Transcript Ligand Field Theory: π Bonding

Ligand Field Theory: σ Bonding
Combination of Metal and Ligand Orbitals in an Octahedral Complex
a1g
t1u
eg
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Ligand Field Theory: σ Bonding
 Overlap of metal orbitals and linear combination of ligand group orbitals (LGOs)
leads to the formation of molecular orbitals (MOs).
 s orbitals transforms as a1g, set of p orbitals as t1u, whereas five d orbitals lose their
degeneracy to form eg (dz2 and dx2-y2) and t2g (dxy, dxz and dyz).
 Spherical a1g orbitals are capable to overlap with ligand group orbitals (LGOs) on all
axes.
 t1u and eg sets have lobes along bond directions and participate in bonding.
 However, t2g set have lobes directed between the bonding axis and thus will yield
no overlap with ligand orbitals.
 Electronic configuration for [Co(NH3)6]3+: a1g2t1u6eg4t2g6
 Electronic configuration for [CoF6]3-: a1g2t1u6eg4t2g4eg*2
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Ligand Field Theory: σ Bonding
MO Diagram for Octahedral ML6 Complex
a1g = s
t1u = px, py, pz
t2g = dxy, dyz, dzx
eg = dx2-y2, dz2
[Co(NH3)6]3+:
a1g2t1u6eg4t2g6
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Ligand Field Theory: π Bonding
 Metal t2g orbitals, which are non-bonding in presence of σ-donor ligands, can form
π-bonds with ligand pπ, dπ, dπ* and dσ* orbitals.
Pi-overlap with a metal
t2g (dxy) orbital with
ligand t2g LGOs
 Types of π Interactions:
pπ-dπ
dπ-dπ
dπ-π*
dπ-σ*
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Ligand Field Theory: π Bonding
Types of π Interactions
Type
Description
Ligand Examples
pπ-dπ
Donation of electrons from the filled
p-orbitals of the ligand to the empty
d-orbitals of the metal
RO-, RS-, O2-, F-, Cl-, Br-, I-,
R2N(π-donors)
dπ-dπ
Donation of electrons from filled
d-orbitals of the metal to the empty
d-orbitals of the ligand
R3P, R3As, R2S
(π-acceptors)
dπ-π*
Donation of electrons from filled
CO, CN-, NO2-, ethylene
d-orbitals of the metal to the empty
(π-acceptors)
π- antibonding orbitals (π*) of the ligand.
dπ-σ*
Donation of electrons from filled
d-orbitals of the metal to the empty
σ-antibonding orbitals (σ*) of the ligand
H2, alkane
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Ligand Field Theory: π Bonding
MO Diagram for Octahedral Complex with π-Donor Ligands
[CoF6]3-
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Ligand Field Theory: π Bonding
MO Diagram for Octahedral Complex with π-Acceptor Ligands
[Cr(CO)6]
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Ligand Field Theory: π Bonding
Simplified picture of how π-acceptor and π-donor interactions affect MO diagram
Only frontier orbitals are shown
(π*)
(σ*)
(σ*)
(π*)
π (t2g)
(π)
[CoF6]3-
(π)
π-donor ligands decrease Δo
π-acceptors ligands increase Δo
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Ligand Field Theory: π Bonding
 π-Donor Ligands: These ligands (e.g., F-) are more electronegative than metal and
has filled p orbitals. Hence, the orbitals are lower in energy than the metal d orbitals.
The 3 t2g metal orbitals and 3 low-lying, filled ligand orbitals form 3 bonding MOs and
3 antibonding MOs.
 The electrons from the ligand p orbitals will fill the bonding π-orbitals. The electrons
from the metal d orbitals will be present in the π* orbitals. The eg* orbitals are not
affected. The previously nonbonding metal t2g orbitals become antibonding and
hence are raised in energy. As a result, Δo decreases. This is the reason for halides
being weak ligands in spectrochemical series in spite of their negative charges.
 π-Acceptor Ligands: These ligands are less electronegative than metal and has
empty orbitals (d orbitals for PR3, π* orbitals for CO). Hence, the orbitals are higher
in energy than the metal d orbitals. The 3 t2g metal orbitals and 3 high-lying, empty
ligand orbitals form 3 bonding MOs and 3 antibonding MOs.
 The electrons from the metal d orbitals will fill the bonding π-orbitals. The eg*
orbitals remain empty and they are not affected. The previously nonbonding metal
t2g orbitals become bonding and hence are lowered in energy. As a result, Δo
increases. This is the reason for PR3 and CO being strong ligands in spectrochemical
series, although they are neutral ligands.
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Ligand Field Theory: π Bonding
Explanation for the sequence of ligands within the spectrochemical series
CO > CN- > PPh3 > NO2- >
en > NH3>
H2O>
strong ligands
empty d or
π*-orbitals
π-acceptor
OH- > F- > Cl- > Br- > I-
weak ligands
no suitable p-orbitals
exclusively σ-donor
(no π effect)
filled p-orbitals
weak π donor
filled p-orbitals
π-donor
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