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Colour and the d block
• UV / Vis frequencies are have photons
with energies of the sort of values needed
to promote electrons from their ground
state energy level to a higher level.
• A typical substance will require UV
photons so does not absorb Visible light.
• Most substances are colourless.
d – block metal complexes
• These have unusually close energy levels
due to the splitting of the 3d orbitals in a
non spherical environment.
3-
Cl
Cl
Cl
Fe
Cl
Cl
Cl
3d orbitals in gas
Energy
But in an octahedral complex two of the
orbitals will be closer to ligands than the
other three.
But ligands usually have a negative charge…
..so electrons in the d orbitals are repelled.
They have a higher energy
3d orbitals in complex
Energy
Further from ligands
Closer to ligands
d electrons will tend to go preferentially
into the lower energy orbitals further
from the ligands.
Ti has the electron configuration
[Ar] 3d2 4s2
So Ti3+ has the electron configuration
[Ar] 3d1
3d orbitals in Ti3+ complex
Energy
3d orbitals in Ti3+ complex
Energy
Absorbs a photon of yellow light
So what d block complexes will not be coloured ?
Ones with no d electrons eg……
Sc 3+, Ti 4+
..or those for which the d shell is full so electrons
cannot move. Eg…
Zn2+ , Cu+
3d orbitals in Cu+ complex
Energy
Different ligands cause different
degrees of splitting of the d orbital
energies.
Ligand exchange therefore usually
causes a change of
colour
3d orbitals in Ni2+ complexes
Absorbs orange appears blue
Absorbs red appears green
Ni(H2O)62+
Ni(NH3)62+