Phys. Lett. A 136, 163 (1989)
The possibility of superconductivity arising from interactions involving nearest neighbor singlet pairs is discussed. It is shown that the strong coupling effective Hamiltonian for the Hubbard model only admits extended s-wave solutions, and it is argued that those are suppressed by the Hubbard U. Neglecting hopping terms in the effective Hamiltonian involving three sites while keeping nearest neighbor superexchange and/or density interactions introduces spurious d- and p-wave solutions. A related but more fundamental interaction describing covalent bonds between polarizable ions can give rise to extended s-wave superconductivity, and the relationship with the resonating valence bond picture is discussed.