As the C-O bond formation proceeds (structure III), their basins separatrix value increases to 0.74, close to the 0.80 value of the two attractors. No covalent bond is formed yet as the two attractors are still separated. At the product state (structure IV) where the C•••C distance is 1.85 Å, attr2 and attr3 merge into a single attractor between C u2 and C u3 , with the value of 0.88, accounting for about 2 electrons. From the ELF perspective, it is clear that C-C bond formed. However, this new attractor is slightly displaced out of the bond line and the ELF shape does not correspond to any single, double or triple typical bond shape, which is noticeably consistent with the particularly stressed geometry this bond has to fit in (see Supplementary Information, Fig. ).