The ditungsten decacarbonyl dianion.

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC)(5)W-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) angstrom] and, surprisingly, in the solid-state the dianion adopts aD(4h)eclipsed rather thanD(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 theD(4d)geometry is energetically preferred in the gas-phase, being similar to 18 kJ mol(-1)more stable than theD(4h)form, since slight destabilisation of the degenerate W-CO pi 5d(xz) and 5d(yz) orbitals is outweighed by greater stabilisation of the W-W sigma-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 degrees. 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.