A model transition metal-carbene system MnCH₂

Abstract

The first transition metal-methylene species, Ta(C₅H₅)₂ CH₃CH₂, has recently been prepared and characterized by Schrock and co-workers. As a model for systems of this type, ab initio electronic structure theory has been applied to the MnCH₂ molecule. After experimentation with several types of basis sets, the following double-zeta contracted gaussian basis was adopted: Mn(12s 8p 4d/8s 6p 2d), C(9s 5p/4s 2p), H(4s/2s). However, in some cases it was found necessary to add a set of more diffuse d functions to the above. Single-configuration self-consistent-field (SCF) potential curves are reported for electronic states dissociating to Mn⁺(4s 3d^{5 7}S), Mn(4s² 3d^{5 6}S), Mn(4s 3d^{6 6}D) and Mn(4s 4p 3d^{5 8}P) plus ³B₁ CH₂ and ¹A₁ CH₂. The electronic ground state of MnCH₂ is of ⁸B₁ symmetry and for this state only a full geometry optimization was performed, yielding r _e(Mn-C) = 2·16 Å, θ _e(HCH) = 109°, and r _e(C-H) = 1·09 Å. In the SCF approximation ⁸B₁ MnCH₂ is bound by 33 kcal/mole relative to infinitely separated ⁶S Mn + ³B₁ CH₂. The predicted dipole moment is + 1·48‡

1 debye ≈ 3·335 64 × 10^-30 C m.

compared to those of isolated triplet and singlet methylene, -0·66 and -2·40 debye. The second low-lying strongly bound state is of ⁸A₁ symmetry, arising from ⁶D Mn + ³B₁ CH₂. The predicted equilibrium geometry is r _e(Mn-C) = 2·18 Å, θ _e(HCH) = 142°, corresponding to a dissociation energy of 39 kcal. Several higher electronic states dissociating to singlet CH₂ are also predicted to be bound. The lowest bound state is of ⁶B₁ symmetry with r _e(Mn-C) = 1·90 Å, θ _e(HCH) = 108°, corresponding to a dissociation energy of 38 kcal/mole. The latter bond distance is reasonably close to the 1·95 and 1·96 Å values found for the two manganese carbenes whose structures have been determined experimentally. The electronic structure of MnCH₂ is discussed in terms of Mulliken populations and orbital energies. The predicted methylene bond angles are readily understood in terms of a Walsh model. Allowing for the possibility of (a) s² d ⁿ and sd ⁿ⁺¹ metal atom ground states and (b) singlet and triplet carbene electronic ground states, a rich spectrum of possible transition metal-carbene properties is predicted.

Work performed under the auspices of the U.S. Energy Research and Development Administration.

Work performed under the auspices of the U.S. Energy Research and Development Administration.

Notes

1 debye ≈ 3·335 64 × 10^-30 C m.

Work performed under the auspices of the U.S. Energy Research and Development Administration.