Oxohalides of Transition Metals

Oxohalides of Transition Metals

Oxohalides are chemical compounds containing a transition metal, oxygen, and at least one halogen atom (F, Cl, Br, or I). They have both metal–oxygen (M=O) and metal–halogen bonds. They are formed when transition metals in high oxidation states react with halogens and oxygen under specific conditions, such as high temperature or in the presence of oxygen-containing reagents.

Key Characteristics

1. Structure: Oxohalides often have a central transition metal atom bonded to oxygen (as an oxo group) and halogens. The oxygen is usually double-bonded to the metal, and the halogen atoms are covalently bonded. The structure can be molecular, polymeric, or ionic, depending on the metal and halogen involved.
   Example: Vanadium oxytrichloride (VOCl₃) has a tetrahedral structure with a V=O bond and three V–Cl bonds.
2. Oxidation States: Transition metals in oxohalides typically exhibit high oxidation states, as the electronegative oxygen and halogen atoms stabilize these states.
   Example: In MoO₂Cl₂ (molybdenum dioxydichloride), molybdenum is in the +6 oxidation state.
3. Formation: Oxohalides are often formed by:
   
   • Direct reaction of a transition metal oxide with a halogen or halide.
   • Partial hydrolysis of metal halides.
   • Reaction of metals or their oxides with halogenating agents in the presence of oxygen.
   • Example: 2V₂O₅ + 3SOCl₂ → 2VOCl₃ + 3SO₂


4. Examples:
   • Vanadium: VOCl₃ (vanadium oxytrichloride), VOF₃ (vanadium oxyfluoride).
   • Molybdenum: MoO₂Cl₂ (molybdenum dioxydichloride), MoOCl₄.
   • Tungsten: WOCl₄ (tungsten oxytetrachloride).
   • Chromium: CrO₂Cl₂ (chromyl chloride).
5. Properties:
   • Reactivity: Oxohalides react easily with water called hydrolysis. For example, CrO₂Cl₂ reacts with water to form chromic acid (H₂CrO₄) and HCl.
   • Volatility: Many oxohalides, like VOCl₃ or CrO₂Cl₂, are volatile liquids or gases due to their covalent nature.
   • Color: They often exhibit vivid colors due to electronic transitions in the metal center (e.g., CrO₂Cl₂ is deep red).
6. Applications:
   • Used as catalysts or intermediates in organic synthesis (e.g., VOCl₃ in polymerization reactions).
   • Precursors for preparing metal oxides or other metal compounds.
   • In industrial processes, for the purification of metals

Oxohalides of Transition Metals by Series

Oxohalides for the first, second, and third transition series are given below.

First Transition Series (3d Series: Sc to Zn)

• Titanium (Ti):
  • TiOCl₂ (titanium oxydichloride)
  • TiOF₂ (titanium oxyfluoride)
• Vanadium (V):
  • VOCl₃ (vanadium oxytrichloride)
  • VOF₃ (vanadium oxyfluoride)
  • VOCl₂ (vanadium oxydichloride)
  • VOF₂ (vanadium oxyfluoride)
• Chromium (Cr):
  • CrO₂Cl₂ (chromyl chloride)
  • CrO₂F₂ (chromyl fluoride)
  • CrOF₄ (chromium oxyfluoride)
• Manganese (Mn):
  • MnO₃Cl (manganese oxychloride or Manganese(VII) trioxide chloride or Permanganyl chloride, unstable)
• Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn): No well-documented oxohalides.

Second Transition Series (4d Series: Y to Cd)

• Zirconium (Zr):
  • ZrOCl₂ (zirconium oxydichloride, often hydrated as ZrOCl₂·8H₂O)
  • ZrOF₂ (zirconium oxyfluoride)
• Niobium (Nb):
  • NbOCl₃ (niobium oxytrichloride)
  • NbOF₃ (niobium oxyfluoride)
  • NbO₂Cl (niobium dioxychloride)
• Molybdenum (Mo):
  • MoO₂Cl₂ (molybdenum dioxydichloride)
  • MoOCl₄ (molybdenum oxytetrachloride)
  • MoOF₄ (molybdenum oxyfluoride)
  • MoO₂F₂ (molybdenum dioxyfluoride)
• Technetium (Tc):
  • TcO₃Cl (technetium oxychloride, rare)
• Ruthenium (Ru):
  • RuO₃Cl₂ (ruthenium oxychloride, rare)
• Rhodium (Rh), Palladium (Pd), Silver (Ag), Cadmium (Cd): No well-documented oxohalides.

Third Transition Series (5d Series: Hf to Hg)

• Hafnium (Hf):
  • HfOCl₂ (hafnium oxydichloride, often hydrated as HfOCl₂·8H₂O)
  • HfOF₂ (hafnium oxyfluoride)
• Tantalum (Ta):
  • TaOCl₃ (tantalum oxytrichloride)
  • TaOF₃ (tantalum oxyfluoride)
  • TaO₂F (tantalum dioxyfluoride)
• Tungsten (W):
  • WOCl₄ (tungsten oxytetrachloride)
  • WOF₄ (tungsten oxyfluoride)
  • WO₂Cl₂ (tungsten dioxydichloride)
  • WO₂F₂ (tungsten dioxyfluoride)
• Rhenium (Re):
  • ReO₃Cl (rhenium oxychloride)
  • ReOF₅ (rhenium oxyfluoride)
  • ReO₂Cl₃ (rhenium dioxychloride)
• Osmium (Os):
  • OsO₃Cl₂ (osmium oxychloride, rare)
• Iridium (Ir), Platinum (Pt), Gold (Au), Mercury (Hg): No well-documented oxohalides.

Stability and Trends

• Oxohalides are more stable for early transition metals (Groups 5–7) in high oxidation states because oxygen and halogens can stabilize the electron-deficient metal centers.
• Fluorine-containing oxohalides (oxyfluorides) are often more stable than chlorides, bromides, or iodides due to fluorine’s higher electronegativity and stronger bonding.

Structures of Oxohalides of Transition Metals

Below are the molecular structures of representative oxohalides of transition metals from the first, second, and third transition series, focusing on their geometry and bonding characteristics.

1. First Transition Series (3d Series)

• Vanadium Oxytrichloride (VOCl₃)
  • Geometry: Tetrahedral
  • Description: Vanadium (V, +5) is bonded to one oxygen (V=O, double bond) and three chlorine atoms (V–Cl). The V=O bond is ~1.57 Å, and V–Cl bonds are ~2.1–2.2 Å.
  • Bond Angles: ∠Cl–V–Cl ≈ 108–110°, ∠O=V–Cl ≈ 110–112°.
  • Note: Volatile reddish-yellow liquid, used in catalysis.


• Chromyl Chloride (CrO₂Cl₂)
  • Geometry: Tetrahedral
  • Description: Chromium (Cr, +6) is bonded to two oxygens (Cr=O) and two chlorines (Cr–Cl). Cr=O bonds are ~1.57–1.60 Å, Cr–Cl bonds are ~2.2–2.3 Å.
  • Bond Angles: ∠O=Cr=O ≈ 108°, ∠Cl–Cr–Cl ≈ 111°, ∠O=Cr–Cl ≈ 109–110°.
  • Note: Deep red liquid, reactive with water, used in oxidations.

2. Second Transition Series (4d Series)

• Molybdenum Dioxydichloride (MoO₂Cl₂)
  • Geometry: Distorted Tetrahedral (monomeric) or Polymeric
  • Description: Molybdenum (Mo, +6) is bonded to two oxygens (Mo=O) and two chlorines (Mo–Cl). Mo=O bonds are ~1.65–1.70 Å, Mo–Cl bonds are ~2.3–2.4 Å. Solid state may involve polymeric chains.
  • Bond Angles (Monomeric): ∠O=Mo=O ≈ 105–108°, ∠Cl–Mo–Cl ≈ 112°, ∠O=Mo–Cl ≈ 108–110°.
  • Note: Yellow solid, used as a catalyst precursor.


• Niobium Oxytrichloride (NbOCl₃)
  • Geometry: Polymeric (tetrahedral in gaseous state and octahedral in solid state)
  • Description: Niobium (Nb, +5) forms a polymeric structure with one terminal oxygen (Nb=O), three chlorines, and bridging ligands. Nb=O bond is ~1.65–1.70 Å, Nb–Cl bonds are ~2.3–2.5 Å.
  • Coordination: Octahedral with terminal and bridging ligands.
  • Note: White solid, used in niobium chemistry.

3. Third Transition Series (5d Series)

• Tungsten Oxytetrachloride (WOCl₄)
  • Geometry: Square Pyramidal (monomeric) or Polymeric
  • Description: Tungsten (W, +6) is bonded to one oxygen (W=O) and four chlorines (W–Cl). W=O bond is ~1.65–1.70 Å, W–Cl bonds are ~2.3–2.4 Å. Oxygen is apical in the square pyramid.
  • Bond Angles (Monomeric): ∠O=W–Cl ≈ 90–95°, ∠Cl–W–Cl (base) ≈ 85–90°.
  • Note: Red-orange solid, used in catalysis.


• Rhenium Oxychloride (ReO₃Cl)
  • Geometry: Tetrahedral
  • Description: Rhenium (Re, +7) is bonded to three oxygens (Re=O) and one chlorine (Re–Cl). Re=O bonds are ~1.65–1.70 Å, Re–Cl bond is ~2.2–2.3 Å.
  • Bond Angles: ∠O=Re=O ≈ 108°, ∠O=Re–Cl ≈ 110°.
  • Note: Volatile compound, used in rhenium chemistry.

Key Points to Remember:

• Oxohalides have both oxo (M=O) and halide (M–X) bonds.
• Metals are in high oxidation states.
• Most are reactive, colored, and can be volatile.
• They have important uses in chemistry and industry.