Fourier transform ion cyclotron resonance mass spectrometry was used to study ion/molecule reactions of six cationic methyl metallocenes : [Cp(2)ZrCH(3)](+) (1; Cp=eta(5)-cyclopentadienyl), [(Ind)(Cp)ZrCH3](+) (2; ind=eta(5)-indenyl), [Ind(2)ZrCH(3)](+) (3), [ansa-(CH3)(2)Si(eta(5)-C5H4)(2)ZrCH3](+) (4), [(Flu)(2)ZrCH3](+) (5; Flu=eta(5)-fluorenyl), and [ansa-(CH3)(2)Si(eta(5)-C(5)Me(4))(N-t-C4H9)ZrCH3](+) (6). Rate constants and product distributions for reactions with dihydrogen and various deuterium-labeled and unlabeled alkenes were determined. The electrophilicity of [L(2)Zr-CH3](+) ions in the gas phase is strongly dependent on the nature of the ancillary ligands L, with electrophilicity decreasing in the order Cp(2)>Cp,Ind>Ind(2)>Flu(2) for reactions with dihydrogen and 1-alkenes. Silyl-bridging of Cp ligands as in 4 appears to increase the electrophilicity somewhat, and the 10-electron ion 6 also has higher electrophilicity than 1. In each reaction used to assess electrophilicity, the probable rate determining transition state involves a 4-center/4-electron species in either C-H or H-H activation. In the reaction of ethylene with [L(2)Zr-CH3](+), labeling studies show partial-to-complete scrambling of H/D labels before elimination of dihydrogen. Scrambling increases with decreasing electrophilicity of the ion. A model for the intramolecular isotope effect leads to k(H)/k(D)=2.2 for the allylic C-H(D) activation step in reaction of 3 with ethylene. Isotopic labeling has shown that the C-H activation of isobutene by [L(2)Zr-CH3](+) proceeds via allylic activation only. Dimerization and hydrolysis of [L(2)Zr-CH3](+) is facile in the gas phase, and the products are analogous to known side reactions in solution chemistry. The polymerization activity of electrophilic zirconium polymerization catalysts does not necessarily correlate with the intrinsic electrophilicity of [L(2)Zr-CH3](+) species as L is varied. The ion pairing tendency of [L(2)Zr-CH3](+) in solution is a key contributor to the overall propagation rate, and the stability (K-ip) of the ion pair for a given counterion is expected to decrease as L becomes more electron donating and larger. If the ion pair preequilibrium trends dominate the kinetics, then more active catalysts can be obtained by introducing larger ancillary ligands with electron-donating substituents despite the reduced electrophilicity of the metallocenium ion.