Bimolecular rate constants, primary products, and kinetic isotope effects for the reactions of Y (4d(1)5s(2), 2D) with C2H4 and C2D4 and with C3H6 and C3D6 are measured in a fast flow reactor at 300 K with He/N-2 buffer gas at 0.8 Torr. The H;? and D-2 elimination products and Y(alkene)-stabilized complexes are detected using single photon ionization at 157 nm and time-of-flight mass spectrometry. We find a small normal isotope effect (k(H)/k(D) = 1.75 +/- 0.12) for the reaction with ethylene but no significant isotope effect (k(H)/k(D) = 1.06 +/- 0.07) for the reaction with propylene. We use density functional theory in its B3LYP and mPW1PW91 forms with a large basis set to characterize stationary points on the doublet potential energy surface for the reaction Y + C2H4 --> YC2H2 + H-2 Theory finds no energy barrier to the formation of a long-range Y-ethylene complex. Subsequent steps involving CH bond insertion by metallacyclopropane complexes are consistent with earlier work. However, a new, low-energy path involves concerted rearrangement of the HYC2H3 insertion intermediate directly to a weakly bound, product-like complex with no exit channel barrier to elimination products. Theory also provides a set of geometries and vibrational frequencies for use in statistical rate models of the hot metallacyclopropane complex decay. The preferred model, consistent with the collection of Y + ethylene experimental data, requires no adjustments to the mPW1PW91 energies. As in earlier work, B3LYP places key transition state energies too high by 6-9 kcal/mol. The available evidence suggests that nonadiabatic and/or steric effects contribute to the reaction inefficiency at room temperature.