The ditungsten decacarbonyl dianion.

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC)W-W(CO)][K(18-crown-6)(THF)] (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 eclipsed rather than D 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 geometry is energetically preferred in the gas-phase, being ∼18 kJ mol more stable than the D form, since slight destabilisation of the degenerate W-CO π 5d and 5d orbitals is outweighed by greater stabilisation of the W-W σ-bond orbital. The gas-phase D structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D isomer directly to the D 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.