Molecular orbital theory approach to

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Molecular orbital theory approach to
bonding in transition metal complexes
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• Molecular orbital (MO) theory considers the
  overlap of atomic orbitals, of matching symmetry
  and comparable energy, to form molecular
  orbitals.
• When atomic orbital wave functions are
  combined, they
• generate equal numbers of bonding and
  antibonding
• molecular orbitals.
• The bonding MO is always lower in energy than
  the
• corresponding antibonding MO.
• Electrons occupy the molecular orbitals in
  order of their
• increasing energy in accordance with the
  aufbau principal.

Bond-Order Electrons in bonding MOs
Electrons in antibonding MOs

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Molecular orbital descriptions of dioxygen
species.
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Molecular orbital approach to bonding in
octahedral complexes, ML6
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__________________________ Combinations of
atomic orbitals Molecular Orbital 4s
1/v6(s1 s2 s3 s4 s5 s6)
a1g 4px 1/v2 (s1 ? s2) 4py 1/v2
(s3 ? s4) t1u 4pz 1/v2
(s5 ? s6) 3dx2 - y2 1/2 (s1 s2 ? s3 ?
s4) eg 3dz2 1/v12 (2 s5
2 s6 ? s1 ? s2 ? s3 ? s4) 3dxy 3dxz
Non-bonding in s complex
t2g 3dyz _________________________________________
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MO diagram for s-bonded octahedral metal complex
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M.O. Diagram for Tetrahedral Metal Complex
Since the metal 4p and t2 orbitals are of the
same symmetry, e ? t2 transitions in Td
complexes are less d-d than are t2g ? eg
transitions in Oh complexes. They are therefore
more allowed and have larger absorbtivity values
(e)
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Metal-ligand P-bonding interactions

• t2g orbitals (dxy, dxz, dyz) are non-bonding
  in a s-bonded octahedral
• complex
• ligands of P-symmetry overlap with the metal
  t2g orbitals to form
• metal-ligand P-bonds.
• P-unsaturated ligands such as CO, CN- or
  1,10-phenanthroline or sulfur
• and phosphorus donor ligands (SR2, PR3)
  with empty t2g-orbitals have
• the correct symmetry to overlap with the
  metal t2g orbitals.

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P-acceptor interactions have the effect of
lowering the energy of the non-bonding t2g
orbitals and increasing the magnitude Doct.

This explains why P-acceptor ligands like CO and
CN- are strong field ligands, and why metal
carbonyl and metal cyanide complexes are
generally low-spin.
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• -interactions involving P-donation of electron
  density from filled p-orbitals of halides (F- and
  Cl-) and oxygen donors, to the t2g of the metal,
  can have the opposite effect of lowering the
  magnitude of Doct. In this case, the t2g
  electrons of the s-complex, derived from the
  metal d orbitals, are pushed into the higher t2g
  orbitals and become antibonding. This has the
  effect of lowering Doct.

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Effect of ligand to metal P-donor interactions
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P-alkene organometallic complexes
Zeises Salt, KPtCl3(C2H4)
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P-acceptor interactions have the effect of
lowering the energy of the non-bonding t2g
orbitals and increasing the magnitude Doct.

This lowering of the energy of the t2g orbitals
also results in 9 strongly bonding M.O.s well
separated in energy from the antibonding orbitals
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Consequences of P-bonding interactions between
metal and ligand

• Enhanced D-splitting for P-acceptor ligands
  makes P-unsaturated ligands
• like CO, CN- and alkenes very strong-field
  ligands.
• Stabilization of metals in low oxidation
  states.
• Delocalization of electron density from low
  oxidation state (electron-rich)
• metals into empty ligand orbitals by
  back-bonding enables metals to exist
• in formally zero and negative oxidation
  states (Fe(CO)5, Ni(CO)42-).
• Accounts for organometallic chemistry of P-Acid
  ligands
• The application of the 18-electron rule to
  predict and rationalize
• structures of many P-acid organometallic
  compounds.

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Electron donation by P-unsaturated ligands
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Examples of 18-electron organometallic complexes
with P-unsaturated (P-acid) ligands


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Scope of 16/18-electron rules for d-block
organometallic compounds
16 or 18 Electrons Co Ni Rh Pd Ir
Pt
Usually less than 18 electrons Sc Ti V Y
Zr Nb
Usually 18 electrons Cr Mn Fe Mo Tc
Ru W Re Os
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Metal-ligand interactions involving bonding and
antibonding molecular orbitals of O2