Radiochim. Acta 2014; 102(12): 1093–1110 Julia Even*, Alexander Yakushev, Christoph Emanuel Düllmann, Jan Dvorak, Robert Eichler, Oliver Gothe, Willy Hartmann, Daniel Hild, Egon Jäger, Jadambaa Khuyagbaatar, Birgit Kindler, Jens Volker Kratz, Jörg Krier, Bettina Lommel, Lorenz Niewisch, Heino Nitsche, Inna Pysmenetska, Matthias Schädel, Brigitta Schausten, Andreas Türler, Norbert Wiehl, and David Wittwer In-situ formation, thermal decomposition, and adsorption studies of transition metal carbonyl complexes with short-lived radioisotopes Abstract: We report on the in-situ synthesis of metal car- was studied. We also studied the stability of some of the bonyl complexes with short-lived isotopes of transition complexes, showing that these start to decompose at tem- metals. Complexes of molybdenum, technetium, ruthe- peratures above 300 ∘C in contact with a quartz surface. nium and rhodium were synthesized by thermalisation Our studies lay a basis for the investigation of such com- of products of neutron-induced fission of 249Cf in a car- plexes with transactinides. bon monoxide-nitrogen mixture. Complexes of tungsten, Keywords: Carbonyl Complexes, Short-lived Isotopes, Car- rhenium, osmium, and iridium were synthesized by ther- bonyl Synthesis, In-Situ Synthesis, Adsorption Stud- malizing short-lived isotopes produced in 24Mg-induced ies, Thermochromatography, Isothermal Chromatography, fusion evaporation reactions in a carbon monoxide con- Thermal Stability, Fission Products, Superheavy Elements, taining atmosphere. The chemical reactions took place at Physical Preseparation. ambient temperature and pressure conditions. The com- plexes were rapidly transported in a gas stream to col- lection setups or gas phase chromatography devices. The DOI 10.1515/ract-2013-2198 Received October 9, 2013; accepted June 13, 2014 physisorption of the complexes on Au and SiO2 surfaces *Corresponding author: Julia Even, Helmholtz-Institut Mainz, 55099 1 Introduction Mainz, Germany; and Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany, e-mail: [email protected] Alexander Yakushev, Willy Hartmann, Egon Jäger, Birgit Kindler, Experimental studies of the heaviest elements are very Jörg Krier, Bettina Lommel, Matthias Schädel, Brigitta Schausten: challenging due to low production rates and generally GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64192 short half-lives. Gas-phase chemical reactions have proven Darmstadt, Germany powerful for such studies as they often proceed quickly Christoph Emanuel Düllmann: Johannes Gutenberg-Universität and with high yield. To date, thermally stable, simple inor- Mainz, 55099 Mainz, Germany; and Helmholtz-Institut Mainz, ganic compounds with the transactinide present in a high 55099 Mainz, Germany; and GSI Helmholtzzentrum für Schwer- ionenforschung GmbH, 64192 Darmstadt, Germany oxidation state, such as halides, oxyhalides, hydroxides, Jan Dvorak: Helmholtz-Institut Mainz, 55099 Mainz, Germany or oxides, have been studied [1–4]. Due to experimental Robert Eichler, Andreas Türler, David Wittwer: Paul Scherrer limitations, it was not possible to synthezise less stable Institut, 5232 Villigen, Switzerland; and University of Bern, 3012 compounds, e.g., organometallic ones [5]. Bern, Switzerland As a first step towards the investigation of complexes Jadambaa Khuyagbaatar: Helmholtz-Institut Mainz, 55099 Mainz, Germany; and GSI Helmholtzzentrum für Schwerionenforschung of the transactinides in oxidation state “0”, we have cho- GmbH, 64192 Darmstadt, Germany sen to focus on metal carbonyl complexes. More than Oliver Gothe, Heino Nitsche: Lawrence Berkeley National 100 years ago, Mond reported the first synthesis of a car- Laboratory, Berkeley, CA 94720, USA bonyl complex – nickel tetracarbonyl [6]. From that time Daniel Hild, Jens Volker Kratz, Lorenz Niewisch, Inna Pysmenetska: on, metal-carbonyl chemistry was developed and became Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany Norbert Wiehl: Johannes Gutenberg-Universität Mainz, 55099 an important topic in basic research and in the applied Mainz, Germany; and Helmholtz-Institut Mainz, 55099 Mainz, sciences; see, e.g., [7]. Stable, binary, mononuclear, neu- Germany tral, volatile metal carbonyl complexes are only known Brought to you by | Lib4RI Eawag-Empa Authenticated Download Date | 12/20/18 11:17 AM 1094 | J. Even et al., Formation, thermal decomposition, and adsorption studies of metal carbonyl complexes with vanadium, nickel, group 6, and group 8 elements [8]. Mononuclear binary carbonyl complexes of palladium, platinum, and group-11 elements were synthesized and studied in matrix isolation experiments [9, 10]. Multinu- clear (like the dimetal decacarbonyl complexes of group 5 elements, the tetrametal dodecacarbonyl complexes of cobalt and iridum, or the octarhodium hexadecacarbonyl complexes) or mixed carbonyl complexes with ligands like nitrosyl, hydrogen or oxygen are known for transition met- Fig. 1: Schematic drawing (after Ref. [11]) of the -donation bond als of almost all groups. (upper picture) from the HOMO of the carbonyl ligand to the atomic Carbon monoxide is a very strong ligand, a good - orbitals of the transition metal (TM) in -symmetry, and donor and -acceptor, and carbonyl complexes are in the -backdonation bond (bottom) from the d-orbital of the transition focus of many theoretical studies; see, e.g., [11–15]. The metal to the LUMO of the carbonyl ligand. dominant interactions in the bonding between the tran- sition metal and the ligand are i) the interaction of the Highest Occupied Molecular Orbital (HOMO) of the CO are discontinuous and time consuming and thus restricted molecule with an empty -symmetric atomic orbital of the to studies of at least moderately long-lived isotopes. metal and ii) the interaction between occupied -type d- The transactinide elements can only be produced in orbitals of the metal and the 2∗ orbital (Lowest Unoccu- nuclear fusion reactions with low cross-sections, which pied Molecular Orbital, LUMO) of CO. The -donor bond allow for production rates from a few atoms per minute strengthens the C−O bond. The -backbonding, on the down to single atoms per month. Furthermore, their half- other hand, weakens the C−O bond, as electron density lives are – with a few exceptions – in the range of millisec- is donated to the antibonding LUMO [11]. onds to about a minute. Therefore, chemical experiments Chemical properties of the transactinide elements are with these elements are conducted under atom-at-a-time strongly influenced by relativistic effects. Their influence conditions, and continuous procedures are preferred. scales approximately proportionally to 2 [1, 16]. Theo- In the past, no studies were performed to investi- retical predictions suggest the carbon-metal bond in the gate whether a high pressure of more than one hun- seaborgium hexacarbonyl complex to be influenced by dred bar is necessary to synthesise metal carbonyl com- these effects. Due to the relativistic contraction ofthe 1/2 plexes in chemical reactions with single, recoiling ions or and orbitals, the -donation bonding is predicted to be- atoms thermalized in a carbon-monoxide containing at- come slightly weaker in carbonyl complexes of transac- mosphere. tinide elements than in their lighter homologs [15]. Only recently, we reported on the successful in-situ Due to the indirect relativistic effect the -symmetric synthesis of carbonyl complexes with short-lived molybde- orbitals are more delocalised. This strengthens the - num, tungsten, and osmium isotopes [20]. In this work, we backbonding in the Sg−CO bond compared to that in the give a complete and more detailed overview on our studies lighter homologs [15]. involving also technetium, ruthenium, rhodium, iridium The synthesis of carbonyl complexes with short- and rhenium, which open the door for studies of carbonyl lived isotopes appeared challenging so far. With a few complexes also with superheavy elements. exceptions, metal carbonyl complexes are formed un- Fission products were directly thermalized in carbon- der high carbon monoxide pressure conditions (around monoxide containing gas-mixtures. In the experiments 300 bar)[8]. with fusion products, the approach of physical presepa- Despite these limitations, hot atom chemistry with ration [5, 21, 22] was used. These experiments were con- carbonyl complexes has been investigated as well [17–19]. ducted at the TransActinide Separator and Chemistry Ap- Mixtures of chromium hexacarbonyl with 235U were ir- paratus TASCA at the GSI Helmholtz Centre for Heavy Ion radiated with neutrons. Molybdenum as fission product Research in Darmstadt, Germany (GSI) [23]. Within TASCA, was found to have replace chromium in these complexes. the evaporation residues were separated in the magnetic Subsequently, the molybdenum hexacarbonyl could be field of a dipole magnet from the primary beam. They evaporated [17, 18]. In other experiments, tungsten was were then focused by quadrupole magnets to the TASCA produced in a nuclear fusion reaction and stopped in focal plane. There, the evaporation residues exited the a chromium hexacarbonyl catcher, from which it could be separator through a thin vacuum window and entered evaporated as tungsten hexacarbonyl [19]. These methods a gas-filled volume∼1 ( bar) called Recoil Transfer Cham- Brought to you by | Lib4RI Eawag-Empa Authenticated Download Date | 12/20/18 11:17 AM J. Even et al., Formation, thermal decomposition, and adsorption studies of metal carbonyl complexes | 1095 ber (RTC), in which they