Transition metal carbyne complex

Transition metal carbyne complexes are most common for the early transition metals, especially niobium, tantalum, molybdenum, tungsten, and rhenium. They can also have low-valence metals as well as high-valence metals.

The first Fischer carbyne complex was reported in 1973.^[4] Two years later in 1975, the first "Schrock carbyne" was reported.^[5]

Many high-valent carbyne complexes have since been prepared, often by dehydrohalogenation of carbene complexes. Alternatively, amino-substituted carbyne ligands sometimes form upon protonation of electron-rich isonitrile complexes. Similarly, O-protonation of μ₃-CO ligands in clusters gives hydroxycarbyne complexes. Vinyl ligands have been shown to rearrange into carbyne ligands. Addition of electrophiles to vinylidene ligands also affords carbyne complexes.^[3]

Bridging alkylidyne ligands in cluster compounds

Some metal carbynes dimerize to give dimetallacyclobutadienes. In these complexes, the carbyne ligand serves as a bridging ligand.

Several cluster-bound carbyne complexes are known, typically with CO ligands. These compounds do not feature MC triple bonds; instead the carbyne carbon is tetrahedral. Tricobalt derivatives are prepared by treating cobalt carbonyl with haloforms:^[6]

2 HCBr₃ + 9⁄2 Co₂(CO)₈ → 2 HCCo₃(CO)₉ + 18 CO + 3 CoBr₂

Monomeric metal carbyne complexes exhibit fairly linear M–C–R linkages according to X-ray crystallography. The M–C distances are typically shorter than the M–C bonds found in metal carbenes. The bond angle is generally between 170° and 180°^[8] Analogous to Fischer and Schrock carbenes; Fischer and Schrock carbynes are also known. Fischer carbynes usually have lower oxidation state metals and the ligands are π-accepting/electron-withdrawing ligands. Schrock carbynes on the other hand typically have higher oxidation state metals and electron-donating/anionic ligands. In a Fischer carbyne the C-carbyne exhibits electrophilic behavior while Schrock carbynes display nucleophilic reactivity on the carbyne carbon^[9] Carbyne complexes have also been characterized by many methods including infrared Spectroscopy, Raman spectroscopy.^[10] Bond lengths, bond angles and structures can be inferred from these and other analytical techniques.

Metal carbyne complexes also exhibit a large trans effect, where the ligand opposite the carbyne is typically labile.

Hexa(tert-butoxy)ditungsten(III) is a catalyst for alkyne metathesis.^[11] The catalytic cycle involves an carbyne intermediate.^[12]

Some carbyne complexes react with electrophiles at C-carbyne followed by association of the anion. The net reaction gives a transition metal carbene complex:

L_nM≡CR + HX → L_n(X)M=CHR

These complexes can also undergo photochemical reactions.

In some carbyne complexes, coupling of the carbyne ligand to a carbonyl is observed. Protonation of the carbyne carbon and conversion of the carbyne ligand into a π-allyl.^[13]

A sulfur-based main group analog of a carbyne complex has been prepared by Seppalt and coworkers.^[14] The compound, trifluoro(2,2,2-trifluoroethylidyne)-λ⁶-sulfurane, F₃C–C≡SF₃, prepared by dehydrofluorination of F₃C–CH=SF₄ or F₃C–CH₂–SF₅, is an unstable gas that readily undergoes dimerization to form trans-(CF₃)(SF₃)C=C(CF₃)(SF₃) at above –50 °C.

• Mayr, A.; Bastos, C. M. (1992). Coupling Reactions of Terminal Two-Faced π Ligands and Related Cleavage Reactions. Progress in Inorganic Chemistry. Vol. 40. pp. 1–98. doi:10.1002/9780470166413.ch1. ISBN 9780470166413. {{cite book}}: |journal= ignored (help)