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.tplbond.C(t-Bu)][CH:CH(t-Bu)]