Lecture 6

Question 1

4 main underpinning reactions

Answer 1

1) Adduct Formation
2) Insertion into M-X bonds
3) Electron Transfer
4) sigma bond metathesis

Question 2

Sigma bond metathesis

Answer 2

• via 4 membered diamond shaped transition state
• can activate inert chemicals such as methane
• exotic chemistry

Question 3

Good Synthetic Starting Points

Answer 3

• [Cp3Ln] not accessible due to extreme crowding caused by steric bulk except [Cp3Sm]
• Th(IV) complexes are diamagnetic so can be studied by NMR
• [Cp*2ThCl2] can be readily prepared from ThCl4
• UCl4 is readily accessible as a starting material, with [Cp4U] and [Cp3UCl] being made
• single electron reduction of U(IV) complexes give U(III) complexes

Question 4

Actinide Carbonyls

Answer 4

• largely U and Th
• mainly +4 but some +3 chemistry
• some dependence on symmetry and availability of the 5f orbitals
• backbonding means the actinides are transition metal like in nature

Question 5

Lanthanide Carbonyls

Answer 5

• 4f orbitals are too contracted to interact with CO pi* orbitals
• unstable Ln(CO)n (n=1-6) have been synthesised by co condensation on lanthanide vapours with CO in an argon matrix at -40 degrees
• C-O stretching frequencies increase (longer bonds) as n increases indicating a competition for back donating electron density
• not definitively characterised as they decompose at ambient temperatures

Question 6

Carbonyl Complexes of Uranium

Answer 6

• Can involve end on bridging of CO with each molecule binding to a U molecule
• can also involve electron transfer coupled to carbon carbon bond formation
• Evidence in bond lengths

Question 7

Reductive coupling of CO by Uranium

Answer 7

• Uranium can form a series of dianions of reduced CO
• Cyclic aromatic oxocarbons of the form [CnOn] 2-
• reaction is thermodynamically unfavourable in the absence of a metal
• Deliberate organic synthesis of these oxocarbons is challenging
• slight changes to ligands or reactive conditions results in formation of different ring sizes for stereochemical reasons
• simple coordination complexes can couple CO to form ynediolate

Question 8

Uranium Carbon Dioxide compounds

Answer 8

• requires electron transfer from U(III)to CO2
• ## exceptional end on CO2 bonding mode

Question 9

Uranium DiNitrogen Complexes

Answer 9

• End on form reversibly, with the complex forming at high pressures
• ## Some Side on binding reversibly reduces N2 to N2(2-)

Question 10

Ln Metallocenes

Answer 10

• [Cp*Ln] Early works and form bet structures
• hard cation centre
• Cp’s polarise cation to give charge gradient
• Maximising bond strength

Question 11

Ln Dinitrogen compounds

Answer 11

• Cp*2Sm can form dinuclear side on bridged complexes
• Reversibly formed
• N2 is weakly bound and can be removed in the solid state under vacuum
• More recently Tm, Dy and Nd have formed compounds with long dinitrogen bonds indicating electron transfer

Question 12

Th cyclopentadienyls

Answer 12

• Diamagnetic so easily studied using NMR
• can form dialkyls and hydrogens from dihalides
• rapid exchange between terminal and bridging hydrides

Question 13

Ln Cyclopentadienyls and hydrogen activation

Answer 13

• by sigma bond metathesis
• tend to decompose by beta hydride elimination
• use sterically demanding ligands or ligands that don’t have any beta protons

Question 14

CH4 activation

Answer 14

• [Cp*2LuMe] can break the CH bonds in methane

- monitored by proton NMR