Localized or Delocalized Excitations

The transition from localized behavior to delocalized behavior is an important one. As an example, nickel oxide (NiO) has about the same color as Ni + complexes in aqueous solution. Cu + complexes in aqueous solution are blue, but teud to be green if the ligands are halide ions or violet if the ligands are ammonia molecules. Copper(II) oxide (CuO), on the other hand, is black. Very likely, the wide absorption in the visible region is from an allowed transition that covers the weak ligand field transitions. It cannot be an LM transition since there is no reason to believe that LM transition are in the visible region just in the case of CuO. 

Photoconduction experiments show that we are dealing with a metal-metal (MM) transition. As we will see in later chapters, MM transition energies are strongly dependent on the distance between the metal ions. For small distances, they go down into the visible region for some metal ions. 

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