3. Molecular Orbital Theory-2011.ppt

MO

1. Molecular orbital theory approach to
bonding in transition metal complexes

2. 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 aufbauprincipal.
Bond-Order = Electrons in bonding MOs –Electrons in antibonding MOs
2

4. Molecular orbital descriptions of dioxygen species.

5. Molecular orbital approach to bonding in octahedral complexes, ML
6
_____________________________________________________________________________________________________________________________ _
Combinations of atomic orbitals MolecularOrbital
4s±1/√6(σ
1
+σ
2
+ σ
3
+ σ
4
+ σ
5
+ σ
6
) a
1g
4p
x
±1/√2 (σ
1
σ
2
)
4p
y
±1/√2 (σ
3
σ
4
) t
1u
4p
z
±1/√2 (σ
5
σ
6
)
3d
x2 -y2
±1/2 (σ
1
+σ
2
σ
3
σ
4
) e
g
3d
z2
±1/√12 (2 σ
5
+2 σ
6
σ
1
σ
2
σ
3
σ
4
)
3d
xy
3d
xz
Non-bondingin σcomplex t
2g
3d
yz
_______________________________________________________________________________________________

6. MO diagram for s-bonded octahedral metal complex

8. Since the metal 4p and t
2orbitals are of the same symmetry, e→ t
2 transitions in
T
d complexes are less “d-d” than are t
2g→ e
gtransitions inO
hcomplexes. They are
therefore more allowed and have larger absorbtivity values (e)
M.O. Diagram for Tetrahedral Metal Complex

9. Metal-ligand P-bonding interactions
t
2gorbitals (d
xy, d
xz, d
yz) are non-bonding in a s-bonded octahedral
complex
ligands of P-symmetry overlap with the metal t
2gorbitals to form
metal-ligand P-bonds.
P-unsaturated ligands such as CO, CN
-
or 1,10-phenanthroline or sulfur
and phosphorus donor ligands (SR
2, PR
3) with empty t
2g-orbitals have
the correct symmetry to overlap with the metal t
2gorbitals.

10. Pacceptor interactions have the effect of lowering the energy of
the non-bonding t
2g
orbitals and increasing the magnitude D
oct
.
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.

11. Metal- d Ligand-
L
p(t
2g)
M
Ligand p (full)
e.g. halide ion, X
-
RO
- P-interactions involving P-donation of electron density from filled p-
orbitals of halides (F
-
and Cl
-
) and oxygen donors, to the t
2g
of the
metal, can have the opposite effect of lowering the magnitude of
D
oct
. In this case, the t
2g
electrons of the s-complex, derived from the
metal dorbitals, are pushed into the higher t
2g
*
orbitals and become
antibonding. This has the effect of lowering D
oct
.

12. Effect of ligand to metal Pdonor interactions

13. P-alkene organometallic complexes
Zeise’s Salt, K[PtCl
3(C
2H
4)]

15. Pacceptor interactions have the effect of lowering the energy of
the non-bonding t
2g
orbitals and increasing the magnitude D
oct
.
This lowering of the energy of the t
2g orbitals also results in 9 strongly bonding
M.O.’s well separated in energy from the antibonding orbitals

17. 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)
4
2-
).
Accounts for organometallic chemistry of P-Acid ligands
The application of the “18-electron rule” to predict and rationalize
structures of many Pacidorganometallic compounds.

18. Electron donation by P-unsaturated ligands

19. Examples of 18-electron organometallic complexes with P-
unsaturated (P-acid) ligands

24. Scope of 16/18-electron rules for
d-block organometallic compounds
Usually less than
18 electrons
Sc Ti V
Y Zr Nb
Usually
18 electrons
Cr Mn Fe
Mo Tc Ru
W Re Os
16 or 18
Electrons
Co Ni
Rh Pd
Ir Pt

25. Fe
O
O
of O2 (filled)
dz
2
of Fe (empty)
O
O
Fe
 of O2 (empty)
t2g (dxz,dyz) of Fe (filled)
*
* Metal-ligand interactions involving bonding and
antibonding molecular orbitals of O
2