Comparing the ⇠ 90 -cases for both molecules, it is clear that the HOMOs of the [9]-and [8]cumulenes are closer to a cylindrical helix than that of the diphenyl-diyne. This underlines that MO helicality is not just a property of the torsion angle, but depends on the chemistry of the end-groups that ultimately controls if a linear carbon chain is cumulenic or polyynic. This lack of perfect helicality may be linked to the more significant bond-length alternation in the diyne. We observe the maximal twist at the single bonds as this is where the occupied frontier MOs are antibonding. Metallacumulenes and -ynes are another class of molecules that may exhibit helical MOs in the linear fragments of the molecules. Synthesis of long [n]cumulenes pose a practical challenge because they are rather unstable . However, by complexing the cumulenes with a transition metal, the reactivity of the cumulenylidene ligands can be brought down due to both electronic and steric effects. Since the first organometallic compounds with cumulene moieties were reported by Fischer et al. and Berke 58 , the synthesis of complexes with great structural variation continues to be an active field. Formally, the metallacumulene shown in Figure has C 1 -symmetry, and any MO-helicality will therefore not be symmetryprotected. Note that the metallacumulene illustrated in Figure has had the phenyl groups replaced with hydrogens to speed up calculations. From the HOMO illustrated in the bottom of Figure , it is clear that this MO is somewhat helical. Still, visual assessment of the helicality of this MO compared with the previous MOs is challenging. Again, we plot the cumulated sum of angles through the entire molecule (blue curve in Figure ). This helical MO deviates from a cylindrical helix as it exhibits the rounded step-like lineshape that we saw previously. We can also see that at the ruthenium center there is a jump in the evolution of the helical MO of approximately ⇡ radians. This jump is due to the inherent sign-change in the metal d -orbitals that couple to the carbon p-orbitals. As we also saw in the previous example, the cumulated sum along the chain provides us with a richer understanding of how the helicality changes compared with visual inspection of the MO.