Oxymercuration can be fully described in three steps (the whole process is sometimes called "deoxymercuration"), which is illustrated in stepwise fashion to the right. In the first step, the nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. The electron pair on the mercury ion in turn attacks a carbon on the double bond, forming a "mercuronium ion" in which the mercury atom bears a positive charge. The electrons in the highest occupied molecular orbital of the double bond are donated to mercury's empty 6s orbital and the electrons in mercury's dxz (or dyz) orbital are donated in the lowest unoccupied molecular orbital of the double bond.