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Chapter 13 Lecture 2 More Ligand Types. Ligands with Extended p Systems Linear p Systems Ethylene (C 2 H 4 ) Single p -bond composed of two overlapping p-orbitals One bonding and one antibonding p molecular orbital Allyl Radical (C 3 H 5 ). 1,3-Butadiene Other extended p systems.

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chapter 13 lecture 2 more ligand types
Chapter 13 Lecture 2 More Ligand Types
  • Ligands with Extended p Systems
    • Linear p Systems
      • Ethylene (C2H4)
        • Single p-bond composed of two overlapping p-orbitals
        • One bonding and one antibonding p molecular orbital
      • Allyl Radical (C3H5)
slide2
1,3-Butadiene
  • Other extended p systems
slide3
Cyclic p Systems
    • Cyclopropene
      • Similar construction of orbitals as in linear systems
      • Degenerate orbitals have the same number of nodes
    • Polygon Method for finding cyclic p system MO’s
      • Draw molecule as a polygon with vertex down
      • One MO per vertex gives energy ordering and degeneracy
      • Number of nodes increases as energy increases
slide5
Bonding Between Metals and Linear p Systems
    • Ethylene Complexes
      • Sidebound geometry is most common
      • Bonding: s-donation from p MO, p-acceptance from p* MO
      • Coordination weakens C=C bond (137.5 pm, 1516 cm-1) compared to free ethylene (133.7 pm, 1623 cm-1)
    • p-Allyl Complexes
      • Can be trihapto: both s- and p-bonding
      • Can be monohapto: s-bonding only from sp2 hybrid orbital (120o bond angle)
slide6
c) The lowest energy MO provides s-bonding, highest energy MO = p-acceptor
  • Other linear p-system coordination

p3 is a p-acceptor

p2 can be donating

or accepting depending

on metal e- distribution

p1 is a s-donor

slide7
Bonding in Cyclic p Systems
    • Cyclopentadienyl = Cp = C5H5- is the most important cyclic ligand
    • Ferrocene Synthesis: FeCl2 + 2 NaC5H5 (h5-C5H5)2Fe + 2 NaCl
      • Called metallocene or sandwich complex
      • 18-electron complex: Fe2+ = d6 and 2 Cp x 6 e-
      • Bonding Group Orbitals of 2 eclipsed Cp rings

D5h

0-Node Group Orbitals

slide8
Matching with metal d-orbitals: dyz orbital example
  • MO Description
    • 6 strongly bonding MO’s hold electrons from Cp ligands
    • 8 antibonding orbitals are empty
    • 5 mid-range energy orbitals holding metal d-electrons
  • Reactivity
    • Follows 18-electron rule, but not inert
    • Ligand reactions on Cp ring are most common reactions
slide10
M—C Single, Double, and Triple Bonds
    • Metal Alkyl Complexes
      • Grignard Reagents: X—M—CH2CH2CH2CH3
      • Bonding in Transition Metal Complexes
        • s-donation from C sp3 hybrid orbital
        • 2 electron, -1 charge for electron counting
      • Synthesis
        • ZrCl4 + 4 PhCH2MgCl Zr(CH2Ph)4
        • Na[Mn(CO)5] + CH3I CH3Mn(CO)5 + NaI
      • Other M—C single bond ligands
slide11
Metal Carbene Complexes
    • M=C counted as 2 electron, neutral ligand in electron counting
    • Schrock Alkylidenes: only H or C attached to the carbene Carbon
    • Fisher Carbenes: heteroatom attached to the carbene Carbon (our focus)
      • s-bond from C sp2 hybrid to metal
      • p-bond from C p-orbital(s)
      • Heteroatom delocalizes p-system to 3 atoms, stabilizing it by resonance
slide12
Metal Carbyne Complexes
    • First synthesis in 1973 by Lewis Acid attack on carbene complex
    • Bonding
      • 180o bond angle and short bond length confirm triple bond
      • 3 electron, 0 charge for electron counting
slide13
Spectroscopy of Organometallic Complexes
    • Infrared Spectroscopy
      • Number of Bands is determined by group theory (chapter 4 procedure)
        • Monocarbonyl = 1 band only
        • Dicarbonyl
          • Linear arrangement = 1 band only
          • Bent arrangement = 2 bands
        • 3 or more Carbonyls: table 13.7 in your book
      • Position of IR Bands
        • Electron Density determines Wavenumbers

Cr(CO)6n = 2000 cm-1 [V(CO)6]-n = 1858 cm-1[Mn(CO)6]+n = 2095 cm-1

        • Bonding Mode
        • Other ligands
slide14
NMR Spectroscopy
    • Proton NMR
      • Hydride Complexes M—H hydrogen strongly shielded (-5 to –20 ppm)
      • M—CH3 hydrogens 1-4 ppm
      • Cyclic p system hydrogens 4-7 ppm and large integral because all the same
    • 13C NMR
      • Useful because “sees” all C ligands (CO) and has wide range (ppm)
      • CO: terminal = 195-225 ppm, bridging slightly larger
slide15

tds

  • Examples
    • [(Cp)Mo(CO)3]2 + tds Product?
      • Data: 1H NMR: 2 singlets at 5.48 (5H) and 3.18 (6H)

IR: 1950, 1860 cm-1

Mass = 339

      • Solution: proton nmr 5.48 = Cp, 3.18 = ½ tds

IR: at least 2 CO’s

Mass: 339 - (Mo=98) – (Cp=65) – 2(CO) = 120 = ½ tds

Product = (Cp)Mo(CO)2(S2CN(CH3)2)

I: proton = 4.83 (4H), carbon = 224, 187, 185, 184, 73

II: proton = 7.62-7.41 m (15H), 4.19 (4H) carbon: 231, 194, 189, 188, 129-134,72

III: proton = 7.70-7.32 m (15H), 3.39 s (2 H) carbon: 237, 201,193,127-134, 69

Solution: 224 = M=C; 184-202 = CO; 73 = CH2CH2