Hassium - Experimental Atomic Gas Phase Chemistry

Experimental Atomic Gas Phase Chemistry

Summary of compounds and complex ions

Formula	Names
HsO4	hassium tetroxide; hassium(VIII) oxide
Na2	sodium hassate(VIII); disodium dihydroxytetraoxohassate(VIII)

Despite the fact that the selection of a volatile hassium compound (hassium tetroxide) for gas-phase chemical studies was clear from the beginning, the chemical characterization of hassium was considered a difficult task for a long time. Although hassium isotopes were first synthesized in 1984, it was not until 1996 that a hassium isotope long-lived enough to allow chemical studies to be performed was synthesized. Unfortunately, this hassium isotope, 269Hs, was then synthesized indirectly from the decay of 277Cn; not only are indirect synthesis methods not favourable for chemical studies, but also the reaction that produced the isotope 277Cn had a low yield (its cross-section was only 1 pb), and thus did not provide enough hassium atoms for a chemical investigation. The direct synthesis of 269Hs and 270Hs in the reaction 248Cm(26Mg,xn)274−xHs (x = 4 or 5) appeared more promising, as the cross-section for this reaction was somewhat larger, at 7 pm. However, this yield was still around an order of magnitude lower than that for the reaction used for the chemical characterization of bohrium. New techniques for irradiation, separation, and detection had to be introduced before hassium could be successfully characterized chemically as a typical member of group 8 in early 2001.

Ruthenium and osmium have very similar chemistry due to the lanthanide contraction, but iron shows some differences from them: for example, although ruthenium and osmium form stable tetroxides in which the metal is in the +8 oxidation state, iron does not. Consequently, in preparation for the chemical characterization of hassium, researches focused on ruthenium and osmium rather than iron, as hassium was expected to also be similar to ruthenium and osmium due to the actinide contraction. However, in the planned experiment to study hassocene (Hs(C₅H₅)₂), ferrocene may also be used for comparison along with ruthenocene and osmocene.

In ruthenocene and osmocene, the cyclopentadienyl rings are in an eclipsed conformation. Hassocene is predicted to also have this structure.

The first chemistry experiments were performed using gas thermochromatography in 2001, using 172Os and 173Os as a reference. During the experiment, 5 hassium atoms were synthesized using the reaction 248Cm(26Mg,5n)269Hs. They were then thermalized and oxidized in a mixture of helium and oxygen gas to form the tetroxide.

269
108Hs + 2 O₂ → 269
108HsO₄

The measured deposition temperature indicated that hassium(VIII) oxide is less volatile than osmium tetroxide, OsO₄, and places hassium firmly in group 8. However, the enthalpy of adsorption for HsO₄ measured, (−46 ± 2) kJ/mol, was significantly lower than what was predicted, (−36.7 ± 1.5) kJ/mol, indicating that OsO₄ was more volatile than HsO₄, contradicting earlier calculations, which implied that they should have very similar volatilities. For comparison, the value for OsO₄ is (−39 ± 1) kJ/mol. It is possible that hassium tetroxide interacts differently with the different chemicals (silicon nitride and silicon dioxide) used for the detector; further research is required, including more accurate measurements of the nuclear properties of 269Hs and comparisons with RuO₄ in addition to OsO₄.

In order to further probe the chemistry of hassium, scientists decided to assess the reaction between hassium tetroxide and sodium hydroxide to form sodium hassate(VIII), a reaction well-known with osmium. In 2004, scientists announced that they had succeeded in carrying out the first acid-base reaction with a hassium compound:

HsO₄ + 2 NaOH → Na₂

The team from the University of Mainz are planning to study the electrodeposition of hassium atoms using the new TASCA facility at the GSI. The current aim is to use the reaction 226Ra(48Ca,4n)270Hs. In addition, scientists at the GSI are hoping to utilize TASCA to study the synthesis and properties of the hassium(II) compound hassocene, Hs(C₅H₅)₂, using the reaction 226Ra(48Ca,xn). This compound is analogous to the lighter ferrocene, ruthenocene, and osmocene, and is expected to have the two cyclopentadienyl rings in an eclipsed conformation like ruthenocene and osmocene and not in a staggered conformation like ferrocene. Hassocene was chosen because it has hassium in the low formal oxidation state of +2 (although the bonding between the metal and the rings is mostly covalent in metallocenes) rather than the high +8 state which has been investigated, and relativistic effects were expected to be stronger in the lower oxidation state. Many metals in the periodic table form metallocenes, so that trends could be more easily determined, and the highly symmetric structure of hassocene and its low number of atoms also make relativistic calculations easier. Hassocene should be a stable and highly volatile compound.