Osmium (Os): Properties, Reactions & Uses | JEE/NEET Chemistry

Introduction

Osmium (Os) is a transition metal within Group 8 of the periodic table, known primarily as the densest naturally occurring element. Its high density, extreme hardness, and high melting point contribute to its specialized applications, particularly in alloys and as a catalyst. It belongs to the platinum group metals (PGMs).

CBSE/JEE Quick Revision Notes

• Symbol: Os
• Atomic Number: 76
• Atomic Mass: 190.23 g/mol
• Group: 8
• Period: 6
• Block: d-block (Transition Metal)
• Density: 22.59 g/cm³ (highest known density)
• Melting Point: 3033 °C
• Boiling Point: 5012 °C
• Nature: Silvery-blue, brittle, hard metal
• Common Oxidation States: +2, +3, +4, +6, +8 (most common and stable are +4 and +8)

Electron Configuration & Bonding Behavior

• Ground State Electron Configuration: [Xe] 4f¹⁴ 5d⁶ 6s²
  • The presence of 4f electrons (which are filled) is characteristic of the 6th period transition metals following the lanthanides.
• Bonding Behavior:
  • Osmium exhibits a wide range of oxidation states, with +8 being the highest known for any transition metal (e.g., in OsO₄).
  • In its higher oxidation states (+6, +8), osmium forms predominantly covalent compounds. Osmium tetroxide (OsO₄) is a molecular compound with a tetrahedral geometry, indicating significant covalent character in its bonds.
  • Lower oxidation states typically involve more ionic or metallic bonding in compounds.

Crucial Chemical Reactions

Osmium is relatively unreactive as a bulk metal but reacts readily when finely divided or heated.

1. Reaction with Oxygen (Formation of Osmium Tetroxide): When heated in air or oxygen, osmium forms its most stable oxide, osmium tetroxide, which is highly volatile and toxic. Os(s) + 2O₂(g) → OsO₄(s) Note: OsO₄ is a powerful oxidizing agent.

2. Reaction with Halogens (e.g., Chlorine): Osmium reacts with halogens like chlorine at elevated temperatures to form osmium halides. Os(s) + 2Cl₂(g) → OsCl₄(s) (Osmium(IV) chloride)

3. Reduction of Osmium Tetroxide: Osmium tetroxide can be reduced to lower oxidation states or to osmium metal. For instance, reduction by hydrogen sulfide: OsO₄(s) + 4H₂S(g) → OsS₂(s) + 4H₂O(l) (Simplified; can form intermediate products)

4. Reaction with Acids: Bulk osmium is largely resistant to acids, including aqua regia, but finely divided osmium can be slowly oxidized by strong oxidizing acids.

Industrial and Biological Importance

Industrial Uses

• Hardening Alloys: Osmium is alloyed with other platinum group metals (especially iridium, forming osmiridium) to produce extremely hard and wear-resistant alloys. These alloys are used in:
  • Pen tips (fountain pens)
  • Electrical contacts
  • Phonograph needles
  • Instrument pivots
  • Heavy-duty electrical switch contacts
• Catalysis: Osmium compounds, particularly osmium tetroxide (OsO₄), serve as significant catalysts in organic synthesis.
  • Dihydroxylation of Alkenes: OsO₄ is crucial for the stereospecific syn-dihydroxylation of alkenes (e.g., in the Upjohn dihydroxylation and Sharpless asymmetric dihydroxylation), converting C=C double bonds into diols.
• Staining for Electron Microscopy: Osmium tetroxide is used as a staining agent for biological tissues in electron microscopy. Its ability to fix and stain lipids provides contrast for viewing cellular structures.

Biological Importance

• Toxicity: Osmium has no known biological role. Osmium compounds, particularly osmium tetroxide (OsO₄), are highly toxic. OsO₄ is volatile and its vapors can cause severe irritation to eyes and respiratory tract, leading to tissue damage and even blindness. It should be handled with extreme caution in a fume hood.