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- 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

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

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

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

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-

- 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

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

- [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

- [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

- 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

- [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

- [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

- 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

- 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

- 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

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

- 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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