A computational study of two-state conformational changes in 16-electron [CpW(NO)(L)] complexes (L = PH3, CO, CH2, HCCH, H2CCH2)

Electronically and coordinatively unsaturated [Cp*W(NO)(L)] complexes have been postulated as intermediates in several related systems. Model [CpW(NO)(L)] compounds (L= PH3, CO, CH2, H2CCH2, HCCH) have been investigated theoretically by means of density functional theory computational techniques. The structural parameters calculated for saturated [CpW(NO)(PH,)(L)] complexes are in good agreement with the solid-state molecular structures determined crystallographically for the corresponding [Cp*W(NO)(PMe3)(L)] compounds. The 16-electron, singlet [CpW(NO)(L)] species have geometries comparable to those of the same fragment in the phosphine adducts and include a highly pyramidal conformation at W. The energy of the triplet spin state is calculated to be close to or even lower than that of the singlet state for these unsaturated compounds, and depends largely on the pi-bonding capabilities of L (Delta Es-t=Delta E-t-Delta E-s = - 3.3 kcalmol(-1) (PH3), + 2.8 (CO), + 2.4 (CH2), +6.3 (H2CCH2), -2.3 (HCCH)). The optimization of partially constrained structures in both spin states allows for a conformational analysis of the [CpW(NO)(L)] species. The inversion of the conformation of the pyramidal singlet [CpW(NO)(L)] complexes via the planar-at-W triplet species (two-state pathway) is calculated to be competitive with the equivalent process solely along the singlet spin hypersurface. Rotation of the W-CH, bond in the singlet carbene species is also found to proceed more readily via a two-state pathway. The preferred alkyne conformation, the unusually stable triplet states, and the strong W-to-L sc-donation observed in these systems may all be rationalized by the relatively high energies of the occupied orbitals of the formally WO compounds.