LANTHANIDES

Comparisons and Contrasts

Yttrium - Why consider it with the Lanthanides?

• Occurs with lanthanides in rare earth minerals, e.g. monazite
• Y occurs effectively exclusively in +3 oxidation state

  combines with non-metals Æ YHal₃ Y₂O₃ , Y³⁺(H^-)₂(e^-), YH₃, etc...

• Y³⁺ has same radius as Ho³⁺ and is difficult to separate from it
• Forms complexes of high coordination number with chelating O-donors, e.g. Y(acac)₃(H₂O)
• Typical organometallics include: Y(C₅H₅)₃ (polymeric in the solid state)

  dimeric Y(C₅H₅)₂Cl, monomeric form is a thf adduct

Scandium too?

6 Reasons why Scandium could be considered with the Lanthanides

1. Sc occurs effectively exclusively in +3 oxidation state

combines with non-metals Æ ScHal₃ Sc₂O₃, etc...

but coordination octahedral (small size)

2. Sc forms reduced halides e.g. Sc₇Cl₁₂ which is Sc³⁺(Sc₆Cl₁₂)^3- with Sc₆ clusters (but c.f. Nb)

3. Scandium Hydride ScH₂ is highly conducting Sc³⁺(H^-)₂(e^-)

4. Forms complexes of high coordination number with chelating O-donors

e.g. Na⁺[Sc(CF₃COCHCOCF₃)₄^-] with C.N. = 8

but forms octahedral complexes with monodentate ligands e.g. mer-ScCl₃(thf)₃

5. Nitrate & Sulphate are obtained as hydrated salts Sc(NO₃)₃•4H₂O & Sc₂(SO₄)₃•5H₂O

6. Typical organometallics include: Sc(C₅H₅)₃ (polymeric in the solid state)

dimeric Sc(C₅H₅)₂Cl, monomeric form is a thf adduct

6 Reasons why Scandium could be considered as main group IIIA

1. Sc³⁺ (r = 74 pm) is appreciably smaller than any of the rare earths

Þ behaviour intermediate between the Lanthanides & Aluminium

2. Sc₂O₃ is more like Al₂O₃ than Ln₂O₃: amphoteric Æ Sc(OH)₆^3- in excess OH^-

3. ScF₃ disssolves in excess F^- Æ ScF₆^3- (N.B. scarcity of halogeno complexes for Lanthanides)

4. Anhydrous ScCl₃ is easily obtained by P₂O₅-dehydration of hydrated halide

but unlike AlCl₃, ScCl₃ is not a Friedel-Crafts catalyst

Scandium may with similarly few exceptions be viewed as a 1st Row Transition Metal

• Six-coordinate complexes are typical
• Aqua-ion is Sc(H₂O)₆³⁺ and is susceptible to hydrolysis Æ O-H-bridged dimers...

Some CONTRASTS between Lanthanides & Pre-Transition & Transition Metals

Pre-Transition Metals	Lanthanides	Transition Metals
Essentially Monovalent - show Group (n+) oxidation state	Essentially Monovalent (+3) +2/+4 for certain configurations	Show Variable Valence (extensive redox chemistry) control by environment ~ ligands, pH etc…
Periodic trends - dominated by (effective nuclear) charge at noble gas config. (i.e. on group valence)	Lanthanide Contraction of Ln3+	Size changes of Mn+ less marked
Similar Properties for a given group (differentiated by size)	Similar Properties (differentiated by size)	Substantial Gradation in Properties
widespread on earth	common mineralogy	diverse mineralogy
No Ligand Field Effects	Insignificant Ligand Field Effects	Substantial Ligand Field Effects
Always 'hard' (O, Hal, N donors) (preferably negatively charged)	Always 'hard' (O, Hal, N donors) (preferably negatively charged)	Later (increasingly from Fe&endash;Cu)/heavier metals may show a 'soft' side
'Ionic' Æ 'Covalent' Organometallics	'Ionic' Organometallics	'Covalent' Organometallics
No ¹-Ligand Effects	Paucity of ¹-Ligand Effects	¹-Acceptor Ligands Æ Extensive Chemistry
Poor Coordination Properties (C.N. determined by size)	High Coordination Numbers (C.N. determined by size)	Extensive Coordination C.N. = 6 is typical maximum (but many exceptions)
Flexibility in Geometry	Flexibility in Geometry	Fixed (by Ligand Field effects) Geometries
No Magnetism from the metal ions - noble gas configurations of ions	Free Ion-like Magnetism ground state magnetism	Orbital Magnetism 'Quenched' by Ligand Fields excited J-states populated
'Ionic' compound formulations Þ large HOMO-LUMO gaps Þ UV CT spectra	Weak, Narrow Optical Spectra forbidden, unfacilitated transitions	Stronger, Broader Optical Spectra forbidden transitions vibronically-assisted