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Organometallic Chemistry Bonding in Coordination Compounds by yurtgc548


									    Organometallic Chemistry
                                                                  Ligands in Organometallic Chemistry
                                                               2 Electron Donors       4 Electron Donor        6 Electron Donors


                               hexacarbonyl                     HC      CH

                                                                 C      O

                                                                halide ions
                                                                                                               cyclopentadienide ion
                                                                PR3, phosphines
                                                Zeise’s salt

  16 & 18 Valence Electron Rule                                  16 & 18 Valence Electron Rule
  C. A. Tolman, Chem. Soc. Rev., 1972, 1, 337                    C. A. Tolman, Chem. Soc. Rev., 1972, 1, 337

  • Diamagnetic organometallic compounds                         • Organometallic reactions, including
    of the transition metals may exist in                          catalytic ones, proceed by elementrary
    significant concentration at moderate                          steps involving only intermediates with
    temperatures only if the metal’s valence                       16 or 18 metal valence electrons.
    shell contains 16 or 18 valence electrons.

                                                                         Bonding in Coordination
  16 & 18 Valence Electron Rule
                                 Cr0 = d6 = 6 e-
                                 6 CO = 6 x 2 e-                 •   Consider Cr(NH3)63+
                                       = 12 e-                   •   Cr3+ = 3 valence electrons
                                 Total = 18                      •   6 NH3 ligands = 12 valence electrons
chromium                           valence                       •   TOTAL valence electrons = 15
   18 valence electrons lead to maximum bonding. All                 Ordinary coordination compounds do not
   bonding molecular orbitals are filled.                            generally have 18 (or 16) valence electrons.

Bonding in M—CO Complexes                                   MO Stabilization by π Acceptor Ligands
                              Metal to ligand π bonding
Ligand to metal ! bonding

    M        C   O                   M             C   O
            Electron flow
                                 electron flow
                            Electron flow from filled M d
                            orbitals into π antibonding
                            ligand orbitals.
                                                            Screen 22.7 of ChemNow CD-ROM
  This SYNERGY leads to stronger bonding than either
            sigma or pi bonding alone.

                                                                        Metal Carbonyls
Bonding in Metal Carbonyls                                   Cr(CO) 6

• M-CO bonding illustrates why OM                                                                           Ni(CO) 4
  compounds of trans metals always use
  low-valent metals (< +2) (usually 0 or +1).
• M ---> L π* bonding leads to
  stabilization.                                                                     Fe(CO) 5

• Low valence state of M means effective                                            Colorless
                                                             White solid            liquid
  nuclear charge of M is low, so M-->L                                                                 Colorless liquid
  charge flow is possible.
                                                               Cr, Fe, and Ni have even number of d electrons

           Metal Carbonyls                                    Metal-Olefin Complexes
                                                                                  Pt 2+ has 8 valence e-
                                                                                                           Zeise’s salt
                                                                                             2e per Cl-

                                                                           2e- donated by π electrons of ligand

  Mn 2(CO) 10
  Yellow solid                         Co2(CO) 8                  Total valence electrons = _______________
                                       Red solid
    Mn and Co have ODD number of d electrons

Metal-Aromatic Complexes                                                             Metal-Aromatic Complexes
                                                 6 π electrons of donated by
                                                 C6H6 ligand                                                        6 π electrons of donated by
                                                                                                                    C5H5– ligand

                                       Cr has 6 valence e-
                                                                                                         Fe2+ has 6 valence e-

                                              Dibenzenechromium                                                      Ferrocene

  Total valence electrons = _______________                                             Total valence electrons = _______________

           Ferrocene Chemistry
                                                                                     16 & 18 Valence Electron Rule
                      CH3 COCl/AlCl3
                                                      C CH3

     Fe                                         Fe

                                                                                     C. A. Tolman, Chem. Soc. Rev., 1972, 1, 337

                                   HCHO/HNMe2                 CH2NMe2
                                                                                    • Organometallic reactions, including
                                                                                      catalytic ones, proceed by elementrary
          Oxidation                              Li                                   steps involving only intermediates with
                                                      Cp2Fe undergoes                 16 or 18 metal valence electrons.
                                                      electrophilic substitution
                                                      much more rapidly than

                                                                                             Reactions & Catalysis
          A Rhodium Catalyst                                                                                        IrCl(CO)(PPh 3)2

                                                 Total valence electrons?

                                                                                   Vaska’s compound

              Oxidative Addition
                                                                                                 Organometallic Chemistry
            & Reductive Elimination                                                                    Problem 12

                            Oxidative addition                                                                                   FeSO4

                                  + CH 3I
                                                                  I           W(CO)6
                                                                                                                                                            W    CO

                                                                                                                                 Na,Hg   OC                     CO
 L    CO                                                   L    CH3                         NaC5 H5              W(CO) 3
                                                                                                                                                    CO    OC

   Ir                                                        Ir                                             CO frequencies                    CO frequencies
Cl    L                                                   Cl    L                                         1744 and 1894 cm -1             1904 and 2010 cm-1
                                  - CH 3I
 Ir+                       Reductive elimination Ir3+
 16 valence e-                                   18 valence e-

  L = PPh3, triphenylphosphine

                  Infrared Spectroscopy
                    of Metal Carbonyls                                                      Metal Alkyls and Aryls
                                                                               Grignard reagents
        •   Bands for CO stretch.                                              RX + Mg in ether -----> RMgX
        •   Uncomplexed CO is at 2140 cm -1                                    Structure is more complex than RMgX.
        •   Number of bands depends on symmetry.
        •   Frequency depends on other ligands.                                Organomercury compounds
                                                                               Hg + 2 Na + 2 RX ---> HgR 2 + 2 NaAl
                                                                               Compounds like Hg(CH 3)2 are EXTREMELY toxic

                                                                               Organolithium compounds
                                                                               2 Li (dispersion) + RCl ---> Rli + LiCl
                                                                               Usually done in pet. Ether, benzene, or cyclohexane
                                                                               RLi can be used to transfer R groups to other compounds

   AlX3: X = Cl, Br, I or R                                                   3-Center, 2-Electron Bond
221 pm            ••           101˚
                  Cl                                In liquid and gas phase                       H3

                                                                                                      C                           Terminal and bridging R
   Cl             ••              Cl                AlCl3 is dimer.
            Al             Al              118˚                                                                                   groups exchange
                                                    AlBr3 and AlI 3 are                                                    sp3
  Cl              ••              Cl                                                                                              positions rapidly at room
                  Cl                                dimers in all phases.

                  ••                                                          R                                            R      temperature.
                                   206 pm                                                                           Al
Replace Cl with alkyl or aryl groups --->                                     R                                              R
organoaluminum compounds (AlR3)2
                                      H3    74.7˚
                                      C                                                               C
                 CH3                                CH3                                               H3
                           Al               Al             123˚
                                                                                       2e- spread over 3 orbitals
                 CH3                  C              CH3
                      196 pm          H3    215 pm

       Organosilanes, R4Si
                           MeSiCl3        25%
Si + CH 3Cl over Cu        Me3SiCl3       65%
catalyst at >200 ˚C        Me3SiCl        5%

This is the origin of silicone polymers

                           Me      Me
Me2SiCl2                   Si O Si O
                           Me   Me   n


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