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Schedule. Last week: p -Acceptor Ligands and Biology CO, O 2 , N 2 and NO complexes, introduction to M-M bonds. Lecture 7: M-M bonds d -bonds and bonding in metal clusters. Lecture 8: Rates of reaction Ligand-exchange reactions, labile and inert metal ions.

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slide2

Schedule

  • Last week: p-Acceptor Ligands and BiologyCO, O2, N2 and NO complexes, introduction to M-M bonds
  • Lecture 7: M-M bondsd-bonds and bonding in metal clusters
  • Lecture 8: Rates of reaction Ligand-exchange reactions, labile and inert metal ions
  • Lecture 9: Redox reactions Inner and outer-sphere reactions
slide3

Summary of Course – week 6

Metal-metal bonding

  • be able to predict bond order for M2Lx dimers using d-electron count and s, p and d molecular orbital diagram
  • be able to predict bond order in larger metal-halide clusters using d-electron count shared over edges of cluster
  • be able to predict bond order in metal carbonyl clusters using 18 e- rule

Reaction mechanisms

  • be able to describe ligand exchange mechanisms
  • be able to explain role of metal charge and LFSE in rate of ligand exchange
  • be able to describe electron transfer reaction mechanisms
  • be able to predict relative rate of outer sphere reaction for different metals

Resources

  • Slides for lectures 7-9
  • Shriver and Atkins “Inorganic Chemistry” Chapter 18.11, 21.20, 20.1-20.13
slide4

Summary of Last Lecture

Metal-N2 complexes

  • N2 is isoelectronic with CO but M-N2 bonding is much weaker
  • N2 is non-polar and bond is strong

NO complexes

  • Can bond as 1 electron donor (NO-: bent M-NO)
  • Can bond as 2 electron donor (NO+ linear M-NO)

Today’s lecture

  • Metal-Metal bonding in complexes
slide5

Maximum Bond Order – d-Block

3dsu

3dpg

3ddu

3ddg

3dpu

  • the maximum bond order is 5
  • but…complexes also contain ligands which use some of the d-orbitals reducing number of bonds

dx2-y2+ dx2-y2

dxy + dxy

2 ×

3d

3d

dxz + dxzdyz + dyz

2 ×

3dsg

dz2+ dz2

slide6

Maximum Bond Order – d-Block

dx2-y2+ dx2-y2

dxy + dxy

2 ×

2 ×

dxz + dxzdyz + dyz

2 ×

2 ×

dz2+ dz2

slide7

Complexes Containing d-d Bonds

  • In complexes, not all of these molecular orbitals will form as some of the d-orbitals will be involved in bonding to ligands
  • [Re2Cl8]2-contains two ~square planar ReCl4 units
    • there must be a Re-Re bond as the two units are attached
    • the geometry is eclipsed
    • [Re2Cl8]2- ≡ 2Re3+ (d4) + 8Cl-
slide8

Quadruple Bonds

  • In complexes, not all of these molecular orbitals will form as some of the d-orbitals will be involved in bonding to ligands
  • [Re2Cl8]2-contains two ~square planar ReCl4 units
    • dx2-y2 bonds with the four ligands
    • leaving:

dz2 (s), dxz, dyz (p) and dxy (d) to form M-M bonds

slide9

Quadruple Bonds

3dsu

3dpg

3dpu

  • 2 × Re3+ (d4)  8 e-
  • (s)2(p)4(d)2
  • quadruple bond:

3ddu

dxy + dxy

3d

3d

3ddg

dxz + dxzdyz + dyz

2 ×

3dsg

dz2+ dz2

slide10

Quadruple Bonds

  • [Re2Cl8]2-quaduple bond:
    • (s)2(p)4(d)2: a s-bond, two p-bonds and one d-bond
    • the sterically unfavourable eclipsed geometry is due to the d-bond
slide11

Quadruple Bonds

  • [Re2Cl8]2-
    • [Re2Cl8]2- ≡ 2Re3+ (d4) + 8Cl-
    • 2 × Re3+ (d4)  8 e-
    • (s)2(p)4(d)2
    • bond order = 4 (quadruple bond)
    • d-bond: eclipsed geometry

3dsu

3dpg

3ddu

  • [Re2Cl8]4-
    • [Re2Cl8]4- ≡ 2Re2+ (d5) + 8Cl-
    • 2 × Re2+ (d5)  10 e-
    • (s)2(p)4(d)2(d*)2
    • bond order = 3 (triple bond)
    • no d-bond: staggered geometry

3ddg

3dpu

3dsg

slide12

Larger Clusters – M3

  • ReCl3 exists Re3Cl9 clusters
    • detailed view of cluster bonding more involved…
    • Re3Cl9 ≡ 3Re3+ (d4) + 9Cl-
    • 3 × Re3+ (d4)  12 e- or 6 pairs
    • 3 × Re-Re connections in triangle
    • number of pairs / number of edges = 6/3 = 2
    • 3 × Re=Re double bonds
slide13

Larger Clusters – M6

  • [Mo6Cl14]2-
    • Mo6 octahedron with 8Cl capping faces and 6 Cl on edges
  • [Mo6Cl14]2- ≡ 6Mo2+ (d4) + 14Cl-
  • 6 × Mo2+ (d4)  24 e- or 12 pairs
  • 12× Mo-Mo connections in octahedron
  • number of pairs / number of edges = 12/12 = 1
  • 12× Mo-Mo single bonds
slide14

Larger Clusters – M6

  • [Nb6Cl12]2+
    • Nb6 octahedron with 12Cl on edges
    • [Nb6Cl12]2+ ≡ [Nb6]14+ + 12Cl-
    • 6 × Nb (d5)  30 e-
    • [Nb6]14+ so…

number for M-M bonding is 30 – 14 = 16 e- or 8 pairs

    • 12× Nb-Nb connections in octahedron
    • number of pairs / number of edges = 8/12 = 2/3
    • 12 × Nb-Nb bonds with bond order = ⅔
slide15

Carbonyl Clusters

  • In carbonyl clusters, some of the metal orbitals are used to s and p-bond to the CO ligands
  • The number and order of the M-M bonds is easily determined by requiring that all of the metals obey the 18 e- rule
    • Mn2(CO)10
    • total number of electrons = 2 × 7 (Mn) + 10 × 2 (CO)  34 e-
    • Mn2 so each Mn has 34/2 = 17 e-
    • to complete 18 e- configuration, a Mn-Mn single bond is formed
slide16

Carbonyl Clusters

  • In some cases, the CO ligands bridge two metals – this does not effect the method as CO always donates 2e- to the cluster
    • Fe2(CO)9
    • total number of electrons = 2 × 8 (Fe) + 9 × 2 (CO)  34 e-
    • Fe2 so each Fe has 34/2 = 17 e-
    • to complete 18 e- configuration, a Fe-Fe single bond is formed
slide17

Carbonyl Clusters

  • Larger clusters are again possible
    • Os3(CO)12
    • total number of electrons = 3 × 8 (Os) + 12 × 2 (CO)  48 e-
    • Os3 so each Os has 48/3 = 16 e-
    • to complete 18 e- configuration, each Os makes two Os-Os bonds
    • Os3 triangle results
slide18

Summary

By now you should be able to

  • Draw out the MO diagram for L4ML4 complexes
  • Complete this diagram by filling in the appropriate number of electrons
  • Explain the appearance of eclipsed geometries due to d-bonding
  • In larger clusters, use the total number of pairs of metal electrons and number of metal-metal connections to work out the bond order
  • In carbonyl clusters, use the 18 e- rule to work out how many bonds have to be formed

Next lecture

  • Ligand-substitution reactions
slide20

Practice

  • What is the Mo-Mo bond order in the complex [Mo2(CH3CO2)4]?
  • What is the Os-Os bond order in the cluster Os4(CO)12?
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