5: Organometallics Flashcards
Bonding and nodes
sigma: an MO is unaffected though any angy about the internuclear axis (0 nodes)
pi: if a MO changes sign on 180 rotation about the internuclear axis (1 node)
delta: Bonds involve the face-on overlap of one orbital with four lobes with another orbital with four orbitals (2 nodes)
phi: bonds involve the face on overlap of one orbital with 6 lobes with another orbital with six orbitals (3 nodes)
General features of Lanthanide Organometallic Chemistry
- not as extensive as organoTM chemistry
- primarily ionic bonding due to contracted 4f
- Cannot act as pi bases and Ln-CO compounds not stable
- Organolanthanides extremely sensitive to O2 and H2O due to the carbanionic character of the organic ligands and the oxophilicity of Ln2+/Ln3+
General features of Actinide Organometallic Chemistry
- largely confined to U and Th and mostly in 4+ although some 3+ compounds are known- tend to be highly reactive
- the behaviour of An organometallics id between d block and Ln, there is some dependancy on symmetry and availability of the 5f orbitals
Lanthanide cyclopentadienyl complexes: general
The bonding is ionic in nature
For Ln3+, 3 main types of LnCp’s
[LnCp3], [LnCp2Cl] and [LnCpCl2]
Synthesised by:
-Anhydrous Ln halide + Na/KCp in stoichiometric amounts in THF
LnCl3 +2NaCp –> LnCp2Cl +2NaCl
Solid state structure of LnCp3 -reflect lanthanide contraction ** Early CN= 10 Mid CN=9 Late CN=8 PICTURES In solution there is rapid exchange of Cp ligands occurs between lanthanide complexes
LnCpX: properties/reactivity
LnCp3 compounds react readily with FeCl2 to form ferrocene
3FeCl2 +2LnCp3 –> 3FeCp2 + 2LnCl3
LnCp2X compounds are usually dimers in solid state (picture drawn in panopto
but also form base adducts w THF for example
(PIcture slide)
Limitation of Ln-Cp complex
Low solubility in hydrocarbon solvents
Prone to facile ligand redistribution reactions
Prone to reactions that deprotonate the C-H bonds of Cp ligand
How are the limitations of Ln-Cp complexes solved?
By replacing use of Cp with bulkier analogues
eg Cp*and C’C5H3(SiMe3)2
these are now less prone to ligand redistribustion and are more soluble in organic solvents
tris complexes (LnCp*3) are inaccessible due to steric bulk of ligand causing extreme crowding apart from w/Sm
however LnCp*2Cl complexes readily synthesised
generally exist in dimers- bridging halides
synthesised in same way
Cyclopentadienyl chemistry w/ actinides
- mainly involves Th(IV) and U(IV)
- UCp4 is the only known example of a compound w/ 4 cyclopentadienyls all bonded in a hapacity 5 fashion
- Smaller metal ions cause ring slippage - 3 x eta5 and 1x eta1
- Th and U cyclopentadienyls do not react with FeCl2 to produce ferrocene, this is due to a higher covalent componant in bonding for Actinides
-Full UCpnXn complexes also known
UCp3X ThX2CP*2 synthetically useful
How is hapacity related to coordination number
hapacity 5 = 3 electron pairs hapacity 2 = 1 electron pair hapacity 3 =2 electron pairs hapacity 1=1 electron pair eg. Picture
How to synthesise arene complexes of the lanthanides
Co-condenstation of lanthanide metal vapours with an excess of the bulky arene C6H3tBu3 yielding a zero valent(Ln(0)), hapacity 6 complex
-200C
technique called metal vapour synthesis
for La: 3-4 and 8-12 and group 3 TM
Bonding reminiscent of the 18 e- Cr metallocene
Explain bonding and stability of the lanthanide arenes
Cr diagram- d6 MO diagram
Sc- minus 3 electrons to produce a partially populated delta bonding orbital 0.75
BY analogy
- A 5d16s2 configuration is required for bonding by the members of the Ln series as 4f orbitals are too contracted for bonding
The stability of the sandwhich complexes mirror the promotion energy from Xe 4fn 5d0 6s2 to Xe 4fn-1 5d1 6s2 - only stable ones can be synth 3/4 8-12
early members an exception - promotion energy low- but dont exist - believed that the ligand is insufficiently buly to kinetically stablise the La and Ce complexes
promotion energy lightning w low points at La and Gd
What are the flaws in this theory
The model rationalises data based on negative results
something hasnt been able to be done
there may be other limitiation or the synthetic technique may be ‘wrong’
What is inverse arene complexes
Reduction of a U(IV) precursor with KC8 in TOLUENE
2x UL4 -> L2-U-toluene-U-L2
u-toluene bond eta-6 coordination
L4: 3x bulky amides+iodide ion
Arene bonding: phi bonding?
no but delta bonding or backbonding- four lobes - d orbitalw/ four lobes
COT: the ligand
COT: cyclooctatetraene
require two electrons to achieve 10e and to become aromatic
COT + 2K –> COT(2-) K(+)2 (puckered chair to planar)
it is a sterically demanding ligand and suited to the formation of complexes with lanthanide ions
eta-8 COT will satisfy two metal valenciesM in +2 with one or M in+4 with two
Sandwich complexes with COT
All Ln form complexes of the type K[Ln(eta8-COT)2]
Ce can form neutral cerocene [Ce(eta8-COT)2] with an apparent OS of 4+ however recent have indicated is more like 3+ bbecause its complexed by 2 COT (-1.5) ligands
The Ln-C bonding is essentially ionic
