Based on exact diagonalization results for small clusters we discuss t
he effect of intersite Coulomb repulsion in Mott-Hubbard or charge tra
nsfers insulators. Whereas the exchange constant J for direct exchange
is enhanced by intersite Coulomb interaction, that for superexchange
is suppressed. The enhancement of J in the single-band models holds up
to the critical value for the charge density wave (CDW) instability,
thus opening the way for large values of J. Single-band Hubbard models
with sufficiently strong intersite repulsion to be near a CDW instabi
lity thus may provide physical realizations of t-J-like models with th
e ''unphysical'' parameter ratio J/t approximate to 1.