimination of methane gas even in the absence of air. Furthermore, many of these compounds are pyrophoric and must be handled in an oxygen-free environment, such as through Schlenk-line techniques or a glovebox.