Reversible switching of the electronic ground state in a pentacoordinated Cu(II) complex

An easy reversible switching of the electronic ground state in a pentacoordinated copper(II) complex is reported for the first time. The simple protonation of a carboxylic group in a Cu(II) complex with a {d(x2-y2)}(1) electronic configuration leads to a flip of the ground electronic configuration from {d(x2-y2)}(1) to {d(z2)}(1) in the metal ion.