Reaction of 2 equivs of MesC CPh with Cp2Zr(eta(2)-Me3SiC CSiMe3)(pyr) afforded the zirconacyclopentadiene Cp2Zr[2,5-Ph-2-3,4-Mes(2)C(4)]. The regiochemistry of this isomer (beta beta with respect to the mesityl substituents) was determined through single-crystal X-ray analysis and 2D (NOESY, HSQC, HMBC) NMR experiments. This selectivity is attributed largely to a steric-based directing effect of the o-methyl ring substituents since coupling of 1,3-dimethyl-2-(phenylethynyl)benzene with zirconocene gave a single regioisomer (o-xylyl groups in both beta-positions) while coupling of 1, 3-dimethyl-5-(phenylethynl) benzene gave a statistical distribution of zirconacyclopentadiene regioisomers. The coupling reaction of 2 equivs of MeC CMes or PrC CMes with Cp2Zr(eta(2)-Me3SiC CSiMe3)(pyr) at ambient temperature gave the beta beta regioisomers, Cp2Zr[2,5-Me-2-3,4-Mes(2)C(4)] and Cp2Zr[2,5-Pr-2-3,4-Mes(2)C(4)], respectively, as the major products. Heating solutions of these zirconacycles at 80 degrees C for several hours resulted in an increase in the amount of the unsymmetrical product. For reaction mixtures of PrC CMes and Cp2Zr(eta(2)-Me3SiC CSiMe3)(pyr) the major (and apparently thermodynamic) product under these reaction conditions was Cp2Zr[2,4-Mes(2)-3,5-Pr2C4]. The steric strain in the mesityl-substituted zirconacycles allowed for facile substitution reactions of MesC CPh or PrC CMes by less bulky alkynes (i.e., tolan and 3-hexyne) to give the unsymmetrical ziconacyclopentadienes Cp2Zr[2,4,5-Ph-3-3-MesC(4)], Cp2Zr[2-Ph-3-Mes-4,5-Et2C4], and Cp2Zr[2-Pr-3-Mes-4,5-Ph2C4]. Reaction of a mesityl-terminated diyne containing a rigid dihexylfluorenylene spacer with zirconocene afforded poly(p-fluorenylenedienylene) after demetalation with benzoic acid.