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Carbene radicals, in which the carbene is bonded to an open-shell metal with the carbene carbon possessing a radical character. Carbene radicals have features of both Fischer and Schrock carbenes, but are typically long-lived reaction intermediates. The "second generation" of the Grubbs catalysts for alkene metathesis features an NHC ligand.
A singlet carbene contains an empty p orbital and a roughly sp 2 hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product. [1] The mechanism for addition of a carbene to an alkene is a concerted [2+1] cycloaddition (see figure).
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Bonding scheme of carbene radical complexes as compared to Schrock and Fischer-type carbene complexes. Carbene radicals are a special class of organometallic carbenes.The carbene radical can be formed by one-electron reduction of Fischer-type carbenes using an external reducing agent, or directly upon carbene formation at an open-shell transition metal complex (in particular low-spin cobalt(II ...
Major/minor compositions are musical compositions that begin in a major key and end in a minor key (generally the parallel minor), specifying the keynote (as C major/minor).). This is a very unusual form in tonal music, [1] [2] although examples became more common in the nineteenth century
Carbene complexes have been synthesized from most transition metals and f-block metals, [2] using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. [1] The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. [2]
The net reaction gives a transition metal carbene complex: L n M≡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]
Carbyne molecules are generally found to be in electronic doublet states: the non-bonding electrons on carbon are arranged as one radical (unpaired electron) and one electron pair, leaving a vacant atomic orbital, rather than being a triradical (the quartet state).