The ditungsten decacarbonyl dianion

Abstract

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC)₅W–W(CO)₅][K(18-crown-6)(THF)₂]₂ (1), completing the group 6 dianion triad over half a century since the area began. The W–W bond is long [3.2419(8) Å] and, surprisingly, in the solid-state the dianion adopts a D_4h eclipsed rather than D_4d staggered geometry, the latter of which dominates the structural chemistry of binary homobimetallic carbonyls. Computational studies at levels of theory from DFT to CCSD(T) confirm that the D_4d geometry is energetically preferred in the gas-phase, being ∼18 kJ mol⁻¹ more stable than the D_4h form, since slight destabilisation of the degenerate W–CO π 5d_xz and 5d_yz orbitals is outweighed by greater stabilisation of the W–W σ-bond orbital. The gas-phase D_4h structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D_4h isomer directly to the D_4d forms, which are produced by rotation around the W–W bond by ±45°. It is therefore concluded that the gas-phase transition state becomes a minimum on the potential energy surface when subjected to crystal packing in the solid-state.