We consider a one-band model effective Hamiltonian H(eff) derived prev
iously from the three-band model H for CuO2 planes. The parameters of
H(eff) are given in terms of those of H by a simple prescription. Usin
g H(eff) and the slave-boson approach of Kotliar and Ruckenstein, we d
etermine the metal-insulator boundary and the magnitude of the charge-
transfer gap as a function of the parameters of H. Our results are in
better agreement with experiment than those obtained applying the slav
e bosons directly to H. For sufficiently large values of the Cu-O repu
lsion U(pd), treated in the Hartree-Fock approximation, there is a cha
rge-transfer instability inside the metallic phase, very near the meta
l-insulator boundary.