Symmetry, molecular orbitals
and spectroscopy in transition
metal compounds
• UV/vis spectroscopy examines the difference in
energy between occupied and unoccupied orbitals.
• Transition metal compounds are well known for
their colours, which are associated with electronic
transitions between d orbitals.
• The split in d-orbital energy results from bonding .
In an octahedral complex the energy gap between
highest occupied (t2g) and lowest empty d orbital (eg)
is symbolised by o.
In tetrahedral complexes the corresponding gap is t
and is between e and t2.
eg
t2
o
t2g
t
e
Crystal Field Theory provides a simple,
ionic bonding, explanation for the d
orbital splitting in a non-spherical
environment:-
The d orbital split explains:•The colour of transition metal compounds
•Lattice energies and ionic radii
•Magnetic properties
•Site selection in spinels.
Ligands arranged according to their ability to split
d orbitals are known as the Spectrochemical
Series
I- < Br- < S2- < SCN- < Cl- < NO3- < F- < OH- <
H2O < NCS- < CH3CN < NH3 < en < NO2- <
CN- < CO
Group theory can identify MOs in transition
metal complexes and explain d-splitting.
Covalent bonding model.
Will now look at:1. σ-bonding in tetrahedral complexes
2. σ-bonding in octahedral complexes
3. π-bonding in octahedral complexes
Matching AO to make MO
• Identify molecule’s point group
• Assign symmetry to valence orbitals of central
atom using point group table.
• Place appropriate orbitals on bonded atoms and
determine their reducible representation (0 if
orbital moves, +1 if it stays, -1 if it stays but
reverses). N.B. only 0 and +1 for sigma bonds
• Reduce
• Does symmetry of irreducible reps match valence
orbitals?
• Yes = MO. No = non-bonded.
• If required. Construct MO diagram
Metal orbitals
Td label
3d
z2 and (x2-y2)
E
3d
xy, xz and yz
T2
4s
A1
4p
T2
x3
x3
1. Tetrahedral complexes:...