Using a monolayer of iron phthalocyanine adsorbed on the Au(111) surface and on a Cu(111)-supported two-dimensional copper nitride layer, we show how the modification of the support can influence the electronic and magnetic properties of an adsorbed transition metal complex. The two systems have been studied by a combination of electron spectroscopy and first principles theoretical calculations, which included modelling of the van der Waals interactions. For adsorption of iron phthalocyanine on the Cu(100)c(2x2)-2N/Cu(111) surface we find several different adsorbate structures at different surface sites, which all have very similar energies. Depending on the adsorption site - above a support nitrogen or a support copper atom - the spin magnetic moment of the molecular iron ion assumes different values. Although pyridine adsorption on the iron phthalocyanine monolayer reduces the spin magnetic moments for all phthalocyanine adsorption sites, the difference between the different sites persists: only for the iron phthalocyanine adsorbates with the central iron ion above a support nitrogen atom the spin is quenched completely, while a residual spin magnetic moment is found for the adsorption structures with the iron ion above a copper atom. The results illustrate clearly the relevance of the support as a ligand to the central ion.