The substitution chemistry of olefin complexes (silox)₃M(ole) (silox = ^tBu₃SiO; M = Nb (1-ole), Ta (2-ole); ole = C₂H₄ (as ¹³C₂H₄ or C₂D₄), C₂H₃Me, C₂H₃Et, cis-2-C₄H₈, iso-C₄H₈, C₂H₃Ph, ^cC₅H₈, ^cC₆H₁₀, ^cC₇H₁₀ (norbornene)) was investigated. For 1-ole, substitution was dissociative (ΔG^⧧_diss), and in combination with calculated olefin binding free energies (ΔG°_bind), activation free energies for olefin association (ΔG^⧧_assoc) to (silox)₃Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement to alkylidenes. Instead, activation free energies for olefin association to (silox)₃Ta (2) were measured, and when combined with ΔG°_bind (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 ΔG°_bind) 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)₃M(C₂H₄) (M = Nb, 1‘-C₂H₄; Ta, 2‘-C₂H₄) are asymmetric and have orbitals consistent with either singlet or triplet states. The rearrangement of (silox)₃Nb(trans-Vy,Ph-^cPr) (1-VyPh^cPr) to (silox)₃NbCHCHCHCH₂CH₂Ph (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)₃M + 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.

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