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)

1,3-Butadiene Other extended p systems

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

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)

Other linear p-system coordination 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

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

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

D5h

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

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

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

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)6 n = 2000 cm-1 [V(CO)6]- n = 1858 cm-1 [Mn(CO)6]+ n = 2095 cm-1 Bonding Mode Other ligands

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

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 tds