Lecture 7: M-M bonds d-bonds and bonding in metal clusters 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 Demos: charles law (2.3), CO2 density (2.1), star wars (4.3) Brief mention of 3 states of matter. 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 the maximum bond order is 5 but…complexes also contain ligands which use some of the d-orbitals reducing number of bonds 3dpu 3dpg 3ddg 3ddu dx2-y2 + dx2-y2 dxy + dxy 2 × 3d 3d dxz + dxz dyz + dyz 2 × 3dsg dz2 + dz2

Maximum Bond Order – d-Block dx2-y2 + dx2-y2 dxy + dxy 2 × 2 × dxz + dxz dyz + 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 2 × 2 × 2 × Re3+ (d4)  8 e- (s)2(p)4(d)2 3dsu 2 × Re3+ (d4)  8 e- (s)2(p)4(d)2 quadruple bond: 3dpu 3dpg 3ddu dx2-y2 + dx2-y2 dxy + dxy 2 × 3d 3d 3ddg dxz + dxz dyz + 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

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?