An unprecedented red shift of more than 200 cm(-1) in the vibrational frequency of the C-O bond in the [Cu(PhO) L-n](+) complex (PhO = phenoxy), dependent on the number n of additional ligands L, is reported. Upon change of n from 1 to 2, the spin density is shifted from the aromatic ring to the oxygen and copper atoms, which is reflected in the bond order and thus vibrational frequency of the C-O bond.