Organotungsten chemistry is the chemistry of chemical compounds with W-C bonds. It shares many similarities with organomolybdenum chemistry, while having more prevalent high oxidation states than the related organochromium chemistry. Notable applications include that in olefin/alkyne metathesis catalysis, and in arene activation. If the compound only has W-C bonds, is it, by definition, a form of tungsten carbide.
Carbonyl & cyanide complexes
Carbonyl complexes The simplest tungsten carbonyl complex is tungsten hexacarbonyl, most commonly prepared via reductive carbonylation (for instance, reaction of WCl6 and zinc powder under a CO atmosphere) of tungsten halides and similar compounds. Tungsten hexacarbonyl itself is able to catalyze alkene metathesis. Being volatile and easily decomposed, it is also widely used in the electron beam-induced deposition technique to deposit tungsten atoms. Reduction of the hexacarbonyl (in liquid ammonia with borohydride and sodium metal, respectively) yields the anionic carbonyl complexes [W2(CO)10]2- & [W(CO)4]4-. Also known are the complexes [W(CO)5]2- & [W3(CO)14]2-. Substitution of the carbonyl ligand can be facilitated thermally or photochemically, for instance, the reaction with cyclopentadienide to yield [CpW(CO)3]-, which can be further derivatized. A roundabout substitution method of first using nitriles to displace the carbonyls and then displacing the nitriles is also viable. Alkane complexes of W(CO)5 can be photochemically produced. A niche catalysis reaction utilizes the strong Lewis acidity of the W(CO)5 fragment, converting thiirane to the sulfur analogs of crown ethers. The other common reactivity of alkyl/aryl containing tungsten carbonyl complexes involve carbonyl insertion.
Isocyanide and cyanide complexes Isocyanide complexes W(CO)6-n(CNR)n (n = 1~3) are prepared via ligand substitution of tungsten hexacarbonyl, catalyzed by palladium oxide or cobalt dichloride. The reactivity regarding migratory insertion is analogous to that of carbonyl complexes. Of the cyanide complexes, [W(CN)8]n- (n = 3, 4) are notable for their photochemical and magnetic properties. The face capped cubic cluster compound Mn9[W(CN)8]6•24EtOH, for instance, has the largest known ground state spin value of S = 39/2 (as of 2011). Such complexes can also be used in constructing coordination polymers, such as {(Me3Sn)4[W(CN)8]}n. The coordination polymers are held together via cyanide bridges, with carbon coordinating the tungsten atoms while nitrogen coordinating the other central atoms.
Hydrocarbyl complexes
Alkyl complexes Simple alkyl complexes of tungsten, as those of molybdenum and chromium, are rather unstable. The simplest, hexamethyltungsten, has no molybdenum or chromium analogs. It is extremely reactive, detonating in air or even in vacuum. It is prepared with methylating reagents and WCl6, and further methylation into [WMe7]- or [WMe8]2- is possible when using methyllithium. Heteroatoms like oxygen can insert into the W-C bond, performing oxidation. WMe6 adopts the geometry of distorted trigonal prismatic, which may be attributed to a second-order Jahn-Teller distortion (for further details, see the article on hexamethyltungsten).
Stabilization of these compounds are possible via dimerization, as in the compound (Me3SiCH2)3W≡W(CH2SiMe3)3. Note that lack of beta hydrogen atoms are necessary to prevent beta-elimination. Neutral mononuclear complexes of different alkyl numbers are known, such as tetrabenzyltungsten (W(CH2Ph)4). For electron deficient alkyl tungsten complexes, one example that demonstrates their bonding interactions and reactivity is shown below:
Aryl complexes As with the alkyl tungsten complexes and most hydrocarbyl organometallics, aryl tungsten complexes can be prepared from tungsten halides and hydrocarbylating agents via transmetallation. The thermolysis of the complexes Cp*W(NO)(aryl)2 results in the loss of an arene and the formation of aryne complexes (similar reactions are observed for other hydrocarbyl ligands). The aryne complexes are unstable and readily activate other C-H bonds (for instance, in solvent molecules).
Vinyl complexes
Vinyl ligands have two different modes of coordination with tungsten atoms, as depicted: Synthesis is facilitated via transmetallation, the deprotonation of tungsten alkene complexes, nucleophilic addition to tungsten alkyne complexes, or alkyne insertion into W-H bonds. The isomerization of the η1 vinyl complexes into carbynes are possible via a [1,2]-hydrogen migration reaction from the alpha carbon, usually via η2 vinyl intermediates. Isomerization of the η2 vinyl complexes into allyl complexes are also known.
Alkynyl complexes
Alkynyl complexes of tungsten can be prepared via transmetallation or via the deprotonation of alkyne or carbene complexes of tungsten. An exotic method of preparation involves the reaction between [CpW(CO)3]- and CH2I2, forming the bridged complex [CpW(CO)3](C≡C)[CpW(CO)3] (along with side products). The main reactivity involves electrophilic attack on the beta-carbon (which forms vinylidene complexes), as explained in the resonance forms, and it is enhanced with the increasing electron density of the complex. Less common are electrophilic attack on the alpha carbon, which produces alkyne complexes, or electrophilic attack on the tungsten atom (as in the case when reacting with allylic halides) to produce allyl tungsten complexes. Alkynyl tungsten complexes, along with propargyl tungsten complexes, have applications as templates during synthesis of cyclic compounds like lactones. For instance:
Carbene and carbyne complexes
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