Lecture 5

Question 1

What are the general features of Ln organometallic chemistry?

Answer 1

• not as extensive as for transition metals
• primarily ionic due to contracted 4f orbitals
• Ln can’t act as pi donor so CO complexes are unstable
• very sensitive to air and moisture due to carbonic character of ligands and oxophilicity of Ln

Question 2

What are the general features of Ac organometallic chemistry?

Answer 2

• largely confined to uranium and thorium
• mostly +4 but some very reactive Ur 3+ compounds are formed
• Behaviour is between d block elements and lanthanides, with dependency on symmetry and 5f availability

Question 3

What orbitals form a phi bond?

Answer 3

The general set of 4f orbitals must be used, as they have 3 nodes. They overlap with a suitable complex ligand/ ligand cell with the right orbital arrangement.

Question 4

How are lanthanide cyclopentadienyl complexes synthesised?

Answer 4

For the trivalent lanthanides 3 types of cyclopentadienyls are known, [LnCpnX3-n] (n = 1, 2 or 3, X = halide). – synthesized by reaction of the anhydrous lanthanide trihalides with NaCp or KCp .

Question 5

How do the structures of lanthanide cyclopentadienyl complexes vary across the period?

Answer 5

• La-Pr have polymeric 10-11 coordinate structures, where each metal centre has 3 η5-ligands and one η3- or η2- ligand
• Tm and Yb form 9-coordinate monomeric structures
• Lu forms an 8-coordinate
  polymeric structure.

Question 6

Bonding and reactivity of lanthanide cyclopentadienyl complexes

Answer 6

• ionic in nature
• will react readily with FeCl2 to form ferrocene
• high polarity causes rapid exchange reactions

Question 7

[LnCp2X] compounds

Answer 7

• generally dimers in the solid state
• form adducts with bases such as THF
• low solubility in hydrocarbon solvents
• prone to facile ligand redistribution reactions
• Prone to reactions that deprotonate the Cp ring
• must be adapted

Question 8

Substituted Cyclopentadienyl Complexes

Answer 8

• Replace Cp– with C5Me5– or C5H3(SiMe3)2–
• more convenient properties
• less prone to redistribution
• more soluble in organic solvents

Question 9

Cp* complexes

Answer 9

• Tris [Cp3Ln] complexes are inaccessible (with the exception of Cp3Sm) because of the extreme crowding
• [Cp2LnCl] are synthesised from LnCl3 and 2 equiv. of CpNa
• ideal starting materials for the exploration of organolanthanide chemistry.
• generally exist as dimers in the solid state with bridging halides.

Question 10

Actinide Cyclopentadienyls

Answer 10

• Mostly Th(IV) and U(IV)
• U(η5-C5H5)4] is the only known tetracyclopentadienyl compound to have all 4 Cp rings bounds in an η5-fashion.
• smaller ions have ring slippage
• don’t react to form ferrocene due to higher covelant character

Question 11

Arene Complexes

Answer 11

• made by cocondensation of lanthanide metal vapours with excess bulky arene
• C6H3tBu3 results in the formation of zerovalent (Ln0) complexes [Ln(η6-C6H3tBu3)2] (Ln = Pr, Nd, Gd, Tb, Dy, Ho, Er and Sc,Y).
• bonding reminiscent of 18 e- complexes
• stability mirrors promotion energy, except La and Ce which are kinetically unstable

Question 12

Cyclooctatetraene Dianion Complexes

Answer 12

• COT needs two additional electrons to become stable
• statically demanding so good at forming complexed with Ln
• Lns form trivalent compounds except Ce which forms a neutral sandwich molecule

Question 13

Ce(COT)2

Answer 13

• oxidation state supposedly +4
• however recent calculations show the Ce is in the +3 state, with a bond valence of 1.5 to each COT
• bonding best regarded as being essentially ionic

Question 14

Actinide COT complexes

Answer 14

• more covelant
• COT rings are planar and oriented in an eclipsed formation giving D(8h) symmetry
• LEARN MO DIAGRAM