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Schedule

• Last week: -Acceptor Ligands and Biology CO, O2, N2 and NO complexes, introduction to M-M bonds

• Lecture 7: M-M bonds-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

Slide 3/20

Summary of Course – week 6

Metal-metal bonding• be able to predict bond order for M2Lx dimers using d-electron count and

and 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

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

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

Slide 4/20

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

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3d 3d

3dg

Maximum Bond Order – d-Block

3du

3du

3dg

3dg

3du

M M

2 ×

2 ×

dz2 + dz2

dx2-y2 + dx2-y2dxy + dxy

dxz + dxz

dyz + dyz

• the maximum bond order is 5

• but…complexes also contain ligands which use some of the d-orbitals reducing number of bonds

Slide 6/20

Maximum Bond Order – d-Block

M M z

x

y

2 ×

2 ×

dz2 + dz2

dx2-y2 + dx2-y2

dxy + dxy

dxz + dxz

dyz + dyz

2 ×

2 ×

Slide 7/20

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-

Slide 8/20

x

y

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 (), dxz, dyz () and dxy () to form M-M bonds

x

y

Slide 9/20

3d 3d

3dg

Quadruple Bonds

3du

3du

3dg

3dg

3du

M M

2 ×

2 ×

dz2 + dz2

dx2-y2 + dx2-y2

dxy + dxy

dxz + dxz

dyz + dyz

• 2 × Re3+ (d4) 8 e-

• ()2()4()2

• quadruple bond: Cl4Re ReCl42-

Slide 10/20

Quadruple Bonds

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

Slide 11/20

Quadruple Bonds

• [Re2Cl8]2-

[Re2Cl8]2- ≡ 2Re3+ (d4) + 8Cl-

2 × Re3+ (d4) 8 e-

()2()4()2

bond order = 4 (quadruple bond) -bond: eclipsed geometry

3dg

3du

3du

3dg

3dg

3du

• [Re2Cl8]4-

[Re2Cl8]4- ≡ 2Re2+ (d5) + 8Cl-

2 × Re2+ (d5) 10 e-

()2()4()2(*)2

bond order = 3 (triple bond) no -bond: staggered geometry

Slide 12/20

Cl2Re

Re

ReCl2

Cl

Cl

Cl

Cl2

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

Cl2Re

Re

ReCl2

Cl

Cl

Cl

Cl2

Slide 13/20

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

Slide 14/20

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

Slide 15/20

Carbonyl Clusters

• In carbonyl clusters, some of the metal orbitals are used to and -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

Slide 16/20

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

Slide 17/20

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

Slide 18/20

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 -

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

Slide 19/20

Practice

Slide 20/20

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?