why is delocalization less effective when there is a separation of charge? I've been reading Peter Syke's book "A guidebook to mechanisms in organic chemistry" wherein he has frequently mentioned that delocatlization is less effective when there is a seperation of charge i.e instead of a double bond formation and a negative charge, there is a positive and a negative charge occurance eg. Formate ion and Formic acid
It takes energy to separate the charges, even when the difference in electronegativity is in the same direction as the proposed charge separation.  The energy that must he put in becomes part of the energy in the charge-separated contributing structure, making that contribution less stable and less likely to contribute to the molecular ground state.