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. View all notes 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.