We consider a one-band model effective Hamiltonian ${\mathit{H}}_{\mathit{e}\mathit{f}\mathit{f}}$ deriv ed previously from the three-band model H for ${\mathrm{CuO}}_{2}$ planes. The parameters of ${\mathit{H}}_{\mathit{e}\mathit{f}\mathit{f}}$ are given in terms of those of H by a simple prescription. Using ${\mathit{H}}_{\mathit{e}\mathit{f}\mathit{f}}$ and the slave-boson approach of Kotliar and Ruckenstein, we determine 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 slave bosons directly to H. For sufficiently large values of the Cu-O repulsion ${\mathit{U}}_{\mathit{p}\mathit{d}}$, treated in the Hartree-Fock approximation, there is a charge-transfer instability inside the metallic phase, very near the metal-insulator boundary.