What is d-d transition of d-block elements?
To define the d-d transition, first, we have to be familiar with the electronic structure of the d-block elements. The d-block elements are ones that have d-orbitals in addition to s and p nuclear orbitals. The occurrence of an electron on the d-orbital is a prerequisite for the origin of d-d type transitions. The d-d transition is an excitation of an electron from one d orbital to another d orbital. In a free metal, all the five d orbitals are degenerate (they are all of the same energy) but in a presence of ligands in the first coordination sphere, the d orbitals split into two sets of different energy making a way for an electron to promote from one set of orbitals to another.