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Quantum Chemical Study of Trivalent Group 12 Fluorides

Sebastian Riedel, Martin Kaupp, Pekka Pyykkö "Quantum Chemical Study of Trivalent Group 12 Fluorides" Inorg. Chem. 2008,
In view of the recent experimental observation of the existence of tetravalent HgIV in HgF4, a quantum chemical study of various trivalent group 12 MIII fluoride complexes has been carried out. The M-F bonds in neutral MF3 are relatively weak, making these species unlikely targets. Some additional stability can be obtained by dimerizing HgF3 to Hg2F6, which has a doubly fluorine-bridged triplet ground state. Anionic [MF4]- and [MF5]2- species are found to be more stable toward F2 elimination and M-F bond breaking than neutral MF3.


Mercury is a Transition Metal: The First Experimental Evidence for HgF4

Xuefang Wang, Lester Andrews, Sebastian Riedel, Martin Kaupp, "Mercury is a Transition Metal: The First Experimental Evidence for HgF4" Angew. Chem. 2007, DOI: 10.1002/ange.200703710
Although the Group 12 elements are generally considered to be post-transition metals with filled d shells, quantum chemical calculations predict a stable HgF4 square-planar d8 complex due to relativistic stabilization of HgIV relative to HgII. In its tetravalent oxidation state, mercury is a true transition metal that utilizes 5d orbitals fully in bonding. The first experimental evidence is reported for HgIVF4 in solid neon by IR spectroscopy, after reaction of mercury with excess fluorine under mercury arc irradiation and subsequent annealing. Assignment of the new bands to HgF4 is supported by coupled-cluster and density functional calculations of structures and vibrational frequencies.

The Figure shows a color-scale plot of the electron localization function (ELF) for HgF2 (up) and HgF4 (down). High ELF values indicate areas with low local kinetic energy densities due to relatively low Pauli repulsion.


Validation of Density Functional Methods for Computing Structures and Energies of Mercury(IV) Complexes

Sebastian Riedel, Michal Straka, Martin Kaupp, "Validation of Density Functional Methods for Computing Structures and Energies of Mercury(IV) Complexes" Phys. Chem. Chem. Phys. 2004, 6, 1122 - 1127.
While quantum chemical predictions have strongly suggested a decade ago the existence of mercury in its oxidation state +IV, no experimental evidence has been found yet. To enable the search for alternative targets and preparation routes by quantum chemical methods, the present work has validated density functional methods against accurate CCSD(T) results for structures, reaction energies and activation barriers for X2-elimination, and atomization energies for three HgXx systems (X=F, Cl, H). Hybrid functionals with ca. 20% Hartree–Fock exchange like B3LYP, B1LYP or MPW1PW91 have provided the best energetics, whereas local or gradient-corrected pure functionals overestimate, and the BHandHLYP hybrid functional underestimates the stability of the HgIV state. Basis sets are suggested that provide a reasonable compromise between accuracy and computational effort in calculations on larger systems.

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

Sebastian Riedel, Michal Straka, Martin Kaupp, "Can Weakly Coordinating Anions Stabilize Mercury in its Oxidation State +IV?" Chem. Eur. J. 2005, 11, 2743-2755
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 HgIV chemistry. The structures and thermochemical stabilities of complexes HgIVX4 and HgIVF2X2 (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 HgII complexes that stabilize the elimination products. Complexes with AsF6 ligands appear more promising than their SbF6 analogues, due to differential aggregation effects in the HgII and HgIV states. HgF2X2 complexes with X-=OSeF5- or OTeF5- exhibit endothermic fluorine elimination and relatively weak interactions in the HgII 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 HgIV target complexes are proposed.