Group 4 metallocene mono- and bis-σ-alkynyl complexes of the type L 2 M(σ-C≡CR) and L 2 M(σ-C≡CR) 2 with M=titanium and zirconium in the oxidation states +3 and +4 and L=Cp (η 5 -cyclopentadienyl) and Cp * (η 5 -pentamethylcyclopentadienyl) are important compounds for stoichiometric and catalytic C C single bond coupling and cleavage reactions. Detailed investigations show five-membered metallacyclocumulenes L 2 M(η 4 -1,2,3,4-RC 4 R) as the key intermediates in both reactions of a C C single bond cleavage of different 1,4-substituted 1,3-butadiynes RC≡C C≡CR to alkynyl groups and the opposite reaction of C C single bond formation starting from alkynyl groups under the formation of 1,4-substituted 1,3-butadiynes. Depending on different metals M and ligands L, coupling or cleavage is favoured. Combination of both reactions offered the first C C single bond metathesis in homogeneous solution, which is photocatalyzed and titanocene-mediated. It proceeds via titanocene-mono-alkynyl complexes, which are interesting species also for other stoichiometric and catalytic C C coupling reactions. Some similarities regarding the σ-to-π conversion exist between the coupling of the alkynyl groups at titano- and zirconocenes to complexed 1,3-butadiynes on one side and the coupling of phenyl groups at chromium to complexed diphenyl on the other side.