Complexes of pBound Ligands

1
Complexes of p-Bound Ligands

• Chapter 10

2
Outline

• Alkene complexes
• Bonding
• Synthesis
• Alkyne complexes
• Allyl complexes
• Cyclopentadienyl (Cp) complexes
• Bonding in metallocenes
• Other metallocenes
• Ligands similar to Cp
• Other cyclic p ligands
• Uranocene

3
Bonding in alkene complexes
Dewar-Chatt-Duncanson model
4
Two bonding extremes

• Two bonding extremes
• Metal fragment can invert the chemical character
  of the alkene umpolung.
• Strained alkenes (cyclopropene or norbornene)
  bind unusually strong to the metal, because the
  rehybridization on binding relieves ring strain.

5
Characterization
NMR 13C-13C coupling constant changes from 67.6
Hz in free ethylene to 39 Hz in
Os(C2H4)(CO)4. 1JCH is insensitive to these
changes.
6
Synthesis by ligand substitution
7
Alkyne complexes

• Similar to alkene complexes, but binding is
  stronger.
• Backdonation is more significant
  metalacyclopropene model (I) is the most
  appropriate.
• Complexation can stabilize highly strained
  alkynes.

IR and Raman free alkynes, 2200 cm-1 2e donors
1700 2000 cm-1 4e donors 1675 1820 cm-1
1H NMR d, 6 14 ppm (10 14 for 4e donors)
from 1 3 ppm 13C NMR d, 100 240 ppm (205
225 for 4e donors) from 60 90 ppm
8
Allyl complexes

• Two binding modes h1 and h3
• The plane of the allyl is canted at an angle (q
  5 - 10) such that orbital interaction is
  improved.
• Fluxionality syn and anti substituents exchange
  through an h1-intermediate.

1H NMR d, 1 3 ppm (Hanti), 2 5 ppm (Hsyn), 4
6 ppm (Hmeso) 13C NMR d, 80 90 ppm
(Cterminal), 110 130 ppm (Cterminal) Fluxional
on the NMR time scale
9
Cyclopentadienyl (Cp) complexes orbitals in Cp-
10
Cp orbital combinations for two ligands
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Metallocenes, MCp2
HOMO of MCp2, dz2 (nonbonding)
Most important interaction e1g
C-C bond in Cp is 1.44 ? (C6H6, 1.35 ?) H atoms
of Cp are bent downwards.
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Reactivity
Electrophilic aromatic substitution faster than
for benzene
Reactions of paramagnetic metallocenes
13
Other metallocenes
Bent metallocenes
Cp C5Me5
Indenyl complexes
Better p-acceptor than Cp
Ansa-metallocenes
Higher field, more bulky, more-electron donating
than Cp
14
Ligands similar to Cp
Tris(pyrazolyl)borate (Tp)
Sometimes these ligands show interesting
reactivity because one arm can open and allow
substrate binding.
15
Other cyclic p ligands
Cyclobutadiene complexes more stable than free
dienes
Benzene more reactive than Cp, more easily lost
from the metal
16
d bonding in uranocene
COT2- HOMO

• UV-PES studies show that bonding in uranocene
  has 5f and 6d contributions.
• e2u interaction shown can only occur via f
  orbitals.

C. Elschenbroich A. Salzer Organometallics, 2nd
ed., VCH, Weinheim, 1992, p. 363