5 Complexes are expected to be colored when the highest occupied orbitals have between one and nine delectrons. Such complexes can therefore have d d transitions (that are usually in the visible part of theelectromagnetic radiation spectrum). The ions V5, Ca2, and Sc3have d0electron configurations and Cu,Zn2, and Pb2have d10 electron configurations: these complexes are colorless. The other complexes haveouter electron configurations of d1to d9and are therefore colored.