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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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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


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


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


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


Maximum Bond Order – d-Block

dx2-y2+ dx2-y2

dxy + dxy

2 ×

2 ×

dxz + dxzdyz + dyz

2 ×

2 ×

dz2+ dz2


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-


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


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


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


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


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


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


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


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


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


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


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



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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