The diazo-coupling reaction dates back to the 1850s (and a close association with Imperial College <em>via</em> the first professor of chemistry there, August von Hofmann) and its mechanism was much studied in the heyday of physical organic chemistry.[1] Nick Greeves, purveyor of the excellent ChemTube3D site, contacted me about the transition state (I have commented previously on this aspect of aromatic electrophilic substitution). ChemTube3D