ca01 only detailed ToC 1..24

2.6.3 Metal–s-Alkyl and –s-Aryl Homoleptic Complexes 111
strong tendency to decompose by Æ-hydrogen elimination. The tetrahedral MR 4 complexes
can only be obtained with sterically demanding ligands and are unexpectedly unreactive.
While M(III) complexes with three unpaired electrons are common for chromium,
no unambiguous example has been reported for molybdenum or tungsten. Delicate
equilibria may exist between different species as a function of the size of R, the nature of
the counterion, or even the solvent. For example, orange-yellow hexaphenylchromate(III),
blue-green pentaphenylchromate(III), and cherry-red tetraphenylchromate(III) have been
isolated under different conditions. [88–90] Neutral trialkylchromium(III) compounds are
known only with very bulky R groups, e.g. bis(trimethylsilyl)methyl [CH(TMS) 2]. [91] Unambiguous
M(II) complexes exist only for chromium. These are either paramagnetic (S =2)
and square planar tetraalkylchromate(2–) monomers or diamagnetic tetraanionic dimers.
The choice of nuclearity is highly dependent on the counterion and the solvent; for example,
the lithium salt of tetramethylchromate(II) adopts a monomeric or a dimeric structure
depending on whether the lithium cations are surrounded by tetramethylethylenediamine
or by diethyl ether molecules. [92]
The relatively high polarity of the metal-alkyl bonds makes these derivatives rather
sensitive toward proton sources including water, especially when the latter can be
activated by coordination to the metal center. Thus, these compounds must generally be
synthesized and handled under scrupulously dry conditions.
Synthesis of Product Subclass 3
2.6.3.1 Method 1:
By Transmetalation
The only synthetic method that allows access to homoleptic alkyl and aryl complexes of
group 6 metals is the transmetalation reaction (Scheme 21). Competitive electron-transfer
processes and equilibria of association/dissociation of the alkylating reagent are often
the reasons for the moderate yields usually associated with these syntheses. There is no
general rule as to the best reagents and conditions to use for a specific product. Lithium
and magnesium reagents are typically used, although other alkylating sources have also
been employed. This is exemplified by the synthesis of hexamethyltungsten(VI) (54) [93,94]
and tetracyclohexylchromium(IV) (55). [95] As a general rule, lithium reagents are more reactive
than Grignard reagents, but have a greater tendency to engage in single-electrontransfer
(SET) side reactions. The choice of solvent may also be a determinant factor.
Tetrahydrofuran, diethyl ether, and toluene are the most commonly used solvents, the
latter disfavoring SET processes. The group 6 metal sources are typically the halides, but
the alkoxides have also been used, the latter usually reducing the SET reactivity. Many
neutral metal(IV) compounds are obtained from lower oxidation state precursors in