The state of a ligand in a molecule is discussed in terms of the quantum field theory operating with wave functions and occupation numbers in the Fock space. Following the basic concepts of the quantum theory, the state of a ligand is considered as an average weighed value of two ionic states given in the form of effective electron charge on ligand. A shift in the ligand state between the ionic states under the influence of an external field is estimated in terms of lability indexes arising as a result of the information statistical theory application to the problem of quantum parameter estimation. The proposed indexes combined with their energetic characteristics are used for predicting cis and trans effects in platinum(II) complexes forming the Werner-Miolatti series.