Topic reserved by: Henrik Stenbrink

•  Re-Re-Recheck the angles/distances.  Hard to see the correct values in vesta. SEEMS TO BE INCORRECT AGAIN
  
•  Recheck the whole neighbouring Ru's concept
•  Figure out how to present/visualize the closest Ru's (if it's a good concept), otherwise it needs to be scrapped.
•  Fix DOI's, they probably shouldn't include the lib-proxy part.
•  A semiconductor? Apparently band gap is about 2.2eV, but not sure how this would handle the electricity due to volatility and melting/vaporization temp.
•  Band gap information added, no pictures (need to find CC/open access figure)
  
•  Should i add structure type?

Introduction

RuO₄, ruthenium tetraoxide is transition metal oxide, yellow in color, borderline solid substance in roomtemperature. However, it will melt when held in your hands due to having a melting point of 25.4 ˚C. It also has a low boiling point of 40.0 ˚C making the window when it's liquid very narrow. 

However, some earlier studies claim that it wouldn't boil, at least under the temperature of 106 ˚C.

These and select other physical properties are shown in table 1. The oxidation state for Ru is VIII in this compound, which is relatively rare and can explain the oxidative properties of RuO₄. There is slight density variation, which could depend on which polymorph is in question. The bonding within the RuO4 molecules are mostly covalent, due to relatively low electronegativity differences between ruthenium and oxygen. An estimate of the bond character can be estimated from the van Arkel-Ketelaar triangle, giving roughly 50 % covalent, 40 % ionic and 10 % metallic character.

As a fun trivial thing to know, when looking for fingerprints on dead bodies, finding fingerprints seems to be effective or even best with RuO4, at least compared to the more common substances (Magnetic jet black etc.).

Properties and Structure

Ruthenium tetraoxide, in both of it's forms, is toxic and volatile. It forms 2 different structures, cubic (space group P-43n) and monoclinic (space group C2/c), In these structures ruthenium is the central cation that is in tetrahedral coordination with oxygens as the anions. It is structurally very similar to OsO_4, as could be expected, but there are differences in reactivity.

The cell parameters for monoclinic and cubic structures are presented in table 2.

RuO4 is not very stable, decomposes in light and it can even explode if heated to over 100 deg C. It can even explode at roomtemperature if it gets in contact with oxidizable organic solvents.

Figures 1 and 2 shows the monoclinic and cubic structures respectively. As can be seen from the figures, ruthenium is tetragonal in both structures.

Figure 1. Cubic RuO4. Figure: Henrik Stenbrink

It is worthwhile noting that the Ru-O distances in the cubic form aren't identical, but rather there are 2 different types. Within the tetrahedron, the Ru-O distance can be either 1.695 Å or 1.696 Å, depending on which location the Ru is positioned in. The rutheniums located in the corners or in the middle of the unit cell (Fig. 1) have 1.695 Å Ru-O bond lengths while the others have the longer 1.696 Å Ru-O bond lengths. Same goes for the angles, the corner and middle RuO4 tetrahedrons have 109.47˚ deg angles, while the rest have 109.34˚.

The angles in the monoclinic RuO4 are also different from the cubic form, however there are no differences within the unit cell of the monoclinic form. Bond angles in the monoclinic are all 110.24˚ or 109.22˚ and the bond lengths are 1.701 Å or 1.699 Å. Two of each angle and bond length is found in every tetrahedron. The monoclinic unit cell, with RuO4 molecules is presented in Fig. 2.

Figure 2. Monoclinic RuO4. Figure: Henrik Stenbrink

The differences between the bond lengths and angles are rather small and close to the ideal tetrahedral values, one can consider them as ideal tetrahedrons.

As one can see from the angles and distances involving oxygen, there is not much of a difference between the monoclinic and cubic RuO₄. The biggest difference between the monoclinic and cubic structures are the distances between the neighboring ruthenium atoms. The shortest Ru-Ru distance in the cubic crystal structure 4.255 Å (Ru2-Ru2) or 4.757 Å (Ru-Ru2) while the monoclinic one has 4.397 Å.

The monoclinic has 8 neighbouring rhutenium atoms while the cubic crystal structures have 12 neighboring Ru atoms. The differences come from the distances between said neighboring atoms. Monoclinic system has distances varying from 4.397 to 5.144 Å while cubic has distances from 4.255 to 5.21 Å. The angles and distances for both crystal systems are presented in table 3.

One interesting aspect of both of the structures presented in Fig. 1 and 2 is that depending which way one is looking, it looks remarkably different on a molecular level. Looking at a 3-by-3 supercell of both the monoclinic and cubic structures, visualizing it along b or c axis. One can see some structural elements, planes for the monoclinic structure as seen on Fig. 3 and a different pattern for the cubic structure in Fig. 4.

Figure 3. Monoclinic RuO4 3x3 supercell viewed along c-axis. Figure: Henrik Stenbrink

Figure 4. Cubic RuO4 3x3 supercell viewed along b-axis. Figure: Henrik Stenbrink

The calculated XRD-diffractogram, based on the parameter values shown in table 2, is shown in figures 5 and 6, for monoclinic and cubic respectively.

Figure 5. Calculated XRD-diffractogram of monoclinic RuO₄. Figure: Henrik Stenbrink

Figure 6. Calculated XRD-diffractogram of cubic RuO₄. Figure: Henrik Stenbrink

The differences in XRD diffractograms between the monoclinic and cubic RuO₄ are minor, yet clearly visible. Both the similarity and minor differences can be explained by their crystal structures, which are as visualized in Fig.1 and 2.

Synthesis

Due to the nature of RuO4, mainly explosivity, it is usually either synthesized in situ or an anionic derivative of the salt TPAP, which in turn is synthesized by oxidizing RuCl3 to get RuO4- anions and countering them with tetrapropylamine cations.

Ruthenium tetraoxide can be synthesized oxidizing elemental ruthenium. While on paper this might sound easy, it isn't as easy in practice. For example, nitric acid alone is not a strong enough oxidizing agent, but can be used to accomplish the oxidation if used in combination with other reagents. One way is to mix ruthenium in a melted mixture of KMnO₄ and KOH and continue the process with KMnO₄ and dilute H₂SO4. After this RuO4 was separated by distillation, resulting in yellow-orange crystals.

Another way is to oxidize Ru(III)Cl, in the presence of KIO4,

{{8}Ru^{3+}(aq)+{5}IO_{4}^{-}(aq)+{12}H_{2}O(l)->{8}RuO_{4} (s)+{5}I^{-}(aq)+{24}H^{+}(aq)}

Synthesis of RuO4 can also be accomplished by electrolytic oxidation of ruthenium sulphate or by the use of chlorine on alkaline ruthenate at elevated temperature of 80-90 ˚C. 

It is important to remember that As ruthenium tetroxide is both toxic and volatile, safety precautions should be used when synthesising it.

Note to self: Photochemical decomposition paper has a nice description of the synthesis, add if/when time allows.

Uses

Ruthenium tetroxide is somewhat versatile and has several possible uses, including but not limited to, functioning as a catalyst and an oxidizing agent. As a catalyst, RuO4 ican be used for example the stereoselective oxidative cyclisation of 1,6-dienes. These synthesis are fast and no extra heat is required, as demonstrated by Piccialli's synthesis needing only 4 minutes at 0 °C temperature. 

As an oxidizing agent, it can be used to oxidize for example sulfones, ether and alkyl amides.

RuO₄ can also be used to produce RuO₂ thinfilms for the use of semiconductor industry or even as a marker for severe nuclear accidents.

However, it is worthwhile to note that the temperature has to be relatively low for RuO2 thinfilm growth by ALD. This is because RuO4 will decompose into RuO2 at 125 ˚C. 

On top of the the uses mentioned, it can be used to characterize polymers, especially rubber toughened plastics. Areas with higher electron density, will apear dark in TEM imaging and graphitized rubber areas that have been treated with RuO4 will have a high electron density.