Transcript
Metal dx2-y2
NH3
Right symmetry for interaction
Metal dxy
Wrong symmetry for interaction
x
y
NH3x
So, (3dz2, & 3dx2-y2) & 2 lone pair orbitals
interact to form sigma bonds
dxy, dxz, dyz do NOT
y
Octahedral Case: Ligands along x, y, z axes
σ-bonding: Molecular Orbital Theory (LCAO-MO)
applied to Coordination Compounds: Ligand Field Theory
Metal Atom Orbitals (ligands along x, y, z axes)
(3dz2, & 3dx2-y2)
Ligands (6 lone pair orbitals)
Ligand Field Theory
SALC
(sigma)6 0 0 2 2 0 0 0 4 2 = A1g + Eg + T1u
Reducible Representation Decompose into three
Irreducible Representations
Ligand Field Theory: Oh Complexes
FIGURE 20.16
SALCs Resource section 5
Symmetry of d orbitals From Oh character table
Crystal Field Splitting of
Tetrahedral Complexes
e = low energy
t2 = high energy (closer to corners)
note: NO “g” subscripts for d orbital symmetry in tetrahedral geometry(the Td point group does not have the inversion symmetry)
Ligand Field Theory: Td Complexes
SALCs from Resource section 5
Metal ML4 4 Ligands
3d
4s
4pT2
A1
E+T2
s
A1+T2
a1
a1
t2
t2
t2
Metal ML4 4 Ligands
3d
4s
4pT2
A1
E+T2
s
A1+T2
a1
a1
t2
t2
t2
e
DT
Crystal Field Splitting
Tetrahedral Complexes
No low-spin tetrahedral complexes!
Dt= -4/9 Do
Extent of splitting from p - bonding: Weak and Strong Field ligands
Consider Cl- (weak), NH3 (intermediate) and CO (strong)
CASE 1: NO p-INTERACTION = σ-DONOR (i.e. NH3)
Metal dx2-y2
NH3x
y
Extent of splitting from p - bonding: Weak and Strong Field ligands
Consider Cl- (weak), NH3 (intermediate) and CO (strong)
Cl M
- bonding as before
Now p - bonding between p & dxy, dxz, dyz
σ - bonding as before
Now p - bonding between CO p * & dxy, dxz, dyz
No p - bonding with CO p
M
CASE 2: p-DONOR (i.e. Cl-)
CASE 3: p -ACCEPTOR (i.e. CO)
CASE 1: NO p-INTERACTION = σ-DONOR (i.e. NH3)
N C
SALCs from Resource section 5
dxy, dxz, dyz
d* = eg
= t2g
σ metal-ligand molecular orbitals
(all filled, mostly ligand character)
6 ligand donor orbitals
(sigma symmetry)
Metal LigandMolecule
Metal-Ligand Bonding: Sigma-DONOR Ligands, NO pi-bonding
Do
σ* metal-ligand molecular orbitals
(all empty, mostly metal character)
d
s
p
A1g + Eg + T1u
Eg + T2g
A1g
T1u
dxy, dxz, dyz
d* = eg
Ligand donor orbitals
(pi symmetry)
always lower energy
than metal orbitals
Metal LigandMolecule
Metal-Ligand Bonding: Sigma-Donor, Pi-Donor Ligands
Do
d
s
p
Pi-Donor Ligands
DECREASE Do
= Weak Field Ligands
p*-antibonding orbitals
p-bonding orbitals
Eg + T2g
A1g
T1u
sigma-donororbital
pi-acceptingorbital(only one of the two)
Classical pi-acceptor:Carbonyl (CO)
dxy, dxz, dyz
d* = eg
Ligand acceptor orbitals
(pi symmetry)
always higher energy
than metal orbitals
Metal LigandMolecule
Metal-Ligand Bonding: Sigma-Donor, Pi-Acceptor Ligands
Dod
s
p
Pi-Acceptor Ligands
INCREASE Do
= Strong-Field Ligands
p*-antibonding orbitals
p-bonding orbitals
Eg + T2g
A1g
T1u
Pi-Backbonding with Pi-Acceptor Ligands
Two contribution to the overall bonding:
The backbonding effect stabilizes the complex because the overall charge transfer can be adjusted to fit both the ligand and metal “needs”: if the metal would like to have more or less electrons, it can adjust the amount of backdonation to the ligand.
Backdonation tends to favor low-oxidation state metals, such as Ti(0) or Cr(0) for instance.
Strong field
Ligands
Weak field
Ligands
The spectrochemical series
Predicting the Crystal Field Splittings (p. 670)
xz yz xy
z2 x2-y2
Do
xz yz xy
z2 x2-y2
Do
Do = hn
I- < Br- < [NCS]- < Cl- < F- < [OH]- < [ox]2- ~ H2O< [NCS]- < NH3 < en < bpy < phen < [CN]- ~ CO
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