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. .
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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
Closer to ligands
Further from 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
Absorbs a photon of yellow light
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+
Different ligands cause different degrees of splitting of the d orbital energies.
Ligand exchange therefore usually causes a change of
Absorbs orange appears blue
Absorbs red appears green