Electronegativity, metals alternation effect

Theories based on cr-effects consider that the more electronegative a ligand the more polarization of the metal occurs, weakening the bond trans to it. This can alternatively be viewed in terms of electronic transmission via a <7-type (p) orbital shared by the two ligands. 

Polarization of the surrounding by the metal ion (either formal or effective) charge can be fit into the electronegativity equilibration scheme by ascribing new electronegativities to the atoms in the external field induced by the ion. However, it must be realized that neither of the characteristic quantum features of polarizability, such as the alternating polarity law [27] can be reproduced by classical methods. 

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