Olefin Substitution in (silox)₃M(olefin) (silox = <i>^t</i>Bu₃SiO; M = Nb, Ta): The Role of Density of States in Second vs Third Row Transition Metal Reactivity

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-VyPhcPr) to (silox)3Nb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)3M + ole) where M = Nb, leading to intersystem crossing
events that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vs
the 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of
4d vs 5d elements.