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.

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 Absorbs a photon of yellow light Energy

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+