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13.6.4: Carbido and Cumulene Complexes

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    Carbido ligand: One single carbon

    A metal carbido (aka carbide, or simply carbon) complex is a metal complex that contains the ligand, carbon (C). Most molecular carbido complexes are clusters, usually featuring carbide as a six-fold bridging ligand. Examples include \(\ce{[Rh6C(CO)15]^2-}\), and \(\ce{[Ru6C(CO)16]^2-}\). The iron carbonyl carbides exist not only in the encapsulated carbon (\(\ce{[Fe6C(CO)16]^2-}\)) but also with exposed carbon centres as in \(\ce{Fe5C(CO)15}\) and \(\ce{Fe4C(CO)13}\).

    Figure \(\PageIndex{1}\): Structure of \(\ce{Fe5C(CO)15}\) containing a carbido bridging five iron centers. (Public domain)

    In rare cases, carbido ligands are terminal ligands (Figure \(\PageIndex{2}\), left). Although it is tempting to extend the model of metal-carbon bonds from the alkyl (\(\ce{M-C}\)), carbene (\(\ce{M=C}\), and carbyne (\(\ce{M \bond{#} C}\)) ligands and evoke the idea of a quadruple bond between metal and carbon in the case of a carbido, experimental data is more consistent with a triple bonded to a metal ion ((\(\ce{M \bond{#} C_1}\))). For example, the metal-carbon bond in the \(\ce{RuC(PCy3)2Cl2}\) complex is similar to the bond length of other complexes with \(\ce{C \bond{3} M}\) (Figure \(\PageIndex{2}\)).

    Figure \(\PageIndex{2}\): Carbido-metal bonds are similar in length to the \(\ce{C \bond{3} M}\) of metal carbynes. Cy = cyclohexane, Bz = benzyl. (CC-BY-SA; Kathryn Haas)


    Cumulene ligands: Chains of carbon atoms with consecutive double bonds

    Chains of carbon atoms that have consecutive (cumulated) double bonds are intriguing ligands for metal complexes due to their potential utility as molecular wires in nano or optical devices. The first reported complex containing a vinylidene ligand was \(\ce{Ph2C2Fe2(CO)8}\). The first monometallic vinylidene complex was \(\ce{(C5H5)Mo(P(C6H5)3)(CO)2[C=C(CN)2]Cl}\). Just as in the case of conjugated pi systems, the longer the length of a cumulated diene, the closer the HOMO/LUMO gap, and thus the more conductive. To date, the longest cumulenylidene ligand reported is heptahexaenylidene (Figure \(\PageIndex{3}\)).

    Figure \(\PageIndex{3}\): Structure of a metal complex with a heptahexaenylidene ligand. (CC-BY-SA; Kathryn Haas)


    This page titled 13.6.4: Carbido and Cumulene Complexes is shared under a CC BY-SA license and was authored, remixed, and/or curated by Kathryn Haas.

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