The vibrational energy of either N-0 or C=N groups in the chelate ring as well as C=O of LM (H2L=N,N'-ethylenebis(isonitrosoacetylacetoneimine), M=divalent Ni, Co Dr Cu) is related to the second ionization potential of the metal ion. This relationship is used for the calculation of the stretching vibration of either nu(N-0), nu(C=N) or nu(C=O) group. The contribution of any of these groups to the stability of the chelate isomers (five- or six-membered chelate ring) is discussed