to Lanthanides & Actinides Menu
  << BACK TO MENU | PERIODIC TABLE >>BibliographyBIBLIOGRAPHY
Lanthanides & Actinides
Halides

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 LnF30.5H2OØ(very insoluble)
    2. heat LnF30.5H2O (under an HF atmosphere for heavy Ln) Æ anhydrous LnF3

    LnCl3

    1. Ln2O3 / Ln2(CO3)3 + HCl(aq) Æ LnCl36-8H2O (rather soluble)
    2. heat LnCl36-8H2O (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

Coordination Environments from LnX3 structures

LnX3 structures (and colours)

La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

F

LaF3

white

LaF3

white

LaF3

green

LaF3

violet

LaF3?

YF3

white

YF3

white

YF3

white

YF3

white

YF3

green

YF3

pink

YF3

pink

YF3

white

YF3

white

YF3

white

Cl

UCl3

white

UCl3

white

UCl3

green

UCl3

mauve

UCl3?

UCl3

yellow

UCl3

yellow

UCl3

white

PuBr3

white

AlCl3

white

AlCl3

yellow

AlCl3

violet

AlCl3

yellow

AlCl3

white

AlCl3

white

Br

UCl3

white

UCl3

white

UCl3

green

PuBr3

violet

PuBr3

?

PuBr3

yellow

PuBr3

grey

FeCl3

white

FeCl3

white

FeCl3

white

FeCl3

yellow

FeCl3

violet

FeCl3

white

FeCl3

white

FeCl3

white

I

PuBr3

white

PuBr3

yellow

PuBr3

PuBr3

green

PuBr3

?

FeCl3

orange

FeCl3

?

FeCl3

yellow

FeCl3

FeCl3

green

FeCl3

yellow

FeCl3

violet

FeCl3

yellow

FeCl3

white

FeCl3

brown

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

      DmH° = 3DLH(MX2) - 2DLH(MX3) + 2I3 - DatmH°(M) - (I1 + I2)

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

        i.e. effectively follow I3 [since variation in DatmH 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 LnII compounds}
  • LnII 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


--Info & DownloadsBibliography  [textbook & online resources]

Source: Dr. S.J. Heyes; University of Oxford
      << BACK TO MENU | PERIODIC TABLE | MEMBERSHIP | HOME >>Top of Page
Join Today!
.:: Radiochemistry.org© - 2003 ::.