Transcript Document

Schedule
• Last week: p-Acceptor Ligands and Biology
CO, O2, N2 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
• Lecture 9: Redox reactions
Inner and outer-sphere reactions
Slide 2/20
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 delectron 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
Slide 3/20
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
Slide 4/20
Maximum Bond Order – d-Block
3dsu
• the maximum bond order is 5
3dpg
• but…complexes also contain
ligands which use some of the dorbitals reducing number of bonds
3ddu
3d
3d
2×
dx2-y2 + dx2-y2
dxy + dxy
2×
dxz + dxz
dyz + dyz
3ddg
3dpu
3dsg
dz2 + dz2
M
M
Slide 5/20
Maximum Bond Order – d-Block
2×
dx2-y2 + dx2-y2
dxy + dxy
2×
2×
dxz + dxz
dyz + dyz
2×
dz2 + dz2
y
x
M
M
z
Slide 6/20
Complexes Containing d-d Bonds
•
In complexes, not all of these molecular orbitals will form as some of the dorbitals 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 7/20
Quadruple Bonds
•
In complexes, not all of these molecular orbitals will form as some of the dorbitals will be involved in bonding to ligands
•
[Re2Cl8]2- contains two ~square planar ReCl4 units
 dx2-y2 bonds with the four ligands
y
y
x
x
 leaving:
dz2 (s), dxz, dyz (p) and dxy (d) to form M-M bonds
Slide 8/20
Quadruple Bonds
3dsu
• 2 × Re3+ (d4)  8 e-
3dpg
• (s)2(p)4(d)2
• quadruple bond: Cl4Re
2-
ReCl4
3ddu
3d
dxy + dxy
3d
3ddg
3dpu
dxz + dxz
dyz + dyz
2×
3dsg
dz2 + dz2
M
M
Slide 9/20
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
Slide 10/20
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
[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
3dsu
3dpg
3ddu
3ddg
3dpu
3dsg
Slide 11/20
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
Cl
Cl2Re
Cl
ReCl2
Re
Cl2
Cl
Slide 12/20
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 13/20
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 14/20
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
Slide 15/20
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 16/20
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 17/20
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 dbonding
• 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 18/20
Practice
Slide 19/20
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?
Slide 20/20