Full title: Carbon-carbon bond formation versus hydrogen transfer in the reaction of alkynes with mono(cyclopentadienyl)zirconium(II) complexes: Regioselective dimerization, cocyclotrimerization, and catalytic cyclotrimerization. The preparation and X-ray structures of Cp(dmpe)XZr[(R)C=C(R′)C(R)=C(R′)] [R = CH3; R′ = H, CH3; X = Cl, CH3; dmpe = 1,2-bis(dimethylphosphino)ethane] and of CpZr(dmpe)Cl[C=C(t-Bu)][CH=CH(t-Bu)]. Under mild conditions, the reaction between CpZr(dmpe)2X [X = Cl (1), Me (2); dmpe = 1,2-bis(dimethylphosphino)ethane] and aliphatic alkynes yields the crystalline CpZr[(R)C=C(R′)C(R)=C(R′)]-(dmpe)X [R = R′ = Me, X = Cl (3), Me (4); R′ = H, R = Me, X = Cl (5)]. In the case of 5, the zirconacyclopentadiene ring has been formed regiospecifically via a quantitative head-to-tail dimerization reaction. Steric factors are probably the origin of the regioselectivity. The structures of the complexes have been demonstrated by X-ray analyses. Crystal data are as follows. 3: monoclinic, C2/c, a = 16.807 (1) ?, b = 9.291 (1) ?, c = 27.557 (1) ?, β = 96.906 (4)°, V = 4271.9 (5) ?3, Z = 8. 4: triclinic, P1?, a = 9.252 (1) ?, b = 9.416 (1) ?, c = 14.104 (1) ?, α = 91.822 (8)°, β = 98.948 (9)°, γ = 118.09 (1)°, V = 1063.0 (2) ?3, Z = 2. 5: monoclinic, P21/n, a = 9.699 (2) ?, b = 13.563 (5) ?, c = 15.361 (4) ?, β = 91.36 (2)°, V = 2020.1 (9) ?3, Z = 4. Hydrogen transfer between two molecules of alkyne has been observed with the bulky HC≡C(t-Bu). The structure of the resulting CpZr(dmpe)Cl[C≡C(t-Bu)][CH=CH(t-Bu)] (6) has been demonstrated by X-ray analysis. Crystal data for 6: monoclinic, P21/a, a = 11.333 (3) ?, b = 15.655 (7) ?, c = 16.051 (3) ?, β = 100.08 (2)°, V = 2804 (1) ?3, Z = 4. Stoichiometric cyclotrimerization reactions and cocyclotrimerization can be achieved by further reaction of complexes 3, 4, and 5 with small alkynes (propyne, acetylene) and acetonitrile, forming random mixtures of isomeric arenes and substituted pyridine (collidine). Catalytic cyclotrimerization has been observed in the case of acetylene (-20°C) and propyne (room temperature) with formation of benzene and a mixture of 1,3,5- and 1,2,4-mesitylene, respectively.