MLB

• η - hapticity, a number of contiguous atoms of a ligand to a metal.

  
• κ - denticity, the number of non-contiguous atoms coordinating from a ligand (often a chelating ligand)

  
• μ - the number of metal atoms bridged by a ligand

  
• Metal oxidation state = charge on complex - sum of charges on ligands
• Some formal charges on ligands:

  +1: NO (linear)

  0: CO, NR3, PR3, N2, O2, H2, C2H4, H2O, RCN, C6H6 (all are molecules)

  -1: H, CH3, F, Cl, Br, I, C5H5, CN, NO2, NR2, NO (bent)

  -2: O, S, CO3, NR, porphyrin

  -3: N, P
• Metal d-electron count = group number - OS
• Total valence electron count (TVEC) at the metal = d-electron count + electrons donated by the ligands + number of metal-metal bonds
• Number of electrons donated by each ligand (using ionic formalism)

  2e - CO, RCN, NR3 (amines), PR3 (phosphines), N2, O2, C2R4 (alkenes), H2O, H-, CH3-(or any alkyl or aryl group, R), F-, Cl-, Br-, I-, CN-, NR2- (bent), η1-C5H5-

  4e - R2PCH2CH2PR2 (bis-phosphines), η4-dienes, NR-2 (linear), (CH3CO2)-, NR2- (bent), O2- (double bonds), S2-

  6e - η6-C6H6, η5-C5H5, NR2- (linear), O2- (triple bond), N3-, P3-
• Why do complexes form?

  ΔG = ΔH -TΔS = RTlnkf (f=formation constant)

  Enthalpic effects: steric factors, charge on complex, electronic configuration

  Entropic effects: number and size of any chelate rings, solvation
• Enthalpic effects (why complexes form):

  Number and strength of metal-ligand bonds: greater number of ligands and stronger bonds, greater thermodynamic stability of complex.

  Steric factors.

  Charge on complex: large positive and negative charges cannot easily be supported. Continually removing electrons from a complex will result in increasingly large ionisation energies and increasing the number of electrons will lead to large e-e repulsive forces.

  Electronic configuration: note CFSE contributes approximately 10% to overall thermodynamic stability.
• Entropic effects (why complexes form):

  Number and size of any chelate rings: 5 and 6 membered rings most stable (enthalpy due to least ring strain) and less likely to dissociate.

  Solvation: requirement for an ordered solvent cage this will lower the entropy
• Metal ligand bonding can be divided into basic classes: σ-donor

  σ-donor, π-acceptor

  π-donor, π-donor
• σ-donors: all have a line pair to donate to an orbital

  e.g. H, CH3 (any alkyl or aryl group R), H2O, NH3, NR2 (bent)
• σ-donor,π-acceptor: have lone pair and additional empty π orbital. E.g. CO, CN, NO, H2, C2H4, N2, O2, PR3, Br2
• σ-donor,π-donor: have a lone pair and additional lone pair with π-symmertry. E.g. F, Cl, Br, I, O, OR, S, SR, N, NR2(linear), NR(bent and linear), P, η3-C3H5, η5-C5H5, η6-C6H6
• In terms of bond strength, σ bonding is more important than π bonding due to better orbital overlap
• σ donor: in these compounds, the bond between the ligand (e.g. H-, hydride) and metal is a σ bond

  Metal hydrides play a very important role in many catalytic reactions including hydrogenation and hydroformylation
• Comparison of molecular orbital and crystal field theory:

  Crystal field theory compares the repulsive interactions between metal and ligand electrons to determine the ordering of metal d-orbitals. It is an electrostatic approach using pure metal d-orbitals.
• σ donor, π acceptor: contain a σ interaction (similar to H-, NH3 etc) and a π acceptor interaction between empty ligand π orbital and a filled d orbital with π symmetry. Metal complexes of CO uses in catalysis and purification of metals and speciality medical uses
• a)All complexes are 18e, d6. The OS of metal reflects electron density on the metal and the C-O stretch reduces as more electron density is transferred to the 2π orbitals as OS decreases.

  b)All complexes d6. As number of CO ligands reduced, a greater π-acceptor interaction per CO required, v decreases.

  c)reflects electronegativity of other ligands. Decrease electronegativity, decrease v.

  
• More metals = more e- = greater π-back blocking.

  
• Trends in v(CO). Always think in terms of CO ligands competing for whatever electrons are available on the metal.
• In these complexes, electron density is not transferred from the metal to the ligand π-accepting orbitals. The major interaction is the σ-donation from the CO 5σ orbital to the metal giving weak M-CO bonding. The CO stretching frequency is > free CO mainly due to electrostatic perturbation.

  
• Ligands expected to have similar bonding to CO are isoelectronic CN- and NO+
• N2 is isoelectronic with CO, but CO orbitals are smaller and symmetrical which leads to much weaker M-L bonding.

  Comparing HOMO orbitals. Coordination of N2 decreases N-N bond strength. N2 can act as a π-acceptor using LUMO 1πg same as CO. The 3σg of N2 is much more bonding than the 5σ of CO therefore σ-donor interaction for N2 weakens the N-N bond.
• π-accepting ligand examples: CO, N2, O2, H2, PR3, alkenes
• As the electron density in the π* orbitals increases, the O-O distance increases, the vibrational frequency decreases
• Why is η1-O2 bent when CO is linear?

  Because O2 has to accommodate an extra pair of electrons in the 1πg* orbital. These occupy 1πgx leaving 1πgy to form a π-acceptor interaction.
• For NO, as the M-N-O angle decreases (becomes more bent) v(N-O) decreases
• Linear NO donates 1 electron NO to M. Gives NO+ and M-

  Bent NO electron goes from M to NO. Gives NO- and M+
• Ligand substitution: most common metal complex reaction, substitution at metal primary coordination sphere.

  
• Dissociation of metal complexes: decreases the metal coordination number

  
• Addition of metal complexes: increases coordination number

  
• Types of ligand substitution:

  Dissociative (D) intermediate reduced coordination

  Interchange (I) undetectable intermediate cannot be isolated common for octahedral

  Associative (A) intermediate increased coordination
• Ligand substitutions

  
• Stoichiometeric mechanism: considers reactants, products and intermediates. Each step has a rate or equilibrium constant associated with it. Each species exists in a potential minimum along the reaction coordinate.

  Intimate mechanism: usually considers transition state of the RDS. Associative M-L bond formation is advanced in the transition state. Dissociative M-L bond cleavage is advanced in transition state.
• Thermodynamically unstable complexes that survive for at least a minute are inert. Complexes that equilibrate quicker are labile.
• Labile - complexes of d10 ions, complexes of 3d M(II) ions

  Inert - d3 and low spin d6 configurations. 4d and 5d complexes due to high CFSE and better metal-ligand overlap. Chelating ligands.
• Timescale of reactions:

  
• Activation parameters: reaction rate examined as a function of temperature.