The ditungsten decacarbonyl dianion†
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 D4h eclipsed rather than D4d 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 D4d geometry is energetically preferred in the gas-phase, being ∼18 kJ mol−1 more stable than the D4h form, since slight destabilisation of the degenerate W–CO π 5dxz and 5dyz orbitals is outweighed by greater stabilisation of the W–W σ-bond orbital. The gas-phase D4h structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D4h isomer directly to the D4d 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.