Halides of the form LnX₂, LnX₃ & LnX₄ {only (Ce,Pr,Tb)F₄} exist

LnX₄

• Only (Ce, Tb, Pr)F₄ are known
  • correlation with I₄ of Ln
  • fluorides only - most oxidizing halogen!
• CeF₄ is comparatively stable e.g. crystallizes as a monohydrate from (aq)
• TbF₄, PrF₄ are thermally unstable and oxidize H₂O
  • i.e. prepare dry!
• MF₄ all white solids with the UF₄/ZrF₄ structure
  • Dodecahedral coordination of M

LnX₃

• All LnX₃ are known (except Pm {not attempted} & possibly EuI₃)
• Typically crystalline / high mpt. / deliquescent
• Typically obtained as hydrates from (aq) e.g. La - Nd 7H₂O

Nd - Lu 6H₂O

• On heating, react with water Æ oxyhalidesLnX₃ + H₂O Æ LnOX + 2HX

  at high temperatures react even with glass2LnX₃ + SiO₂ Æ 2LnOX + SiX₄

• Preparation of anhydrous LnX₃

  LnF₃

  1. Ln(NO₃)₃(aq) + 3HF Æ LnF₃•0.5H₂O_Ø(very insoluble)
  2. heat LnF₃•0.5H₂O (under an HF atmosphere for heavy Ln) Æ anhydrous LnF₃

  LnCl₃

  1. Ln₂O₃ / Ln₂(CO₃)₃ + HCl(aq) Æ LnCl₃•6-8H₂O (rather soluble)
  2. heat LnCl₃•6-8H₂O (under HCl atmosphere for heavy Ln) Æ anhydrous LnCl₃

     or

     Heat at 300°C, Ln₂O₃ + 6NH₄Cl Æ 2LnCl₃ + 3H₂O + 6NH₃

  LnBr₃ / LnI₃

  1. Best by direct combination (susceptible to hydrolysis to oxyhalides)
  2. purify by sublimation (but avoid contact with hot silica!)
• Structures: Ln coordination varies from 9 for light trifluorides to 6 for heavy iodides

  e.g. LnCl3	La - Gd	UCl3 structure	9-coordinate Ln	tri-capped trigonal prism (ttp)
  	Tb, Dy(form I)	PuBr3 structure	8-coordinate Ln	bi-capped trigonal prism (btp)
  	Dy-Lu	AlCl3 structure	6-coordinate Ln	octahedron

LnX₂

• Preparation
  • Typically from comproportionation:- Ln + 2LnX₃ 3LnX₂
  • {(Sm,Eu,Yb)I₂ are obtained from thermal decomposition of LnX₃
  • (Sm,Yb)I₂ from Ln + ICH₂CH₂I LnI₂ + CH₂=CH₂ }
• LnX₂ are easily oxidized
  • liberate H₂ from H₂O {Except for EuX₂ }
• Occurrence of dihalides
  • parallels high values for I₃
  • depends upon the oxidizing power of the halogen (iodides most numerous!)

• Trends in the Stability of MX₂
  • Consider 3MX₂(s) M(s) + 2MX₃(s)
    • the reverse of the preparation of MX₂
    • the most likely decomposition route for MX₂

    D_mH° = 3D_LH(MX₂) - 2D_LH(MX₃) + 2I₃ - D_atmH°(M) - (I₁ + I₂)

    • Irregularities should follow [ 2I₃ - - D_atmH°(M) ]

      i.e. effectively follow I₃ [since variation in D_atmH follows closely variation in I₃

      More? see D.A. Johnson, Some Thermodynamic Aspects of Inorganic Chemistry, p. 160-162, 165]

    

    explains occurrence of MCl₂

    • La, Ce, Pr MCl₂ unknown
    • (Sm, Eu, Yb)Cl₂ are the most stable MCl₂

      ~ may be prepared from LnCl₃(s) + ¹/₂H₂ Æ LnCl₂(s) + HCl

    • Gd, Tb low I₃ Þ MCl₂ unstable to disproportionation
    • Nd, Dy, Tm MCl₂ known
    • Ho, Er MCl₂ unknown
• Occurrence:
  • all X (Sm,Eu, Yb)
  • X=Cl,Br,I (Nd,Dy,Tm,)
  • X=I only (La,Ce,Pr,Gd)
• Structures
  • Coordination numbers from 9 to 6 (see: Wells)
  • Fluorides are Fluorite (CaF₂) [C.N. = 8]
  • Nd,Sm,Eu chlorides are PbCl₂type [C.N. = 7 + 2]
  • Nd,Sm,Eu bromides and iodides are SrBr₂ type [mixed C.N. = 7 & 8]
  • (Dy,Tm,Yb)I₂ have layer structures (CdCl₂,CdI₂) [C.N. = 6] polarization effects
• Two Classes of Dihalide

  1. Most LnX₂ are regarded as "salt-like" halides (insulators)

  2. (Ce,Pr,Gd)I₂ have metallic lustre, high conductivity

  • Þ formulation as Ln³⁺(I^-)₂(e^-) with the electron in a delocalized conduction band
    • see Cox, Electronic Properties of Solids, {p. 142-145 explanation of metallic Ln^II compounds}
• Ln^II Compounds are finding increasingly more uses

Lower Halides

• LnX₃/Ln melts yield phases of reduced halide formulae e.g. Ln₂X₃ & LnX
• "Reduced Halides" contain "condensed metal clusters"
• Black & metallic Þ delocalization of electrons through the metal-metal bonded networks

  Gd2X3 single chains of edge-sharing metal octahedra with M6X8-type environment (i.e.face-capped by X )

• Lowest Halides are stabilized by H, C or N atoms encapsulated in Ln₆ cluster octahedra

  e.g. Gd₂Cl₂C₂ Layers of edge-sharing M₆C units

  

  e.g. Gd₃Cl₃CFramework of M₆C units

Bibliography