Section 22.5. Bonding in Coordination Complexes: Ligand Field Theory.
The most modern theory, an amalgamation of the older ‘crystal field theory’ and molecular
orbital theory. It explains well the observed properties of coordination complexes.
The theory assumes that the attraction between M and L is largely (but not completely)
ionic/electrostatic i.e. that the ML bond is primarily due to an electrostatic (charge) attraction
between the positive charge of M and negative (or partial negative) charge of L or L , e.g.
Cl, Br, CN, NH , H O, etc.
The great thing about Ligand Field Theory is that it considers the change in energy of the metal
dorbitals caused by the approach of the L to the M. This explains the observed colors
and many other props.
As the ligands L approach the M positions that they will occupy for that coordination
geometry, they destabilize (raise the energy of) those dorbitals occupying that region of space
more than they destabilize those orbitals that do not, i.e. all dorbitals are destabilized, but
some more than others. Thus, the dorbitals do not all have the same energy anymore (they do in
the free M ion).
The shapes of the
five d orbitals
are shown in blue
in figures BF.
Only the d anz2
towards the L
dx2y2and d z2rbitals
lie on the axes and
are more affected by
ligands L, because they point straight at them. Therefore:
1 Δ = “crystal field splitting energy”. For octahedral geometry, we often write Δ (o for
The available d electrons for that M (i.e. d ) occupy these dorbitals, lowest energy ones first.
This explains the colors of transition metal compounds – movement of e from lower to upper
energy orbitals absorbs visible light – gives colored solutions!
3+ 3+ 1 + green-
[Ti(H 2) ]6 contains Ti (d ) __ __ __ __
absorbs greenyellow light, yellow
therefore appears purple __ __ __ __ __ __
ground state excited state
(ΔE = E = Δ = hc/λ)
electron photon O
λ = wavelength of light: λ decreases with increasing energy
h = Planck’s constant c = speed of light in a vacuum
Different ligands L cause different splittings Δ
“strongfield ligands” give big Δ
“weakfield ligands” give small Δ
A ranking of different L according to what Δ they give is known as “the spectrochemical
2 Understanding Ligand Field Theory and the spectrochemical series also allows us to
understand the magnetic properties of transition metal compounds, our final topic.
The Magnetic Properties of Transition Metal Complexes
Magnetism is due to the presence of unpaired electrons i.e. only one electron in an orbital. The
number of unpaired electrons that a particular M ion will have depends on:
1. The identity of the metal (3d vs 4d/5d)
2. The oxidation state of the metal
3. The d (the total number