![]() Carbenes have a formally vacant p(π) orbital that can accept electrons in donor-acceptor complexes M⇄CR 2 whereas carbones are double (σ and π) donors in complexes ⇇CL 2. Thus, carbones CL 2 are four-electron donor ligands whereas carbenes CR 2 are two-electron donors. One type of ligand are carbones CL 2, which are carbon(0) compounds with two dative bonds to a carbon atom in the excited 1D state L→ C ¯ ‾ ←L where the carbon atom retains its four valence electrons as two lone pairs that can serve as four-electron donors. This review deals with transition metal complexes with metal-carbon bonds to two types of ligands, which have only recently been isolated and theoretically studied. Fischer-type carbene and carbyne complexes are best described in terms of dative bonds following the Dewar–Chatt–Duncan (DCD) model ⇄CR 2 and CR (+), whereas Schrock-type alkylidenes and alkylidynes are assumed to have electron-sharing double and triple bonds =CR 2 and ≡CR. Two types of compounds with metal-carbon double or triple bonds having different types of bonds are generally distinguished, which are named after the people who isolated them first. ![]() ![]() Molecules with a =CR 2 double bond (carbene complexes) or a ≡CR triple bond (carbyne complexes) were synthesized much later. Molecules with M-C single bonds are already known since 1849 when Frankland reported the accidental synthesis of diethyl zinc while attempting to prepare free ethyl radicals. Transition metal compounds with metal-carbon bonds are the backbone of organometallic chemistry. ![]()
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