Can weakly coordinating anions stabilize mercury in its oxidation state plus IV?

While the thermochemical stability of gas-phase HgF4 against F-2 elimination was predicted by accurate quantum chemical calculations more than a decade ago, experimental verification of "truly transition-metal" mercury(IV) chemistry is still lacking. This work uses detailed density functional calculations to explore alternative species that might provide access to condensed-phase Hg-IV chemistry. The structures and thermochemical stabilities of complexes (HgX4)-X-IV and (HgF2X2)-F-IV (X- = AlF4-, Al2F7-, AsF6-, SbF6-, As2F11-, Sb2F11-, OSeF5-, OTeF5-) have been assessed and are compared with each other, with smaller gas-phase HgX4 complexes, and with known related noble gas compounds. Most species eliminate F-2 exothermically, with energies ranging from only about -60 kJ mol(-1) to appreciable -180 kJ mol(-1). The lower stability of these species compared to gas-phase HgF4 is due to relatively high coordination numbers of six in the resulting Hg-II complexes that stabilize the elimination products. Complexes with AsF6 ligands appear more promising than their SbF6 analogues, due to differential aggregation effects in the Hg-II and Hg-IV states. HgF2X2 complexes with X- = OSeF5- or OTeF5- exhibit endothermic fluorine elimination and relatively weak interactions in the Hg-II products. However, elimination of the peroxidic (OEF5)(2) coupling products of these ligands provides an alternative exothermic elimination pathway with energies between -120 and -130 kJ mol(-1). While all of the complexes investigated here thus have one exothermic decomposition channel, there is indirect evidence that the reactions should exhibit nonnegligible activation barriers. A number of possible synthetic pathways towards the most interesting condensed-phase Hg-IV target complexes are proposed.