A DENSITY FUNCTIONAL-STUDY ON THE ACTIVATION OF HYDROGEN HYDROGEN AND HYDROGEN CARBON BONDS BY CP2SC-H AND CP2SC-CH3

Density functional calculations have been carried out on Cp2Sc-R (R = H, methyl, ethyl, propyl, vinyl, and acetylide). Geometry optimizations reveal an agostic interaction for R = ethyl, whereas methyl and propyl are bound to the metal center without agostic interactions. The Sc-R bond energies are calculated as D(e)(Sc-acetylide) = 540 kJ mol-1 > D(e)(Sc-H) = 340 > D(e)(Sc-vinyl) = 338 > D(e)(Sc-methyl) = 295 > D(e)(Sc-ethyl) = 283 > D(e)(Sc-propyl) = 240. A study on the sigma-bond metathesis reaction (I) (Cp2Sc-R + H-R' --> Cp2Sc-R + H-R) reveals the following reaction enthalpies [DELTAH(I)(R,R'), in kJ mol-1]: DELTAH(I)(CH3) acetylide) = -128 < DELTAH(I)(H, acetylide) = -86 < DELTAH(I)(CH3, H) = -28 < DELTAH(I)(CH3), Vinyl) = -26 < DELTAH(I)(H, vinyl) = 16. Reaction I proceeds from an adduct (a) between Cp2Sc-R and H-R' over a kite-shaped four-center transition state (b) with a Sc-R-H-R' core to an adduct (c) between CP2Sc-R and H-R'. The activation energies, DELTAH(double dagger)I(R,R'), are calculated in kJ mol-1 as DELTAH(t)mo(I)(H, acetylide) = -29 < DELTAH(double dagger)I(H, H) = -7 < DELTAH(double dagger)I(CH3, acetylide) = -4 < DELTAH(double dagger)I(CH3, H) = 8 < DELTAH(double dagger)I(H, vinyl) = 14 < DELTAH(double dagger)I(CH3, vinyl) = 39 < DELTAH(double dagger)I(CH3, CH3) = 45. The activation energies increase with the number of vinyl and methyl groups in the Sc-R-H-R' core as the directional sigma-orbitals on these groups are unable to maintain optimal interactions with both neighbors in the Sc-R-H-R' core. The transition states for reactions with negative activation energies are below the reactants in energy but still above the adduct (a). The formally forbidden [2sigma + 2sigma] reaction is facilitated by a pool of s-, p-, and d-type orbitals on scandium which maintain optimal interactions with the neighboring groups throughout the reaction.