We present a simple model describing resonant tunneling through adsorbed molecules in scanning tunneling microscopy, extending the Newns-Anderson Hamiltonian used in chemisorption studies. The problem is viewed as a double chemisorption situation, in which various parameters depend adiabatically on the position of the adsorbate orbital of interest, leading to a position-dependent adsorbate Green's function. The characteristic inelastic spectrum peaks are obtained, at positions depending on the vibrational energies, and their intensities depend on the occupancies of the respective adsorbate orbitals