The substitution chemistry of olefin complexes (silox)(3)M(ole) (silox = Bu3SiO; M = Nb (1-ole), Ta (2-ole); Ole = C2H4 (as (C2H4)-C-13 or C2D4), C2H3Me, C2H3Et, cis-2-C4H8, iso-C4H8, C2H3Ph, (C5H8)-C-c, (C6H10)-C-c, (c)C7H(10) (norbornene)) was investigated. For 1-ole, substitution was dissociative (Delta G(diss)double dagger), and in combination with calculated olefin binding free energies (AG(bind)degrees), activation free energies for olefin association (Delta G(assoc)double dagger) to (Silox)(3)Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement to alkylidenes. Instead, activation free energies for olefin association to (silox)(3)Ta (2) were measured, and when combined with Delta G(bind)degrees (calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despite stronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than that from 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships (with respect to Delta G(bind)degrees) characterize olefin dissociation from 1-ole, but not olefin dissociation from 2-ole, and olefin association to 2, but not olefin association to 1. Calculated transition states for olefin dissociation from (HO)(3)M(C2H4) (M = Nb, l'-C2H4; Ta, 2'-C2H4) are asymmetric and have orbitals consistent with either singlet or triplet states. The rearrangement of (silox)(3)Nb(trans-Vy,Ph-Pr-c) (1-Vy,(PhPr)-Pr-c) to (silox)(3)Nb=CHCH= CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transtion state for substitution. The disparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater density of states (DOS) for the products (i.e., (silox)(3)M + Ole) where M = Nb; leading to intersystem crossing events that facilitate dissociation. At the crux of the DOS difference is the greater 5d(z2)/6s mixing for Ta vs the 4d(z2)/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of 4d vs 5d elements.