One of the most important classifications in chemistry and within the periodic table is the concept of formal oxidation states. The preparation and characterization of compounds containing elements with unusual oxidation states is of great interest to chemists. The highest experimentally known formal oxidation state of any chemical element is at present VIII, although higher oxidation states have been postulated. Compounds with oxidation state VIII include several xenon compounds (for example XeO4 and XeO3F2) and the well-characterized species RuO4 and OsO4 (refs 2-4). Iridium, which has nine valence electrons, is predicted to have the greatest chance of being oxidized beyond the VIII oxidation state. In recent matrix-isolation experiments, the IrO4 molecule was characterized as an isolated molecule in rare-gas matrices. The valence electron configuration of iridium in IrO4 is 5d(1), with a formal oxidation state of VIII. Removal of the remaining d electron from IrO4 would lead to the iridium tetroxide cation ([IrO4](+)), which was recently predicted to be stable and in which iridium is in a formal oxidation state of IX. There has been some speculation about the formation of [IrO4](+) species, but these experimental observations have not been structurally confirmed. Here we report the formation of [IrO4](+) and its identification by infrared photodissociation spectroscopy. Quantum-chemical calculations were carried out at the highest level of theory that is available today, and predict that the iridium tetroxide cation, with a Td-symmetrical structure and a d(0) electron configuration, is the most stable of all possible [IrO4](+) isomers.

The in-depth knowledge about on-surface reaction mechanisms is crucial for the tailor-made design of covalently bonded organic frameworks, for applications such as nanoelectronic or -optical devices. Latest developments in atomic force microscopy, which rely on functionalizing the tip with single CO molecules at low temperatures, allow to image molecular systems with submolecular resolution. Here, we are using this technique to study the complete reaction pathway of the on-surface Ullmann-type coupling between bromotriphenylene molecules on a Cu(111) surface. All steps of the Ullmann reaction, i.e., bromotriphenylenes, triphenylene radicals, organometallic intermediates, and bistriphenylenes, were imaged with submolecular resolution. Together with density functional theory calculations with dispersion correction, our study allows to address the long-standing question of how the organometallic intermediates are coordinated via Cu surface or adatoms.

The terminal oxo species OUF(2) and OThF(2) have been prepared via the spontaneous and specific OF(2) molecule reactions with laser ablated uranium and thorium atoms in solid argon and neon. These isolated molecules are characterized by one terminal M-O and two F-M-F (M = U or Th) stretching vibrational modes observed in matrix isolation infrared spectra, which are further supported by density functional frequency calculations and CASPT2 energy and structure calculations. Both molecules have pyramidal structures with singlet (Th) and triplet (U) ground states. The molecular orbitals and metal-oxygen bond lengths for the OUF(2) and OThF(2) molecules indicate triple bond character for the terminal oxo groups, which are also substantiated by NBO analysis at the B3LYP level and by CASPT2 molecular orbital calculations. Dative bonding involving O(2p) → Th(6d) and U(df) interactions is clearly involved in these oxoactinide difluoride molecules. Finally, the weak O-F bond in OF(2) as well as the strong U-O, U-F and Th-O, Th-F bonds make reaction to form the OUF(2) and OThF(2) molecules highly exothermic.

need further improvement. First, the total sample fabrication time is presently dominated by sample handling, that is, print setup, sample transport and development. These steps can be automated, for example, by using microfluidics 42 , or by automatically immersing the sample in liquid containers 43 . Second, the photoresin as presented here limits the total exposed volume. After prolonged printing, out-of-focus areas polymerize due to proximity effects. Such accumulation or depletion effects can likely be suppressed by using further improved photoresin formulations. Directions for optimization can be deduced from the rate model analysis presented here. In addition, the in situ replenishment of the used photoresin by using microfluidics, polymerization inhibition by an oxygen-permeable membrane (as used in CLIP 8 ), or volumetric polymerization inhibition patterning 10 can potentially further suppress out-of-focus polymerization. Third, the red solid-state laser can be replaced by inexpensive high-power laser diodes, as is already the case for the blue laser. Compared with the amplified femtosecond pulsed lasers used in FP-TPL, the continuous-wave lasers used in this work are already much less expensive and easier to operate. The development of two-colour two-step photoinitiators improved with respect to sensitivity and tuned to readily available laser wavelengths would be highly desirable for this aspect.The underlying logical-AND-type functionality is not limited to light-sheet 3D printing, but could also prove beneficial in combination with CAL [5][6][7] , where a two-colour projection system was recently demonstrated for multimaterial 3D printing 44 . Related CAL setups could be used to simultaneously expose the photoresin using two-colour two-step absorption, with the two colours impinging from different directions, thereby reducing the proximity effect and voxel size.

Polyfluoride anions have been investigated by matrix-isolation spectroscopy and quantum-chemical methods. For the first time the higher polyfluoride anion [F5 ](-) has been observed under cryogenic conditions in neon matrices at 850 cm(-1) . In addition, a new band for the Cs(+) [F3 ](-) complex in neon is reported.