The mechanism of reversible alkyne coupling at zirconium was investigated by examination of the kinetics of zinconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings studied possess trimethylsilyl substituents in the a-positions, and the ancillary Cp-2, Me2C(eta(5)-C5H4)(2), and CpCp* (Cp* = eta(5)-C5Me5) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the zirconacyclopentadiene ring in Cp2Zr[2,5-(Me3Si)(2)-3,4-Ph2C4] with PMe3, to produce Cp2Zr(eta(5)-PhC CSiMe3)- (PMe3) and free PhC CSiMe3, is first-order in initial zirconacycle concentration and zero-order in incoming phosphine (k(obs) = 1.4(2) x 10(-5) s(-1) at 22 degrees C), and the activation parameters determined by an Eyring analysis (Delta H-double dagger = 28(2) kcal mol(-1) and Delta S-double dagger = 14(4) eu) are consistent with a dissociative mechanism. The analogous reaction of the ansa-bridged complex Me2C(eta(5)-C5H4)(2)Zr[2,5-(Me3Si)(2)-3,4-Ph2C4] is 100 times faster than that for the corresponding Cp-2 complex, while the corresponding CpCp* complex reacts 20 times slower than the Cp-2 derivative. These rates appear to be largely influenced by the steric properties of the ancillary ligands.