ca01 only detailed ToC 1..24

90
2.6 Product Class 6:
Organometallic Complexes of Chromium, Molybdenum,
and Tungsten without Carbonyl Ligands
R. Poli and K. M. Smith
General Introduction
Almost all of the complexes described in this product class are air- and/or moisture-sensitive,
both as solids and in solution. Prior to use all solvents should be dried and distilled
under nitrogen or argon, and the compounds should be synthesized, handled, and stored
under an inert atmosphere using Schlenk or glovebox techniques. The bonds between
group 6 metals and carbon are often readily hydrolyzed, with the notable exception of
several alkylchromium(III) species. In general, these compounds are less sensitive to oxygen
when the metals are formally in the +6 oxidation state, and are thus incapable of being
oxidized further, although the extreme atmospheric sensitivity of molybdenum(VI)
ring-closing metathesis catalysts (Section 2.6.1.5.3) provides a striking exception to this
trend. [1]
Due in part to their hydrolytic instability, the toxicity of the organometallic complexes
described in this product class have generally not been investigated. A prominent
exception is dichlorobis(cyclopentadienyl)molybdenum(IV), which has been studied as an
antitumor agent. [2,3] Purely inorganic compounds of chromium(VI) are well-established
carcinogens, in contrast to the relatively low toxicity characteristic of chromium(III) species.
While chromium(III) compounds are not readily transported through cell walls, the
negative charge and tetrahedral structure of the chromate dianion makes it analogous to
the phosphate and sulfate ions, and so chromium(VI) is brought into the cell via nonspecific
anion transport channels. Chromium(VI) is then reduced to chromium(III) inside the
cell, leading to the DNA lesions responsible for the carcinogenic activity. [4] In the absence
of more detailed toxicity studies for organometallic group 6 complexes, care should be
taken when handling all the compounds in this product class.
2.6.1 Product Subclass 1:
Metal–Carbene Complexes
While group 6 complexes containing carbonyl ligands (Fischer-type) are most common
for chromium, those without carbonyl ligands (Schrock-type, also called alkylidene complexes)
are more typical of molybdenum and tungsten. Although a few chromium examples
are known, [5] our attention will be almost completely devoted to molybdenum and
tungsten systems. These complexes are generally found in high oxidation states (‡4) and
supported by electronegative, ð-donor ligands (alkoxo, amido, imido). These ligands have
the possibility of stabilizing low-coordination environments by ð-donation in excess of
the valence requirement (e.g., ó +2ð M”OorM”NR for oxo and imido derivatives), resulting
in tetrahedral species. Often, however, these complexes allow expansion of the coordination
sphere by formation of dimers (e.g., halide bridged) or by addition of a two-electron
donor with formation of five-coordinate and occasionally six-coordinate species, the
formation of which is more likely for tungsten than for molybdenum and when the metal
bears electron-withdrawing ligands. [6]
Group 6 metal–carbene complexes are most stable when devoid of â-hydrogen atoms
on the carbene ligand, the latter leading to decomposition by 1,2-H migration and formation
of alkene derivatives. [7] The carbene ligand usually bears hydrogen or alkyl substitu-