Can Weakly Coordinating Anions Stabilize Mercury in Its Oxidation State +IV?

Graphical Abstract

The long-running quest for mercury(IV) and thus the goal of turning Group 12 of the periodic table into a true transition-metal group in which the d orbitals are involved in bonding is given new momentum in this article, which provides suggestions for possible target species (see picture for two examples), based on state-of-the-art quantum-chemical calculations.

Abstract

While the thermochemical stability of gas-phase HgF₄ against F₂ 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 Hg^IVX₄ and Hg^IVF₂X₂ (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 F₂ 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.