Ruthenium (Ru)

Ruthenium is a hard, brittle, silvery-gray platinum-group transition metal used to harden platinum and palladium, in wear-resistant electrical contacts, and as a catalyst.

Atomic Number
44
Atomic Mass
101.07
Phase (STP)
Solid
Block
D
Electronegativity (Pauling)
2.2

Bohr Atomic Model

Protons
44
Neutrons
44
Electrons
44
Identity
Atomic Number44
SymbolRu
NameRuthenium
Group8
Period5
Position
Period5
Group Label8
Grid X8
Grid Y5
Physical Properties
Atomic Mass (u)101.07
Density (g/cm³)12.1
Melting Point (K)2606 K 2334 °C
Boiling Point4420 K 4150 °C
Phase at STPSolid
CategoryTransition Metals
Liquid Density (g/cm³)—
Molar Volume (cm³/mol)8.12
Emission Spectrum (nm)—
Discovery
English NameRuthenium
English Pronunciationruːˈθiːniəm
Latin NameRuthenium
Latin Pronunciationroo-THEE-nee-um
Year1844
DiscovererKarl Karlovich Klaus
CountryRussia
CAS Number7440-18-8
CID Number—
RTECS Number—
Atomic Properties
Electron Shell—
Electron Configuration[Kr] 4d^75s^1
Oxidation States-2 0 +1 +2 +3 +4 +5 +6 +7 +8
Ion ChargeRu3+, Ru4+
Ionization Potential (eV)7.36
Electronegativity (Pauling)2.2
Electron Affinity (kJ/mol)101.31
Electrons44
Protons44
Neutrons—
Valence—
BlockD
Atomic Radius (pm)—
Covalent Radius (pm)136
van der Waals Radius (pm)213
Thermodynamic Properties
PhaseSOLID
Heat of Fusion (kJ/mol)—
Specific Heat (J/g·K)0.238
Thermal Expansion (1/K)—
Heat of Vaporization (kJ/mol)—
Mechanical Properties
Brinell Hardness—
Mohs Hardness6.5
Vickers Hardness—
Bulk Modulus (GPa)—
Young's Modulus (GPa)—
Shear Modulus (GPa)—
Poisson Ratio—
Sound Speed (m/s)—
Refractive Index—
Thermal Conductivity (W/m·K)—
Electromagnetic Properties
Electrical Conductivity (S/m)14000000
Electrical TypeCONDUCTOR
Magnetic TypePARAMAGNETIC
Volume Magnetic Susceptibility—
Mass Magnetic Susceptibility—
Molar Magnetic Susceptibility—
Resistivity (Ω·m)0
Superconducting Point (K)—
Crystal Properties
StructureHexagonal close-packed (hcp) — solid Phase I
SystemHEXAGONAL
Space GroupP6₃/mmc
a (Å)2.705
b (Å)2.705
c (Å)4.281
α (°)90
β (°)90
γ (°)120
Debye Temperature (K)—
Nuclear Properties
RadioactiveNo
Half-life—
Lifetime—
Neutron Cross-section (barn)—
Safety Information
Health Hazard—
Reactivity Hazard—
Specific HazardElemental Ru is relatively inert; ruthenium tetroxide (RuO4) is highly toxic and volatile.
Prevalence
Universe—
Sun—
Oceans—
Human Body—
Earth Crust—
Meteorites—


FAQs about Ruthenium

Ruthenium exhibits oxidation states from −2 to +8, with +2, +3, +4 most common in coordination chemistry and +8 in volatile ruthenium tetroxide \(\mathrm{RuO_4}\).

  • +2/+3: Polypyridyl and ammine complexes, e.g., \(\mathrm{[Ru(bpy)_3]^{2+}}\).
  • +4: Ruthenium dioxide \(\mathrm{RuO_2}\), a conductive, chemically robust oxide.
  • +8: \(\mathrm{RuO_4}\), a powerful oxidant (handle with extreme care).

The ground-state configuration is \([\mathrm{Kr}]\,4d^7\,5s^1\), which is an "anomalous" arrangement compared to the simple filling order (you might expect \(4d^6\,5s^2\)). The \(4d\)/\(5s\) near-degeneracy favors a configuration that enhances exchange stabilization. In complexes, Ru commonly engages its \(4d\) electrons in bonding across multiple oxidation states.

Small additions of Ru form solid solutions and intermetallics that increase hardness, wear resistance, and high-temperature stability of Pt and Pd. This improves lifetime of electrical contacts and jewelry alloys without severely compromising corrosion resistance.

  • \(\mathrm{RuO_4}\): Ruthenium tetroxide is volatile, highly oxidizing, and toxic. It can oxidize alkenes and alcohols (e.g., to carbonyls or acids). Handle only with stringent safety protocols.
  • \(\mathrm{RuO_2}\): Ruthenium dioxide is a conductive, stable ceramic used in thick-film resistors, electrochemical electrodes, and as a catalyst support.

\(\mathrm{Ru} \xrightarrow[O_2]{\text{oxidation}} \mathrm{RuO_2} \xrightarrow[\text{strong}\,O]{ } \mathrm{RuO_4}}\)

Ru forms versatile organometallic catalysts that mediate hydrogenation, transfer hydrogenation, C–C coupling, and olefin metathesis. Grubbs-type metathesis catalysts feature a Ru–carbene center that exchanges alkylidene fragments between alkenes.

\(\mathrm{R^1CH{=}CH_2 + R^2CH{=}CH_2 \xrightarrow[\text{Ru}]{\text{metathesis}} R^1CH{=}CH\,R^2 + CH_2{=}CH_2}\)

Complexes such as \(\mathrm{[Ru(bpy)_3]^{2+}}\) (bpy = 2,2′-bipyridine) have intense metal-to-ligand charge-transfer (MLCT) bands, long-lived excited states, and reversible redox behavior. These features enable photoredox catalysis, dye-sensitized solar cells, and luminescent probes.

\(\mathrm{[Ru^{II}(bpy)_3]^{2+} + h\nu \rightarrow [Ru^{II}(bpy)_3]^{2+*}}\)

Ru and Os (Group 8) both form high-valent oxo-species (e.g., \(\mathrm{RuO_4}\), \(\mathrm{OsO_4}\)). Osmium tetroxide is more widely known as a staining reagent; ruthenium tetroxide is similarly powerful but less commonly used due to handling risks. Compared with Pt/Pd, Ru is generally more affordable and often chosen for specific catalytic niches or alloy strengthening.

Ruthenium is a by-product of platinum, palladium, and nickel/copper refining. After primary metal recovery, the platinum-group metal residue is chemically separated to isolate Ru, which can then be converted to \(\mathrm{RuO_2}\) or metallic Ru.

  • Thick-film resistors: RuO2-based pastes with stable resistivity.
  • Electrodes/capacitors: Conductive, corrosion-resistant \(\mathrm{RuO_2}\) layers.
  • Electrocatalysis: Ru-containing oxides for oxygen evolution/reduction in alkaline or acidic media.
  • Solar/LEDs: Ru–polypyridyl dyes in photoactive systems.

Metallic Ru and \(\mathrm{RuO_2}\) are relatively stable, but \(\mathrm{RuO_4}\) is highly hazardous (volatile, strong oxidant). Use fume hoods, cold traps, and proper PPE. Ruthenium-containing dusts should be minimized to avoid inhalation; wastes must be collected and treated under local regulations.