Zinc, cadmium, and mercury are sometimes excluded from the transition metals, as they have the electronic configuration [ ]"d"10s2, with no incomplete "d" shell. In the oxidation state +2, the ions have the electronic configuration [ ]…d10. Although these elements can exist in other oxidation states, including the +1 oxidation state, as in the diatomic ion , they still have a complete "d" shell in these oxidation states. The group 12 elements Zn, Cd and Hg may therefore, under certain criteria, be classed as post-transition metals in this case. However, it is often convenient to include these elements in a discussion of the transition elements. For example, when discussing the crystal field stabilization energy of first-row transition elements, it is convenient to also include the elements calcium and zinc, as both and have a value of zero, against which the value for other transition metal ions may be compared. Another example occurs in the Irving–Williams series of stability constants of complexes.