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Schedule

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

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  1. 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

  2. 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

  3. 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

  4. 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

  5. Maximum Bond Order – d-Block dx2-y2+ dx2-y2 dxy + dxy 2 × 2 × dxz + dxzdyz + dyz 2 × 2 × dz2+ dz2

  6. 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-

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. Practice

  19. 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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