Metal-Metal Bonding in Inorganic Chemistry

Metal-Metal Bonding Covalent: Electron precise bonds. M-M bond counts as one e- from each metal center. Most common type of M-M bonding. Where one metal uses a filled dorbital lone pair to coordinate to an empty orbital on a second, more unsaturated metal. Most dative bonding situations can also be electron-counted as covalent bonds. Symmetry: Weak metal-metal interactions caused by molecular orbital symmetry interactions of filled & empty M-M bonding and/or antibonding orbitals. Typically seen for d8 metals. Not at all common. Dative: dz2 dyz dxz dxy the d- orbitals (not shown) are used for M-L bonding x y 2 2

L M L L L L M-M antibonding orbitals M L L L dxy dxy dyz dxz dz2 dyz dxz dz2 M-M bonding orbitals the d - , s and p orbitals are not shown since they are used for M-ligand bonding 2 2 x y x,y Electron Count d1- d1 d2- d2 d3- d3 d4- d4 d5- d5 d6- d6 d7- d7 d8- d8 Resulting M-M Bond Single bond Double bond Triple bond Quadruple bond optimum Triple bond Double bond (M-L bonding usually dominates) Single bond No bond (symmetry interaction)

Some Covalent Multiple Bonded Examples: Double Bonds R t-Bu t-Bu O O R Cl Cl Cl Cl O O Ta Ta Cl Os Os Cl O R O O O Cl Cl O O R Ta=Ta = 2.68 Os=Os = 2.30 Triple Bonds d5-d5Triple Bond Chisholm d3-d3Triple Bonds O O PhH2C PhH2C C C CH2Ph Cr Cr Mo Mo CH2Ph C C O PhH2C O CH2Ph Mo-Mo = 2.17 Cr-Cr = 2.27

Quadruple Bonds (Cotton) d4-d4 electronic configurations often lead to the formation of quadruple M-M bonds. Prof. F. Albert Cotton at Texas A&M was famous for his discovery and extensive studies of M-M quadruple bonds (and other M-M bonded systems). 2- H3C CH3 Re H3C CH3 H3C CH3 Re H3C CH3 F. Albert Cotton Texas A&M University Re-Re = 2.18 Me Me O O 4 Cr Cr Cr-Cr = 1.85

Dative M-M Bonds (unsymmetrical M-M bonded complexes) t-Bu t-Bu Ni-P = 2.16 Ni-P = 2.24 P tetrahedral coordination like Ni(0) planar coordination like Ni(+2) O O C O C Ni Ni C P Ni-CO = 1.70 Ni-CO = 1.78 t-Bu t-Bu Ni-Ni = 2.41 Covalent M-M Bonding Dative Left Ni Ni(+1) [ -PR2] -PR2 CO M-M Right Ni Ni(+1) [ -PR2] -PR2 2CO M-M Left Ni Ni(+2) 2[ -PR2] 4e- CO Ni Ni(0) 2e- Right Ni Ni(0) 2 -PR2 2CO d9 2e- 2e- 2e- 1e- d9 2e- 2e- 4e- 1e- d8 d10 4e- 4e- 2e- Total 16e- Total 18e- Total 16e- Total 18e-

Problem: Electron-count the following complex using both the covalent and dative M-M bonding methods: R2 P Me3P Me3P CO CO W Re CO Me3P CO CO OC Problem: Electron-count the following complex. What is the order of the Re- Re bond? Why wouldn t it be appropriate to use the dative bond method for this complex? PR3 Cl Cl Re R3P R3P Cl Re Cl PR3

Weak M-M Interactions by Symmetry Based on the MO diagram at the beginning of this section, d8-d8systems shouldn t have any M-M bonding due to the filling of all the M-M antibonding orbitals, which cancels out the M-M bonding orbitals. But Harry Gray and others noted that more than a few bi- or polymetallic d8 complexes do show the presence of weak M-M bonding interactions, both in solution and the solid-state. Harry Gray Caltech L L NR C L L RNC N R CNR Ir M M L L C L L Cl R3P Ir pz pz C C N NR R NR C RNC N R CNR Ir dz2 dz2 C