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. Dative: Where.

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Presentation transcript:

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. Dative: Where one metal uses a filled d orbital “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.

Electron Count Resulting M-M Bond d1 - d1 Single bond d2 - d2 Double bond d3 - d3 Triple bond d4 - d4 Quadruple bond optimum d5 - d5 d6 - d6 Double bond (M-L bonding usually dominates) d7 - d7 d8 - d8 No bond (symmetry interaction)

Some Covalent Multiple Bonded Examples:

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). F. Albert Cotton Texas A&M University

Dative M-M Bonds (unsymmetrical M-M bonded complexes) Covalent M-M Bonding Left Ni Right Ni Ni(+1) d9 [m-PR2]- 2e- m-PR2 2e- CO 2e- M-M 1e- 2CO 4e- Total 16e- Total 18e- Dative Left Ni Right Ni Ni(+2) d8 2[m-PR2]- 4e- CO 2e- Ni←Ni(0) 2e- Ni(0) d10 2m-PR2 4e- 2CO 4e- Total 16e- Total 18e-

Problem: Electron-count the following complex using both the covalent and dative M-M bonding methods: 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?

Weak M-M Interactions by Symmetry Based on the MO diagram at the beginning of this section, d8-d8 systems 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