Using X-ray photoelectron spectroscopy (XPS), we demonstrate that free-base tetraphenylporphyrin (2HTPP) on Ag(1 0 0) forms a theoretically predicted adcomplex with post-deposited iron at 100 K. The formation of the adcomplex, as a precursor for metalation, can be deduced from the N Is spectra showing clearly not-metalated 2HTPP molecules, but with strongly shifted iminic nitrogen peaks, caused by the interaction of the iminic nitrogen atoms with the Fe adatoms. (C) 2015 Elsevier B.V. All rights reserved.