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Lecture 26
MO’s of Coordination Compounds MLx (x = 4,6)
1) Octahedral complexes with M-L s-bonds only
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2t1u
Group orbitals of 6 L’s suitable for s-bonding
transform as a1g, eg and t1u (Lecture 18; p. 4).
Look at the options available for the metal
complex electron count:
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6 L GO's
M
Consider an example of an octahedral complex
ML6 where metal and ligands are s-bound.
2a1g
p
s
If the metal configuration is d5 electrons or
less the metal complex will have the electron
count less than 18 electrons (TiF62-,
d
[Fe(OH2)6]3+ etc.).
If Do is small the antibonding 2eg orbital can
be filled so increasing the complex electron
count up to 19-22 electrons (Co(NH3)62+,
Ni(NH3)62+, Cu(NH3)62+, Zn(NH3)62+).
t1u
a1g
eg
t2g
D
2eg
t2g
1eg
1t1u
1a1g
t1u
eg
a1g
2) Octahedral complexes with M-L p-bonds. p-Donating ligands
The symmetry of metal and ligand group orbitals
suitable for M-L p-bonding in octahedral
complexes can be found using group theory:
Gr(p) = T1g + T2g + T1u + T2u (see Lecture 18).
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Transition metal atom has orbitals of the t2g
(dxy, dyz, dxz) and t1u (px, py, pz) symmetry and
no orbitals of the t1g or t2u symmetry (see the
character table below).
Metal orbitals of the t1u symmetry are already D

involved in s-bonding with 6 L’s (see the
previous diagram).
Therefore, p-bonding is only possible between
metal and ligand orbitals of the t2g symmetry.
Consider the case of the ligand-to-metal pdonation when the ligands t2g orbitals are
completely filled (and of low energy) while the
metal t2g orbitals are not completely filled.
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We will get two new t2g-MO’s, 1t2g and 2t2g.
The energy gap between the partially filled
2t2g and eg MO’s and thus Do are now smaller.
So p-donating ligands are weak field ligands
(halogeno ligands, OH-, H2O etc).
6 L p-GO's
M
eg
D
eg
2t2g
t2g
t2g
1t2g
Oh
A1g
x2+y2+z2
Eg
(2z2-x2-y2, x2-y2)
T1g
(Rx,Ry,Rz)
T2g
T1u
T2u
(xz, yz, xy)
(x,y,z)
3) Octahedral complexes with M-L p-bonds. p-Accepting ligands
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In the case of p-accepting ligands like
CO, CN- etc. the ligands group orbitals
of t2g symmetry are empty and higher
in energy than corresponding t2g metal
orbitals.
2t2g
The metal-ligand p-bonding stabilizes
the metal complex and increases Do.
Therefore the ability of a ligand to be a
p-acceptor makes the ligand a
stronger field ligand.
Increased Do prevents the eg level to
be filled and the metal valence shell to
be “overfilled” and helps it obey 18
electron rule.
6 L p-GO's
M
eg
eg
D
D
t2g
1t2g
t2g
4) Tetrahedral complexes with M-L s- and p-bonds
Consider first the case of s-only bonded tetrahedral
metal complex ML4.
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The ligand group orbitals of a1 and t2 symmetry
are similar to those considered for 4H GO’s
(Lecture 18).
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The 2tg MO is slightly antibonding and that is why
the electron count less than 18 is quite common
for ML4 (FeCl4-, CoCl42- etc.).
p
From the other hand, Dt is usually small and the
electron count up to 18 electrons for ML4 is
s
possible but rare.
The case of tetrahedral metal complex ML4 with M-L d
p-bonds.
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The orbitals of e (dz2, dx2-y2), t1 and t2 (px, py, pz;
dxz, dyz, dxy) symmetry are suitable for p-bonding
with L’s (Lecture 14, p. 5).
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Two e metal orbitals are only available to form M-L
p-bonds since the t2 orbitals are involved in M-L sbonding.
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p-acceptors will stabilize MO’s of e-symmetry so
increasing Dt (not shown on the diagram).
4 L GO's
M
3t2
2a1
t2
a1
e
t2
Dt
2t2
e
1t2
1a1
t2
a1