Following from much discussion over the last decade about the nature of C2, a diatomic molecule which some have suggested sustains a quadruple bond between the two carbon atoms, new ideas are now appearing for molecules in which such a bond may also exist between carbon and a transition metal atom. A suggested, albeit hypothetical example was C⩸Fe(CO)3[1]. Iron has a [Ar].3d6.4s2 electronic configuration and if we ionise to balance a C4- ligand, the iron becomes formally FeVI or [Ar].3d4. By adding 14 electrons deriving from the seven “bonds” to the 3d4, including a quadruple count from carbon, the Fe formally completes its 18-electron valence shell, as also found in e.g. Ferrocene.
References
- A.J. Kalita, S.S. Rohman, C. Kashyap, S.S. Ullah, and A.K. Guha, "Transition metal carbon quadruple bond: viability through single electron transmutation", Physical Chemistry Chemical Physics, vol. 22, pp. 24178-24180, 2020. http://dx.doi.org/10.1039/d0cp03436c