Metal-Carbonyls and Metal-Carbon pi-Bonds Summary

Metal-Carbonyls / Metal-Carbon p-Bonds

Metal-Carbonyl Complexes

  • Neutral M<em>x(CO)</em>yM<em>x(CO)</em>y complexes exist for most of the d-block and uranium (not group 3, lanthanides, or other actinides).
  • Metal carbonyls are a key class of organometallic compounds.
  • Bonding is similar to diatomic molecules with 10 electrons (e.g., CN−CN^-, NO+NO^+, N2N_2).
  • Synergic bonding: both σ\sigma donation and π\pi backbonding are required and reinforce each other (Dewar-Chatt-Duncanson model).
  • M–CO bond is relatively weak.
  • M–CO π\pi-backbonding is the most important component.
  • Consequences of backbonding:
    • M–CO σ\sigma-donation: strengthens M–C , strengthens C–O
    • M–CO π\pi-donation: strengthens M–C, weakens C–O
    • Increased C–O bond length decreases νCO\nu_{CO} in IR spectra.
  • Backbonding indicates a move from a 'reduced' state to complete oxidation of the metal: M–−C≡O+↔M=C=OM^–-C≡O^+ ↔ M=C=O
    • Need low oxidation state metals for backbonding.

Factors Affecting νCO

  • Charge on Complex: Isoelectronic complexes
    • Fe(CO)42−Fe(CO)_4^{2-}: 1790
    • Co(CO)_4^{-}$: 1890
    • Ni(CO)_4:2060</li></ul></li><li>Ligand: 2060</li></ul></li> <li>Ligand\sigma−Donorand-Donor and\pi−AcceptorProperties:<ul><li>-Acceptor Properties:<ul> <li>fac[Mo(CO)3(PMe3)_3]:1835</li><li>: 1835</li> <li>fac[Mo(CO)3(PPh3)_3]:1888</li><li>: 1888</li> <li>fac[Mo(CO)3(PF3)_3]:2055</li></ul></li><li>Coordinationmode:<ul><li>FreeCO:2143</li><li>TerminalM−CO:2120−1850</li><li>: 2055</li></ul></li> <li>Coordination mode:<ul> <li>Free CO: 2143</li> <li>Terminal M-CO: 2120-1850</li> <li>\mu_2−Bridging:1850−1750</li><li>-Bridging: 1850-1750</li> <li>\mu_3−Bridging:1730−1620cm−1</li></ul></li></ul><h4id="synthesisofmetalcarbonylcomplexes">SynthesisofMetal−CarbonylComplexes</h4><ul><li>Fromthemetal:<ul><li>e.g.,-Bridging: 1730-1620 cm-1</li></ul></li> </ul> <h4 id="synthesisofmetalcarbonylcomplexes">Synthesis of Metal-Carbonyl Complexes</h4> <ul> <li>From the metal:<ul> <li>e.g.,Ni + 4CO → Ni(CO)_4(1bar,RT)</li><li>(1 bar, RT)</li> <li>Fe + 5CO → Fe(CO)_5(100bar,200°C)</li></ul></li><li>Reductionofmetalsalts:<ul><li>e.g.,(100 bar, 200 °C)</li></ul></li> <li>Reduction of metal salts:<ul> <li>e.g.,VCl3 + 4Na + xs CO + diglyme → [Na(diglyme)2]^+[V(CO)_6]^-</li><li></li> <li>CrCl3 + Al + xs CO → Cr(CO)6 + AlCl_3(300bar)</li><li>(300 bar)</li> <li>MnCl2 + 2Na/Ph2C=O → Mn2(CO){10}(200bar,200°C)</li><li>(200 bar, 200 °C)</li> <li>2CoCO3 + 2H2 + 8CO → Co2(CO)8 + 2CO2 + 2H2O(300bar,130°C)</li></ul></li></ul><h4id="structures">Structures</h4><ul><li>Mononuclear,Binuclear,Tri−andTetra−nuclearexamples</li></ul><h4id="reactivity">Reactivity</h4><ul><li>Dissociation(heatorphotolysis)</li><li>Ligandsubstitution</li><li>Oxidation/Reduction</li><li>MetalCarbonylAnionscanbeformedbyreductionornucleophilicattack.</li></ul><h4id="synthesisofmetalcarbonylanions">SynthesisofMetalCarbonylAnions</h4><ul><li>Byreduction:<ul><li>e.g.,(300 bar, 130 °C)</li></ul></li> </ul> <h4 id="structures">Structures</h4> <ul> <li>Mononuclear, Binuclear, Tri- and Tetra-nuclear examples</li> </ul> <h4 id="reactivity">Reactivity</h4> <ul> <li>Dissociation (heat or photolysis)</li> <li>Ligand substitution</li> <li>Oxidation/Reduction</li> <li>Metal Carbonyl Anions can be formed by reduction or nucleophilic attack.</li> </ul> <h4 id="synthesisofmetalcarbonylanions">Synthesis of Metal Carbonyl Anions</h4> <ul> <li>By reduction:<ul> <li>e.g.,Co2(CO)8 + 2Na → 2 Na[Co(CO)_4]</li></ul></li><li>ByNucleophilicattack:<ul><li>e.g.,</li></ul></li> <li>By Nucleophilic attack:<ul> <li>e.g.,Fe(CO)5 + NaOH (OC)4Fe C O O H Na -CO2 Na[HFe(CO)4]</li></ul></li></ul><h4id="reactivityofmetalcarbonylanions">ReactivityofMetalCarbonylAnions</h4><ul><li>ExamplesincludereactionswithR−X,RCOOH,etc.toformvariousorganiccompounds.</li></ul><h4id="grouptheoryandirspectroscopy">GroupTheoryandIRSpectroscopy</h4><ul><li>UseGroupTheorytodeterminethenumberofIR</li></ul></li> </ul> <h4 id="reactivityofmetalcarbonylanions">Reactivity of Metal Carbonyl Anions</h4> <ul> <li>Examples include reactions with R-X, RCOOH, etc. to form various organic compounds.</li> </ul> <h4 id="grouptheoryandirspectroscopy">Group Theory and IR Spectroscopy</h4> <ul> <li>Use Group Theory to determine the number of IR\nu_{CO}bands.</li><li>Examplecalculationshownforaspecificcase(Cr(CO)6).</li></ul><h4id="metalcarbonbonds">Metal–Carbonπ−Bonds</h4><ul><li>Dewar−Chatt−Duncansonmodel(synergicbonding).<ul><li>Donor–alkeneHOMObands.</li> <li>Example calculation shown for a specific case (Cr(CO)6).</li> </ul> <h4 id="metalcarbonbonds">Metal–Carbon π-Bonds</h4> <ul> <li>Dewar-Chatt-Duncanson model (synergic bonding).<ul> <li>Donor – alkene HOMO\pi−bondingorbital.</li><li>Acceptor–alkeneLUMO-bonding orbital.</li> <li>Acceptor – alkene LUMO\pi^*−antibondingorbital.</li></ul></li><li>Betterbalancedbondingcomparedtometalcarbonylcomplexes.</li></ul><h4id="consequencesofbackbondingmetalalkenecomplexes">Consequencesofbackbonding(Metal−AlkeneComplexes):</h4><ul><li>LengtheningoftheC=Cbond.</li><li>ReductionofanglesatCfrom 120°(-antibonding orbital.</li></ul></li> <li>Better balanced bonding compared to metal carbonyl complexes.</li> </ul> <h4 id="consequencesofbackbondingmetalalkenecomplexes">Consequences of backbonding (Metal-Alkene Complexes):</h4> <ul> <li>Lengthening of the C=C bond.</li> <li>Reduction of angles at C from ~120° (sp^2)to 109°() to ~109° (sp^3).</li><li>Extentofbackbondingdependson:<ul><li>EnergyofthefrontierorbitalsoftheMfragment.</li><li>Stericeffects.</li><li>Alkeneelectronacceptorability.</li></ul></li><li>Electronwithdrawinggroupsincrease).</li> <li>Extent of backbonding depends on:<ul> <li>Energy of the frontier orbitals of the M fragment.</li> <li>Steric effects.</li> <li>Alkene electron acceptor ability.</li></ul></li> <li>Electron withdrawing groups increase\pi−backdonationanddecreasethe-backdonation and decrease the\sigma−donation.</li></ul><h4id="synthesisofmetalalkenecomplexes">SynthesisofMetal−AlkeneComplexes</h4><ul><li>Addition,Substitutionreactions.</li></ul><h4id="reactivityofmetalalkenecomplexes">ReactivityofMetal−AlkeneComplexes</h4><ul><li>Reactivitywithnucleophiles(intra−orintermolecular).</li></ul><h4id="metalalkynecomplexes">Metal−AlkyneComplexes</h4><ul><li>Alkynesarestronger-donation.</li> </ul> <h4 id="synthesisofmetalalkenecomplexes">Synthesis of Metal-Alkene Complexes</h4> <ul> <li>Addition, Substitution reactions.</li> </ul> <h4 id="reactivityofmetalalkenecomplexes">Reactivity of Metal-Alkene Complexes</h4> <ul> <li>Reactivity with nucleophiles (intra- or intermolecular).</li> </ul> <h4 id="metalalkynecomplexes">Metal-Alkyne Complexes</h4> <ul> <li>Alkynes are stronger\pi−acceptorsthanalkenes.</li><li>Alkyneshavetwoorthogonal-acceptors than alkenes.</li> <li>Alkynes have two orthogonal\pi−bondsandcanactas2or4electrondonorligands.</li><li>Alkynesfrequentlyundergoinsertionsandusuallyoligomeriseratherthanpolymerise.</li><li>Bondingsimilartoalkenes,butanextra-bonds and can act as 2 or 4 electron donor ligands.</li> <li>Alkynes frequently undergo insertions and usually oligomerise rather than polymerise.</li> <li>Bonding similar to alkenes, but an extra\pi$$-bond is available.
    • Extent of backbonding heavily dependent on how electron rich the metal is.

    Synthesis of Metal-Alkyne Complexes

    • By displacement, reduction, or deprotonation.

    Reactivity of Metal-Alkyne Complexes

    • Rearrangement to a carbene (vinylidene) or carbyne.
    • Insertion into metal hydride bonds to give vinyl complexes.

    Oligomerisation (cyclotrimerisations)

    • The Pauson-Khand reaction.
    • Bönnemann cyclisation.