ca01 only detailed ToC 1..24

112 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
Hexamethyltungsten(VI) (54): [94]
CAUTION: Hexamethyltungsten(VI) is known to decompose explosively. Proper safety precautions
should be taken during its synthesis, storage, and handling.
A 50-mLglass container with two openings was attached to a vacuum line, and WF 6 (1.1 g,
3.7 mmol) and pentane (10 mL) were condensed into it in vacuo and with cooling (liq N 2).
A 1 M soln of ZnMe 2 in heptane (11.5 mL) was slowly added dropwise at –788C. The mixture
was stirred at –358C for 2 d, filtered at –10 8C, and concentrated in vacuo at –708Cto
afford an orange soln. The yield (53%) was determined by the quantitative reaction of 54
with NO and weighing the resulting product [WMe 4{ON(Me)NO} 2].
2.6.4 Product Subclass 4:
Metal–ó-Alkyl and –ó-Aryl Non-homoleptic Complexes
Non-homoleptic (heteroleptic) complexes containing alkyl and aryl ligands are much
more common, exist in a wider variety of formal oxidation states and coordination environments,
and are more versatile in organic synthesis than the homoleptic complexes.
The use of ancillary ligands with strong ð-donating properties increases the relative stability
of high oxidation state derivatives. Thus, M(V) and M(VI) (M = Cr, Mo) alkyl and aryl
complexes exist when supported by oxo, imido, or nitrido ligands, [96–99] whereas no homoleptic
counterparts are known. Non-homoleptic tungsten(VI) complexes are more common
and stable than the homoleptic ones.
Like the homoleptic complexes (Section 2.6.3), the non-homoleptic complexes tend
to decompose more readily when they bear hydrogen atoms on the â-position, via the
ubiquitous â-hydrogen elimination pathway. As this pathway necessitates an empty metal
orbital cis relative to the alkyl group and a coplanar transition state, stable complexes
with â-hydrogen-bearing alkyl ligands may only be obtained when one or more of the
above requirements are not met. [100] In particular, these compounds may be isolated
when each valence-shell metal orbital is occupied by at least one electron and when the
ligands cis to the alkyl group do not easily dissociate. Complexes whose alkyl substituents
bear hydrogen atoms on the Æ-position may also decompose, this process being especially
favored for high-valent molybdenum and tungsten complexes, providing good synthetic
methods for carbene and carbyne complexes (see Sections 2.6.1 and 2.6.2, respectively).
Other methods of decomposition are associated with intramolecular C-H bond activation
of ancillary ligands, which is promoted by the metal electron richness, e.g. see
Scheme 22. [101] Finally, homolytic cleavage of the metal-alkyl bond may occur with production
of radicals and reduced metal complexes. Aryl derivatives are more robust than
the alkyl complexes toward this decomposition pathway.
Scheme 22 Decomposition of Alkyl Complexes by Alkane Elimination [101]
Mo
Me3P Me
Me 3P PMe 3
benzene-d6, >40
− CH4 oC Me3P
Mo
Me3P P
Me2
Like the homoleptic complexes, the transmetalation reaction represents the most convenient
entry to alkyl and aryl non-homoleptic complexes of group 6 metals. The metal-alkyl
and metal-aryl bonds may also be formed, however, by oxidative addition reactions.