The electronic interaction between a porphyrin-coordinated Co ion and a silver surface was switched by an axial nitric oxide (NO) ligand on the Co ion. This process is reversible and was monitored with X-ray and UV-photoelectron spectroscopy (XPS and UPS). The Co-Ag interaction leads to an electron transfer from the Ag surface to the Co ion and to a new electronic state close to the Fermi level. Both effects vanish if an axial NO ligand coordinates to the Co ion, indicating that the Co-Ag interaction is suppressed. As a possible explanation, we discuss a competition, similar to the trans effect, between the two axial ligands on the Co ion: the NO molecule and the Ag surface. These findings have important implications for sensor technology and for catalysis on supported metalloporphyrins, in which the catalytic activity of the metal center is controlled by the interaction with the underlying surface.